Exercise Physiology

Overview

Exercise represents one the highest levels of extreme stresses to which the body can be exposed. For example, in a person who has an extremely high fever approaching the level of lethality, the body metabolism increases to approximately 100% above normal; by comparison, the metabolism of the body during a marathon race increases to 2000% above normal.[1, 2, 3]

The following illustration depicts the cell structures of the skeletal muscle.

Excitation contraction coupling. DHP is dihydropyridine.

This article describes the basic physiology of exercise. The focus of this article is mainly at a subspecialty level; however, more detailed descriptions of various basic mechanisms are also provided for the casual reader.

For excellent patient education resources, visit eMedicineHealth's Healthy Living Center. Also, see eMedicineHealth's patient education articles Walking for Fitness and Resistance Training.

NextBasic Concepts -- Sex Differences

In general, the exercise-related measurements established for women follow the same general principles as those established for men, except for the quantitative differences caused by differences in body size, body composition, and levels of testosterone.

In women, the values of muscle strength, pulmonary ventilation, and cardiac output (all variables related with muscle mass) are generally 60-75% of the exercise physiology values recorded in men. When measured in terms of strength per square centimeter, the female muscle can achieve the same force of contraction as that of a male.[4, 5]

PreviousNextMusculoskeletal SystemFunctions of Muscle Tissue

Muscle tissue has 4 characteristics[1, 2, 3] that assume roles in homeostasis, as follows:

Excitability – Property of receiving and responding to stimuli such as the following: Neurotransmitters: Acetylcholine (ACh) stimulates skeletal muscle to contract.Electrical stimuli: Applying electrical stimuli between cardiac and smooth muscle cells causes the muscles to contract. Applying a shock to skeletal muscle causes contraction. Hormonal stimuli: Oxytocin stimulates smooth muscle in the uterus to contract during labor.Contractility – Ability to shortenExtensibility – Ability to stretch without damageElasticity – Ability to return to original shape after extension

Through contraction, muscle provides motion of the body (skeletal muscle), motion of blood (cardiac muscle), and motion of hollow organs such as the uterus, esophagus, stomach, intestines, and bladder (smooth muscle).

Muscle tissue also helps maintain posture and produce heat. A large amount of body heat is produced by metabolism and by muscle contraction. Muscle contraction during shivering warms the body.

Histology of Skeletal Muscle Tissue

Skeletal muscle consists of fibers (cells).[1, 2, 3] These cells are up to 100 µm in diameter and often are as long as the muscle. Each contains sarcoplasm (cytoplasm) and multiple peripheral nuclei per fiber. Skeletal muscle is actually formed by the fusion of hundreds of embryonic cells. Other cell structures (see image below) include the following:

Excitation contraction coupling. DHP is dihydropyridine. Each fiber is covered by a sarcolemma (plasma membrane).The sarcoplasmic reticulum (smooth endoplasmic reticulum) stores calcium, which is released into the sarcoplasm during muscle contraction. Transverse tubules (T tubules), which are extensions of the sarcolemma that penetrate cells, transmit electrical impulses from the sarcolemma inward, so electrical impulses penetrate deeply into the cell. Besides conducting electricity along their walls, T tubules contain extracellular fluid rich in glucose and oxygen. The sarcoplasm of fiber is rich in glycogen (glucose polymer) granules and myoglobin (oxygen-storing protein). It also is rich in mitochondria.

Each fiber contains hundreds to thousands of rodlike myofibrils, which are bundles of thin and thick protein chains termed myofilaments. From a cross-sectional view of a myofibril, each thick filament is surrounded by a hexagonal array of 6 thin filaments. Each thin filament is surrounded by a triangular array of thick filaments.

Thin myofilaments are composed of 3 proteins: actin, tropomyosin, and troponin (see image below).Thin filament. Thick myofilaments consist of bundles of approximately 200 myosin molecules (see image below). Myosin molecules look like double-headed golf clubs (both heads at the same end). The heads of the golf clubs are called myosin heads; they are also called cross-bridges because they link thick and thin filaments during contraction. They contain actin and adenosine triphosphate (ATP) binding sites. Myosin heads project out from the thick filaments, allowing them to bind to the thin filaments during contraction. Thick filament. Actin is a long chain of multiple globular proteins, similar in shape to kidney beans. Each globular subunit contains a myosin-binding site. Tropomyosin is a long strand of protein that covers the myosin-binding sites on actin when the muscle is relaxed.Troponin is a polypeptide complex that binds to tropomyosin, helping to position it over the myosin-binding sites on actin. During muscle contraction, calcium binds troponin, which causes tropomyosin to roll off of the myosin binding sites on actin.

Muscle contraction (overview of the sliding filament mechanism)

A muscle action potential travels over sarcolemma and enters the T tubules, causing the sarcoplasmic reticulum to release calcium into the sarcoplasm. This triggers the contractile process.

Myosin cross-bridges pull on the actin myofilaments, causing the thin myofilaments of a sarcomere to slide toward the centers of the H zones, as shown below.

Sarcomere. Other Components of Skeletal Muscle

Connective-tissue components

Deep fascia is a broad band of dense irregular connective tissue beneath and around muscle and organs.[1, 2, 3] Deep fascia is different from superficial fascia, which is loose areolar connective tissue.

Other connective-tissue components (all are extensions of deep fascia) include epimysium, which covers the entire muscle; perimysium, which penetrates into muscle and surrounds bundles of fibers called fascicles; and endomysium, which is delicate, barely visible, loose areolar tissue covering individual fibers (ie, individual cells).

Tendons and aponeuroses are tough extensions of epimysium, perimysium, and endomysium. Tendons and aponeuroses are made of dense regular connective tissue and attach the muscle to bone or other muscle. Aponeuroses are broad, flat tendons. Tendon sheaths contain synovial fluid and enclose certain tendons. Tendon sheaths allow tendons to slide back and forth next to each other with lower friction. Tenosynovitis is inflammation of the tendon sheaths and tendons, especially those of the wrists, shoulders, and elbows. Tendons are not contractile and not very stretchy; furthermore, they are not very vascular and they heal poorly.

Nerve and blood supply

Nerves convey impulses for muscular contraction. Nerves are bundles of nerve cell processes. Each nerve cell process (ie, axon) divides at its tip into a few to 10,000 branches called telodendria. At the end of each of these branches is an axon terminal that is rich in neurotransmitters.

Blood provides nutrients and oxygen for contraction. An artery and a vein usually accompany a nerve that penetrates skeletal muscle. Arteries in muscles dilate during active muscular activity, thus increasing the supply of oxygen and glucose.

Motor units

A motor nerve is a bundle of axons that conducts nerve impulses away from the brain or spinal cord toward muscles. Each axon transmits an action potential (ie, nerve impulse), which is a burst of electricity. The nerve impulse travels along the axons at a steady rate, like fire travels along a fuse; however, nerve impulses travel extremely fast. Each axon has 4-2000 or more branches (ie, telodendria), with an average of 150 telodendria. Each separate branch supplies a separate muscle cell. Thus, if an axon has 10 branches, it supplies 10 muscle fibers. Small motor units are for fine control of muscles; large motor units are for muscles that do not require such fine control.

Neuromuscular junction

The neuromuscular junction is made of an axon terminal and the portion of the muscle fiber sarcolemma it nearly touches (called the motor endplate). The neurotransmitter released at the neuromuscular junction in skeletal muscle is ACh. The motor endplate is rich in thousands of ACh receptors; the receptors are integral proteins containing binding sites for ACh and sodium channels.

Physiology of ContractionNerve impulse (action potential) reaches the axon terminal, which triggers calcium influx into the axon terminal.[1, 2, 3] Calcium influx causes synaptic vesicles to release ACh via exocytosis.ACh diffuses across synaptic cleft.ACh binds to the ACh receptor on the sarcolemma. Succinylcholine, a drug used to induce paralysis during surgery, binds to ACh receptors more tightly than ACh. Succinylcholine initially causes some depolarization, but then it binds to the receptor, preventing ACh from binding. Therefore, it blocks the muscle's stimulation by ACh, causing paralysis. Another drug that acts in a similar fashion is curare. These drugs do not cause pain relief or unconsciousness; thus, they are combined with other drugs during surgery. When ACh binds the receptor, it opens chemically regulated ion channels, which are sodium channels through the receptor molecule. Sodium, which is in high concentration outside cells and in low concentration inside cells, rushes into the cell through the channels. The cell, whose resting membrane potential along the inside of the membrane is negative when compared with the outside of the membrane, becomes positively charged along the inside of the membrane when sodium (a positive ion) rushes in. This change from a negative charge to a positive charge along the inner membrane is termed depolarization. The depolarization of one region of the sarcolemma (the motor endplate) initiates an action potential, which is a propagating wave of depolarization that travels (propagates) along the sarcolemma. Regions of membrane that become depolarized rapidly restore their proper ionic concentrations along their inner and outer surfaces in a process termed repolarization. (This process of depolarization, propagation, and repolarization is similar to dominoes that topple each other but also spring back into the upright position shortly afterward.) The action potential also propagates along the membrane lining the T tubules entering the cell.This action potential traveling along the T tubules causes the sarcoplasmic reticulum to release calcium into sarcoplasm.Calcium binds with troponin, causing it to pull on tropomyosin to change its orientation, exposing myosin-binding sites on actin. An ATPase, which also functions as a myosin cross-bridging protein, splits ATP into adenosine diphosphate (ADP) + phosphate (P) in the previous contraction cycle. This energizes the myosin head. The energized myosin head, or cross-bridge, combines with myosin-binding sites on actin. Power stroke occurs. The attachment of the energized cross-bridge triggers a pivoting motion (ie, power stroke) of the myosin head. During the power stroke, ADP and P are released from the myosin cross-bridge. The power stroke causes thin actin myofilaments to slide past thick myosin myofilaments toward the center of the A bands (see the following image). Sarcomere. ATP attaches to the myosin head again, allowing it to detach from actin. (In rigor mortis, an ATP deficiency occurs. Cross-bridges remain, and the muscles are rigid.) ATP is broken down to ADP and P, which cocks the myosin head again, preparing it to perform another power stroke if needed.Repeated detachment and reattachment of the cross-bridges results in shortening without much increase in tension during the shortening phase (isotonic contraction) or results in increased tension without shortening (isometric contraction). Release of the enzyme acetylcholinesterase in the neuromuscular junction destroys ACh and stops the generation of a muscle action potential. Calcium is taken back up (resequestered) in the sarcoplasmic reticulum, and myosin cross-bridges separate. ATP is required to separate myosin-actin cross-bridges. The muscle fiber resumes its resting state. Muscle Metabolic Systems During Exercise

The chemical energy that fuels muscular activities is ATP.[1, 2, 3] For the first 5 or 6 seconds of muscle power, muscular activity can depend on the ATP that is already present in the muscle cells. Beyond this time, new amounts of ATP must be formed to enable the activation of muscular contractions that are needed to support longer and more vigorous physical activities.

For activities that require a quick burst of energy that cannot be supplied by the ATP present in the muscle cells, the next 10-15 seconds of muscle power can be provided through the body’s use of the phosphagen system, which uses a substance called creatine phosphate to recycle ADP into ATP.[6] For longer and more intense periods of physical activity, the body must rely on systems that break down the sugars (glucose) to produce ATP.

The complete breakdown of glucose occurs in 2 ways: through anaerobic respiration (does not use oxygen) and through aerobic respiration (occurs in the presence of oxygen). The anaerobic use of glucose to form ATP occurs as the body increases its muscle use beyond the capability of the phosphagen system to supply energy. In particular, the glycogen lactic acid system, through its anaerobic breakdown of glucose, provides approximately 30-40 seconds more of maximal muscle activity. For this system, each glucose molecule is split into 2 pyruvic acid molecules, and energy is released to form several ATP molecules, providing the extra energy. Then, the pyruvic acid partially breaks down further to produce lactic acid. If the lactic acid is allowed to accumulate in the muscle, one experiences muscle fatigue. At this point, the aerobic system must activate.

The aerobic system in the body is used for sports that require an extensive and enduring expenditure of energy, such as a marathon race. Endurance sports absolutely require aerobic energy. A large amount of ATP must be provided to muscles to sustain the muscle power needed to perform such events without an excessive production of lactic acid. This can only be accomplished when oxygen in the body is used to break down the pyruvic acid (that was produced anaerobically) into carbon dioxide, water, and energy by way of a very complex series of reactions known as the citric acid cycle. This cycle supports muscle usage for as long as the nutrients in the body last.

The breakdown of pyruvic acid requires oxygen and slows or eliminates the accumulation of lactic acid. In summary, the 3 different muscle metabolic systems that supply the energy required for various activities are as follows:

Phosphagen system (for 10- to 15-sec bursts of energy)Glycogen lactic acid system (for another 30-40 sec of energy)Aerobic system (provides a great deal of energy that is only limited by the body's ability to supply oxygen and other important nutrients)

Many sports require the use of a combination of these metabolic systems. By considering the vigor of a sports activity and its duration, one can estimate very closely which of the energy systems are used for each activity.

Postexercise Recovery

Oxygen debt

During muscular exercise, blood vessels in muscles dilate and blood flow is increased in order to increase the available oxygen supply.[1, 2, 3] Up to a point, the available oxygen is sufficient to meet the energy needs of the body. However, when muscular exertion is very great, oxygen cannot be supplied to muscle fibers fast enough, and the aerobic breakdown of pyruvic acid cannot produce all the ATP required for further muscle contraction.

During such periods, additional ATP is generated by anaerobic glycolysis. In the process, most of the pyruvic acid produced is converted to lactic acid. Although approximately 80% of the lactic acid diffuses from the skeletal muscles and is transported to the liver for conversion back to glucose or glycogen, some lactic acid accumulates in muscle tissue, making muscle contraction painful and causing fatigue. Ultimately, once adequate oxygen is available, lactic acid must be catabolized completely into carbon dioxide and water.

After exercise has stopped, extra oxygen is required to metabolize lactic acid; to replenish ATP, phosphocreatine, and glycogen; and to replace (“pay back”) any oxygen that has been borrowed from hemoglobin, myoglobin (an iron-containing substance similar to hemoglobin that is found in muscle fibers), air in the lungs, and body fluids. The additional oxygen that must be taken into the body after vigorous exercise to restore all systems to their normal states is called oxygen debt. The debt is paid back by labored breathing that continues after exercise has stopped. Thus, the accumulation of lactic acid causes hard breathing and sufficient discomfort to stop muscle activity until homeostasis is restored.[7]

Recovery of muscle glycogen postexercise

Eventually, muscle glycogen must also be restored. Restoration of muscle glycogen is accomplished through diet and may take several days, depending on the intensity of exercise. The maximum rate of oxygen consumption during the aerobic catabolism of pyruvic acid is called maximal oxygen uptake.

Maximal oxygen uptake is determined by sex (higher in males), age (highest at approximately age 20 y), and size (increases with body size). Highly trained athletes can have maximal oxygen uptakes that are twice that of average people, probably owing to a combination of genetics and training. As a result, highly trained athletes are capable of greater muscular activity without increasing their lactic acid production and have lower oxygen debts, which is why they do not become short of breath as readily as untrained individuals.

Fuel Usage

Fuel usage (light exercise)

The best examples of light exercise are walking and light jogging.[1, 2, 3] The muscles that are recruited during this type of exercise are those that contain a large amount of type I muscle cells, and, because these cells have a good blood supply, it is easy for fuels and oxygen to travel to the muscle. ATP consumption makes ADP available for new ATP synthesis.

The presence of ADP (and the resulting synthesis of ATP) simulates the movement of hydrogen (H+) into the mitochondria; this, in turn, reduces the proton gradient and thus stimulates electron transport. The hydrogen on the reduced form of nicotinamide adenine dinucleotide (NADH) is used up, nicotinamide adenine dinucleotide (NAD) becomes available, and fatty acids and glucose are oxidized.

Incidentally, the calcium released during contraction stimulates the enzymes in the Krebs cycle and stimulates the movement of the glucose transporter 4 (GLUT-4) from inside of the muscle cell to the cell membrane. Both these exercise-induced responses augment the elevation in fuel oxidation caused by the increase in ATP consumption.

Fuel usage (moderate exercise)

An increase in the pace of running simply results in an increased rate of fuel consumption, an increased fatty acid release, and, therefore, an increase in the rate of muscle fatty acid oxidation. However, if the intensity of the exercise increases even further, a stage is reached in which the rate of fatty acid oxidation becomes limited.

The reasons why the rate of fatty acid oxidation reaches a maximum are not clear, but it is possible that the enzymes in the beta-oxidation pathway are saturated (ie, they reach a stage in which their maximal velocity [Vmax] is less than the rate of acetyl-coenzyme A [acetyl-CoA] consumption in the Krebs cycle). Alternatively, it may be that the availability of carnitine (the chemical required to transport the fatty acids into the mitochondria) becomes limited.

Whatever the reason, the consequence is that as the pace rises, the demand for acetyl-CoA cannot be met by fatty acid oxidation alone. The accumulation of acetyl-CoA that was so effective at inhibiting the oxidation of glucose is no longer present, so pyruvate dehydrogenase starts working again and pyruvate is converted into acetyl-CoA. In other words, more of the glucose that enters the muscle cell is oxidized fully to carbon dioxide. Therefore, the energy used during moderate exercise is derived from a mixture of fatty acid and glucose oxidation.

Fuel usage (strenuous exercise)

As the intensity of the exercise increases even further (ie, running at the pace of middle-distance races), the rate at which the muscles can extract glucose from the blood becomes limited. In other words, the rate of glucose transport reaches Vmax, either because the blood cannot supply the glucose fast enough or the number of GLUT-4s becomes limited. ATP generation cannot be serviced completely by exogenous fuels, and ATP levels decrease. Not only does this stimulate phosphofructokinase, it also stimulates glycogen phosphorylase. This means that glycogen stored within the muscle cells is broken down to provide glucose. Therefore, the fuel mix during strenuous exercise is composed of contributions from blood-borne glucose and fatty acids and from endogenously stored glycogen.

Fuel usage in individuals who are unfit

Being fit (biochemically speaking) means that the individual has a well-developed cardiovascular system that can efficiently supply nutrients and oxygen to the muscles. Fit people have muscle cells that are well perfused with capillaries (ie, they have a good muscle blood supply). Their muscle cells also have a large number of mitochondria, and those mitochondria have a high activity of Krebs cycle enzymes, electron transport carriers, and oxidation enzymes.

Individuals who are unfit must endure the consequences of a poorer blood supply, fewer mitochondria, less electron transport units, a lower activity of the Krebs cycle, and poorer activity of beta-oxidation enzymes. To generate ATP in the mitochondria, a steady supply of fuel and oxygen and decent activity of the oxidizing enzymes and carriers are needed. If any of these components are lacking, the rate at which ATP can be produced by mitochondria is compromised. Under these circumstances, the production of ATP by aerobic means is not sufficient to provide the muscles with sufficient ATP to sustain contractions. The result is anaerobic ATP generation using glycolysis. Increasing the flux through glycolysis but not increasing the oxidative consumption of the resulting pyruvate increases the production of lactate.

PreviousNextPulmonary Physiology During Exercise

The purpose of respiration is to provide oxygen to the tissues and to remove carbon dioxide from the tissues.[1, 2, 3] To accomplish this, 4 major events must be regulated, as follows:

Pulmonary ventilationDiffusion of oxygen and carbon dioxide between the alveoli and the bloodTransport of oxygen and carbon dioxide in the blood and body fluids and to and from the cellsRegulation of ventilation and other aspects of respiration: Exercise causes these factors to change, but the body is designed to maintain homeostasis.

When one goes from a state of rest to a state of maximal intensity of exercise, oxygen consumption, carbon dioxide formation, and total pulmonary and alveolar ventilation increase by approximately 20-fold. A linear relationship exists between oxygen consumption and ventilation. At maximal exercise, pulmonary ventilation is 100-110 L/min, whereas maximal breathing capacity is 150-170 L/min. Thus, the maximal breathing capacity is approximately 50% greater than the actual pulmonary ventilation during maximal exercise. This extra ventilation provides an element of safety that can be called on if the situation demands it (eg, at high altitudes, under hot conditions, abnormality in the respiratory system). Therefore, the respiratory system itself is not usually the most limiting factor in the delivery of oxygen to the muscles during maximal muscle aerobic metabolism.

VO2max is the rate of oxygen consumption under maximal aerobic metabolism. This rate in short-term studies is found to increase only 10% with the effect of training. However, that of a person who runs in marathons is 45% greater than that of an untrained person. This is believed to be partly genetically determined (eg, stronger respiratory muscles, larger chest size in relation to body size) and partly due to long-term training.

Oxygen diffusing capacity is a measure of the rate at which oxygen can diffuse from the alveoli into the blood. An increase in diffusing capacity is observed in a state of maximal exercise. This results from the fact that blood flow through many of the pulmonary capillaries is sluggish in the resting state. In exercise, increased blood flow through the lungs causes all of the pulmonary capillaries to be perfused at their maximal level, providing a greater surface area through which oxygen can diffuse into the pulmonary capillary blood. Athletes who require greater amounts of oxygen per minute have been found to have higher diffusing capacities, but the exact reason why is not yet known. Although one would expect the oxygen pressure of arterial blood to decrease during strenuous exercise and carbon dioxide pressure of venous blood to increase far above normal, this is not the case. Both of these values remain close to normal.

Stimulatory impulses from higher centers of the brain and from joint and muscle proprioceptive stimulatory reflexes account for the nervous stimulation of the respiratory and vasomotor center that provides almost exactly the proper increase in pulmonary ventilation to keep the blood respiratory gases almost normal. If nervous signals are too strong or weak, chemical factors bring about the final adjustment in respiration that is required to maintain homeostasis.

PreviousNextCardiovascular System and Exercise

Regular exercise makes the cardiovascular system more efficient at pumping blood and delivering oxygen to the exercise muscles.[1, 2, 3, 8, 9, 10] Releases of adrenaline and lactic acid into the blood result in an increase of the heart rate (HR).

Basic definitions of terms are as follows:

VO2 equals cardiac output times oxygen uptake necessary to supply oxygen to muscles.The Fick equation is the basis for determination of VO2 (see image below).[10] Fick equation.

Exercises increase some of the different components of the cardiovascular system, such as stroke volume (SV), cardiac output, systolic blood pressure (BP), and mean arterial pressure. A greater percentage of the cardiac output goes to the exercising muscles. At rest, muscles receive approximately 20% of the total blood flow, but during exercise, the blood flow to muscles increases to 80-85%.

To meet the metabolic demands of skeletal muscle during exercise, 2 major adjustments to blood flow must occur. First, cardiac output from the heart must increase. Second, blood flow from inactive organs and tissues must be redistributed to active skeletal muscle.

Generally, the longer the duration of exercise, the greater the role the cardiovascular system plays in metabolism and performance during the exercise bout. An example would be the 100-meter sprint (little or no cardiovascular involvement) versus a marathon (maximal cardiovascular involvement).

General functions of the cardiovascular system

The cardiovascular system helps transport oxygen and nutrients to tissues, transport carbon dioxide and other metabolites to the lungs and kidneys, and distribute hormones throughout the body. The cardiovascular system also assists with thermoregulation.

Cardiac cycle

The pumping of blood by the heart requires the following 2 mechanisms to be efficient:

Alternate periods of relaxation and contraction of the atria and ventriclesCoordinated opening and closing of the heart valves for unidirectional flow of blood

The cardiac cycle is divided into 2 phases: ventricular diastole and ventricular systole.

Ventricular diastole This phase begins with the opening of the atrioventricular (AV) valves. The mitral valve (located between the left atrium and left ventricle) opens when the left ventricular pressure falls below the left atrial pressure, and the blood from left atrium enters the left ventricle. Later, as the blood continues to flow into the left ventricle, the pressure in both chambers tends to equalize.At the end of the diastole, left atrial contractions cause an increase in left atrial pressure, thus again creating a pressure gradient between the left atrium and ventricle and forcing blood into the left ventricle. Ventricular systole Ventricular systole begins with the contraction of the left ventricle, which is caused by the spread of an action potential over the left ventricle. The contraction of the left ventricle causes an increase in the left ventricular pressure. When this pressure is higher than the left atrial pressure, the mitral valve is closed abruptly. The left ventricular pressure continues to rise after the mitral valve is closed. When the left ventricular pressure rises above the pressure in the aorta, the aortic valve opens. This period between the closure of the mitral valve and the opening of the aortic valve is called isovolumetric contraction phase. The blood ejects out of the left ventricle and into the aorta once the aortic valve is opened. As the left ventricular contraction is continued, 2 processes lead to a fall in the left ventricular pressure. These include a decrease in the strength of the ventricular contraction and a decrease in the volume of blood in the ventricle. When the left ventricular pressure falls below the aortic pressure, the aortic valve is closed. After the closure of the aortic valve, the left ventricular pressure falls rapidly as the left ventricle relaxes. When this pressure falls below the left atrial pressure, the mitral valve opens and allows blood to enter left ventricle. The period between the closure of the aortic valve closure and the opening of the mitral valve is called isovolumetric relaxation time. Right-sided heart chambers undergo the same phases simultaneously.Pressure changes during the cardiac cycle

Most of the work of the heart is completed when ventricular pressure exists. The greater the ventricular pressure, the greater the workload of the heart. Increases in BP dramatically increase the workload of the heart, and this is why hypertension is so harmful to the heart.

Arterial BP is the pressure that is exerted against the walls of the vascular system. BP is determined by cardiac output and peripheral resistance. Arterial pressure can be estimated using a sphygmomanometer and a stethoscope. The reference range for males is 120/80 mm Hg; the reference range for females is 110/70 mm Hg.

The difference between systolic and diastolic pressure is called the pulse pressure. The average pressure during a cardiac cycle is called the mean arterial pressure (MAP). MAP determines the rate of blood flow through the systemic circulation.

During rest, MAP = diastolic BP + (0.33 X pulse pressure). For example, MAP = 80 + (0.33 X [120-80]), MAP = 93 mm Hg.During exercise, MAP = diastolic BP + (0.50 X pulse pressure). For example, MAP = 80 + (0.50 X [160-80]), MAP = 120 mm Hg.Coordinated control of the heart

The heart has the ability to generate its own electrical activity, which is known as intrinsic rhythm. In the healthy heart, contraction is initiated in the sinoatrial (SA) node, which is often called the heart's pacemaker. If the SA node cannot set the rate, then other tissues in the heart are able to generate an electrical potential and establish the HR.

Control of cardiac output (HR)

The parasympathetic nervous system and the sympathetic nervous system affect a person's HR.

Parasympathetic nervous system: The vagus nerve originates in the medulla and innervates the SA and AV nodes. The nerve releases ACh as the neurotransmitter. The response is a decrease in SA node and AV node activity, which causes a decrease in HR. Sympathetic nervous system: The nerves arise from the spinal cord and innervate the SA node and ventricular muscle mass. The nerves release norepinephrine as the neurotransmitter. The response is an increase in HR and a force of contraction of the ventricles. Control of sympathetic and parasympathetic activity

At rest, sympathetic and parasympathetic nervous stimulation are in balance. During exercise, parasympathetic stimulation decreases and sympathetic stimulation increases. Several factors can alter sympathetic nervous system input.

Baroreceptors are groups of neurons located in the carotid arteries, the arch of aorta, and the right atrium. These neurons sense changes in pressure in the vascular system. An increase in BP results in an increase in parasympathetic activity except during exercise, when the sympathetic activity overrides the parasympathetic activity.

Chemoreceptors are groups of neurons located in the arch of the aorta and the carotid arteries. These neurons sense changes in oxygen concentration. When oxygen concentration in the blood is decreased, parasympathetic activity decreases and sympathetic activity increases.

Temperature receptors are neurons located throughout the body. These neurons are sensitive to changes in body temperature. As temperature increases, sympathetic activity increases to cool the body and to reduce internal core temperature.

Control of cardiac output (SV)

SV is controlled by end-diastolic volume, average aortic BP, and the strength of ventricular contraction.

End-diastolic volume: This is often referred to as the preload. If the end-diastolic volume increases, the SV increases. With an increased end-diastolic volume, a slight stretching of the cardiac muscle fibers occurs, which increases the force of contraction Average aortic BP: This is often referred to as the afterload. The BP in the aorta represents a barrier to the blood being ejected from the heart. The SV is inversely proportional to the aortic BP. During exercise, the afterload is reduced, which allows for an increase in SV. Strength of ventricular contraction: Epinephrine and norepinephrine can increase the contractility of the heart by increasing the calcium concentration within the cardiac muscle fiber. Epinephrine and norepinephrine allow for greater calcium entry through the calcium channels in cardiac muscle fiber membranes. This allows for greater myosin and actin interaction and an increase in force production. Control of cardiac output (venous return)

Venoconstriction occurs as a response to sympathetic nervous system stimulation. Sympathetic stimulation constricts the veins that drain skeletal muscle. This causes greater blood to flow back to the heart.

The muscle pump is the rhythmic contraction and relaxation of skeletal muscle that compresses the veins and thus drains the skeletal muscle. This causes greater blood flow back to the heart. The muscle pump is very important during both resting and exercise conditions.

During exercise, the respiratory pump helps increase venous return. The pressure within the chest decreases and abdominal pressure increases with inhalation, thus facilitating blood flow back to the heart. Because of the increased respiratory rate and depth of breathing during exercise, this is an effective way to increase venous return.

Hemodynamics

The circulatory system is a closed-loop system, and flow through the circulatory system is the result of pressure differences between the 2 ends of the system, the left ventricle (90 mm Hg) and the right atrium (approximately 0 mm Hg).

Systemic blood flow affects hemodynamics. The control of blood flow during exercise is extremely important to ensure that blood and oxygen are transported to the tissues that need them most. Blood flow to tissues is dependent on the relationship between BP and the resistance provided by the blood vessels.

Blood flow at rest is equal to the change in pressure divided by the resistance of the vessels (ie, BF = P/R, where BF is blood flow, P is pressure, and R is resistance). Blood flow during exercise is regulated by changing BP and altering the peripheral resistance of the vessels.

The pressure change at rest in the cardiovascular system is 93 mm Hg, as follows: Mean aortic pressure = 93 mm Hg, mean right atrial pressure = 0 mm Hg, and driving pressure in the system = 93 mm Hg.

During exercise, BP increases so that blood flow through the body increases. Blood flow is also increased during exercise by decreasing the resistance of the vessels in the systemic circulation of active skeletal muscle. Resistance is determined by the following formula:

Resistance = (length of tube X viscosity of blood)/radius4.

Changing the radius of the vessels has the most profound effect on blood flow. Doubling the radius of a blood vessel decreases resistance by a factor of 16. Decreasing the radius of a blood vessel by half increases resistance by a factor of 16. The arterioles have the most control over blood flow in the systemic circulation.

Changes in oxygen delivery to muscle during exerciseBP increases as exercise intensity increases, rising from approximately 120 mm Hg to approximately 200 mm Hg.SV increases during exercise until 40% of VO2max (maximum oxygen uptake level) is reached, rising from approximately 80 mL/beat to approximately 120 mL/beat.HR increases with intensity until VO2max is reached, rising from approximately 70 beats per minute to approximately 200 beats per minute.Cardiac output increases with intensity until VO2max is reached, rising from approximately 5 L/min to approximately 25-30 L/min.

The arterial-venous oxygen difference is the amount of oxygen extracted from the blood as it passes through the capillary bed. This difference rises from approximately 4 mL of oxygen per 100 mL of blood at rest to approximately 18 mL of oxygen per 100 mL of blood during high-intensity aerobic exercise.

Redistribution of blood flow during exercise

At rest, 15-20% of blood goes to skeletal muscle; during exercise, this amount increases to 80-85%. The percentage of blood to the brain decreases, but the absolute amount increases. The same percentage of blood goes to cardiac muscle, but the absolute amount increases. Blood flow to visceral tissues and inactive skeletal muscle reduces. In addition, the cutaneous blood flow initially decreases, but it later increases during the course of exercise.

The redistribution of the blood is brought about by several mechanisms. During exercise, generalized vasodilatation occurs because of the accumulation of vasodilatory metabolites. This leads to a decrease in the peripheral resistance, which, in turn, elicits a strong increase in the sympathetic activity through the activation of baroreceptors. The increase in sympathetic activity leads to vasoconstriction in the visceral organs, whereas the vasodilatation predominates in the blood vessels of the muscles and the coronary circulation because of the local vasodilatory metabolites. The cutaneous blood vessels initially respond to the sympathetic activity by vasoconstriction. As the exercise continues, temperature reflexes are activated and cause cutaneous vasodilatation to dissipate the heat produced by the muscle activity, resulting in an increase in the cutaneous blood flow.

Regulation of blood flow at the local level

The local blood flow is controlled by chemical factors, metabolites, paracrines, physical factors such as heat or cold, stretch effects on endothelial membrane, active hyperemia, and reactive hyperemia. The paracrine regulation is mainly regulated by nitric oxide, histamine release, and prostacyclin. Nitric oxide diffuses to smooth muscle and causes vasodilation by reducing calcium entry into smooth muscle.

Regulation of cardiovascular function

HR and blood flow are controlled by various centers in the brain. These centers receive input from receptors located throughout the body. The centers work to initiate the appropriate response from tissues and organs in the body.

Aerobic exercise requires oxygen to be present for the generation of energy from fuels such as glucose or glycogen. Aerobic exercise results in no buildup of lactic acid as a result of metabolism. This process is more efficient than anaerobic metabolism. During normal rest and aerobic exercise, carbohydrates and fats are used as fuels. A high degree of aerobic fitness requires a well-adapted ability to take in, carry, and use oxygen. Laboratory measurements are most accurate, but they are expensive. An individual's fitness level may be estimated according to these measurements.

Anaerobic exercise produces lactic acid and is usually of short duration. Anaerobic exercise is high intensity and has a greater inherent risk of injury. Individuals who are unfit have a lower anaerobic threshold than athletes who are aerobically trained. The well-trained athlete may be able to approach 80% of the VO2max aerobically without lactate production.

The usual VO2 measurements are in L/min; however, if the size of the individual needs to be accounted for, the measurements may be in mL/kg/min. The values for the average person aged 20 years are 37-48 mL/kg/min. Male athletes who are highly trained may approach measurements in the high 70s to low 80s. Training enhances the ability of the body, in particular the muscle cells, to better handle oxygen. Muscle must be able to use oxygen efficiently to keep anaerobic metabolism at a given level of effort to a minimum.

Cardiac output is a major determinant of oxygen uptake. VO2max declines with age as the maximum HR declines. This is one of the major factors causing the approximately 7% decline with each decade of life after age 30 years. Muscle training and use of oxygen at the end organ, muscle, is the second factor that affects oxygen uptake. The arterial-venous oxygen difference comes about as a combination of arterial oxygen content, shunting of blood to muscles, and the muscle extraction of oxygen.

Training results in a more efficient heart and an increase in the maximum SV. An increase in VO2 results in an ease in the stress of a given workload. When maximum SV is increased, the heart can work more efficiently at a given pulse rate. This lessens the necessity of an increased pulse at a given workload. Resting pulse is lower, as is the pulse at any given workload.

One metabolic unit (MET) equals the VO2 at rest. The estimate of the value of one MET is 3.5 mL of oxygen per kg/min. Conversion of VO2 measurements may be obtained by dividing the value of the VO2 in mL of oxygen per kg/min by the value of one MET or 3.5. For example, a VO2 measurement of 35 mL of oxygen per kg/min is equivalent to an output of 10 METs.

Cardiovascular changes with isometric exercise

Cardiovascular changes during isometric exercise differ from those during dynamic exercise. Static exercise causes compression of the blood vessels in the contracting muscles, leading to a reduction in the blood flow in them. Therefore, total peripheral resistance, which normally falls during dynamic exercise, does not fall and may, in fact, increase, especially if several large groups of muscles are involved in the exercise. The activation of the sympathetic system with exercise thus leads to an increase in HR, cardiac output, and BP.

Because the total peripheral resistance does not decrease, the increase in HR and cardiac output is less and an increase in the systolic, diastolic, and mean arterial pressure is more compared with those seen with dynamic exercise. Because BP is a major determinant of afterload, the left ventricular wall stress, and thus the cardiac workload, is significantly higher during static exercise compared with the cardiac workload achieved during dynamic exercise.

Cardiac changes following training

In most cases, the SV plateaus at a VO2 of approximately 40-60% of the maximum. This applies to both trained and untrained males and females. The SV for untrained males may approach 100-120 mL/beat/min. For trained males, this value is 150-170 mL/beat/min. For highly trained athletes, maximal SV may reach or even exceed 200 mL/beat/min. The values for women are lower than those for men. Maximal SV for untrained women is usually between 80 mL/beat/min, and for trained women, it is usually between 100 mL/beat/min. These changes translate into an increase in the circulation blood volume and in cardiac output, with a corresponding decrease in the resting HR and the resting and exercise BP.

The heart undergoes certain morphologic changes in response to chronic exercise, commonly seen via echocardiography. These morphologic changes define what is commonly referred to as an "athletic heart." Athletic heart syndrome is characterized by hypertrophy of the myocardium (ie, an increase in the mass of the myocardium).

Although the hypertrophy in athlete's heart is morphologically similar to that seen in patients with hypertension, several important differences exist. In contrast to the hypertension-induced hypertrophy, the hypertrophy in the athletic heart is noted in absence of any diastolic dysfunction, with a normal isovolumetric relaxation time, with no decrease in the peak rate of left ventricular filling, and with no decrease in the peak rate of left ventricular cavity enlargement and wall thinning. Because the wall stress in the athlete's heart is normal, sometimes the hypertrophy seems to be disproportionate to the level of resting BP.

In addition, the rate of decline in the left ventricular hypertrophy and mass is much more rapid when the training is stopped compared with the regression in the same parameters in treated hypertension. On average, the decline in these parameters is seen 3 weeks after stopping exercise, and these morphologic changes can be seen on echocardiograms.

Sometimes, these morphologic changes are confused with the changes seen in patients with hypertrophic cardiomyopathy (HCM). A few important morphologic differences exist. In athletic heart syndrome, the hypertrophy is usually symmetrical, as opposed to the asymmetrical hypertrophy in HCM. Also, the left ventricular size is generally normal or increased, and the left atrial size is normal, as opposed to a small left ventricular cavity with a larger left atrial cavity size (usually >4.5 cm) in HCM. Despite these differences, sometimes making a distinction between 2 conditions is a challenge.

PreviousNextSummary

In summary, exercise is accomplished by alteration in the body response to the physical stress (exercise physiology). These responses to exercise include an increase in the HR, BP, SV, cardiac output, ventilation, and VO2. The metabolism at the cellular level is also modulated to accommodate the demands of exercise. These changes occur temporarily during the exercise. Long-term changes also occur in the body metabolism and function.

Previous, Exercise Physiology

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Exercise Prescription

Background

Exercise prescription commonly refers to the specific plan of fitness-related activities that are designed for a specified purpose, which is often developed by a fitness or rehabilitation specialist for the client or patient. Due to the specific and unique needs and interests of the client/patient, the goal of exercise prescription should be successful integration of exercise principles and behavioral techniques that motivates the participant to be compliant, thus achieving their goals.[1, 2, 3, 4, 5, 6, 7]

Components of exercise prescription

An exercise prescription generally includes the following specific recommendations:

Type of exercise or activity (eg, walking, swimming, cycling)Specific workloads (eg, watts, walking speed)Duration and frequency of the activity or exercise sessionIntensity guidelines – Target heart rate (THR) range and estimated rate of perceived exertion (RPE)Precautions regarding certain orthopedic (or other) concerns or related comments

For an explanation of terms, see Glossary of Terms.

NextOverview

Substantial data are available regarding the benefits of physical activity.[1, 2, 3, 4, 5, 6, 7] For primary preventative benefits, physical activity patterns should begin in the early school years and continue throughout an individual's life. Schools must specifically designate physical education programs with aerobic activities for children at early ages. Programs should include recreational sports (eg, running, dancing, swimming). Support at home for an active lifestyle for children helps to promote healthy physical activity patterns.

In the clinical setting, discuss physical activity and provide exercise prescriptions for patients and their families. In some instances, suggestions could be made about implementing physical activity recommendations at the work site.

Consider intensity, duration, frequency, mode, and progression in all types of physical activity programs. As children and adolescents become adults and discontinue the athletic endeavors of school and college, primary prevention must include a plan for a lifetime of appropriate physical activity. Ideally, this activity should be performed for at least 30-60 minutes, 4-6 times weekly or 30 minutes on most days of the week. The frequency, duration, and intensity of activity should be individualized (exercise prescription) to personal satisfaction, mode, and progression.

Subjects may use individual end points of exercise, such as breathlessness and/or a fatigue level ranging from somewhat hard to hard on the Borg perceived exertion scale (see Glossary of Terms, Borg rating of perceived exertion [RPE]). Standardized charts that designate heart rates may help by providing heart rate end points that can be measured immediately after exercise, but these are not necessary. Exercise should include aerobic activities, such as bicycling (stationary or routine), walk-jog protocols, swimming, and other active recreational or leisure sports. Shoes and clothing should be appropriate for extremes of heat, cold, and humidity.

Resistive exercises using free weights or standard equipment should be performed 2-3 times per week. These exercises should include 8-10 exercise sets that consist of 10-15 repetitions per set (including arms, shoulders, chest, trunk, back, hips, and legs) and are performed at a moderate intensity. If free weights are used, 15-30 lb is generally adequate or resistance that requires a perceived effort that is relatively hard (ie, an RPE 15-16). Resistive exercises tend to complement aerobic exercise in that some training effect is realized.[8] However, as adults age, development of muscle tone and strengthening of body musculature is more important.

The long-term effect of any physical activity program is affected by compliance. In today's mobile society, an exercise plan must include activities for business trips and vacations. Exercise facilities may not be convenient in such settings, which may mean improvising. For example, a walk-jogger should bring walking or running shoes and find a safe place to walk or run at a pace that approximates the usual activity level. Many hotels or motels have exercise facilities with a track or treadmill, exercise cycle, and weights, enabling travelers or others away from their usual routine to maintain an exercise program.

Physical activity measured in total time or kilocalories (kcal) or kilojoules (kJ) per week is appropriate and may be achieved with various combinations of scheduling, such as 10-15 minutes in the morning, at noon, and/or an afternoon/evening session. Many persons may schedule longer, less frequent periods of exercise. As intensity decreases, frequency and duration should increase and vice versa. The dosage or total energy (calorie) expenditure per week must be individualized (exercise prescription).

Persons with influenza syndromes or respiratory illnesses should decrease or stop exercise until they have recovered. If the recovery time is greater than 2-3 weeks, activity should be resumed at a lower level to compensate for the slight loss in training level. Maintenance of the cardiovascular training effects of exercise has been shown to be more related to the exercise intensity than to exercise frequency or duration. In other words, if the intensity is maintained, even though the exercise sessions are less frequent or shorter in duration, transient reductions in conditioning from the decreased exercise appears to be minimized.

Various exercise testing measures of functional capacity should be used in special populations but are not necessary for primary prevention. Traditionally, many athletically inclined persons like to have periodic oxygen-uptake (VO2) measurements to assess their level of training. Physical activity levels are associated with long-term peak oxygen uptake (VO2peak). Highly active individuals have higher VO2peak compared with individuals who are inactive, and modification of activity levels from low to high can lead to substantially higher VO2peak compared with continuing to have low activity levels.[9]

Additionally, one study developed a nonexercise model of cardiorespiratory fitness by assessing age, waist circumference, leisure time physical activity, and resting heart rate. This model proved to be fairly accurate in predicting VO2peak in a healthy population of both men and women, and it may be a valid means of assessing cardiorespiratory fitness in an outpatient setting.[10]

However, recent technologic advances have not only made cardiopulmonary (CPX) or metabolic (CMET) testing more commonplace among medical practices, but they have also become increasingly popular as a part of a routine physical and stress test evaluation, especially for individuals who are considered at high risk for cardiovascular disease (eg, those with hypertension, shortness of breath, chest discomfort, or abnormal blood lipid levels).

A lifestyle of physical activity from childhood throughout the adult years fosters health and longevity. Even brisk walking as a physical activity/exercise habit promotes health benefits. This is the simplest program for most individuals and has clear benefits. This improved state of health is enhanced by weight control, restricted intake of saturated fat and cholesterol, abstinence from cigarette smoking, and control of high blood pressure and glucose intolerance.

For patient education resources, visit the Public Health Center, as well as Walking for Fitness and Strength Training.

PreviousNextBenefits of Exercise

Routine exercise improves tissue VO2 affects the following:

Improves insulin sensitivityImproves glycemic control in persons with type 2 diabetes (and, hence, decreases overall mortality)[11] Decreases blood pressureDecreases low-density lipoprotein and triglyceride levelsIncreases high-density lipoprotein levels

Considerable data also support evidence that exercise may decrease the prevalence of colon cancer and endometrial cancer. Exercise also helps with osteoarthritis and obesity, as well as reportedly benefits persons with migraine headaches and fibromyalgia.

Middle-aged men and women who work in physically demanding jobs or perform moderate to strenuous recreational activities have fewer manifestations of coronary artery disease than their less active peers. Meta-analysis studies of clinical trials reveal that medically prescribed and supervised exercise can reduce mortality rates for persons with coronary artery disease.[1, 7, 12, 13, 14, 15, 16]

The results from one study noted that moderate-to-vigorous physical activity at least 3 times per week was associated with lower risk of secondary cardiovascular disease (CVD) events in patients previously diagnosed with CVD. Researchers suggest improvement of metabolic and inflammatory risk markers as biological mechanisms.[17]

In a meta-analysis of 33 studies that included almost 200,000 participants and cases, Kodama et al quantified the relationship between better cardiorespiratory fitness and lower rates of coronary heart disease (CHD) and CVD events, as well as deaths from all causes.[18] When cardiorespiratory fitness was estimated as maximal aerobic capacity (MAC) expressed in metabolic equivalent (MET) units, a MAC of ≥7.9 METs was associated with substantially lower rates of CHD/CVD events and all-cause mortality.[18]

Furthermore, categorization of cardiorespiratory fitness as low (P [18]

Several studies suggest that relatively small amounts of physical activity show considerable reductions in mortality and improved health outcomes among participants when compared with sedentary control subjects. These findings imply that a minimal activity (ie, exercising once per week) may have positive health benefits even though fitness may not be measurably improved. Some authors have suggested that a threshold of physical activity may be necessary for maintaining optimal health and that future investigations should involve control subjects who participate in at least minimal activity levels rather than comparing exercise treatment groups to control subjects who are completely sedentary.

In addition to the physical benefits of exercise, both short- and long-term aerobic exercise training is associated with improvements in various indexes of psychologic functioning. Cross-sectional studies reveal that compared with sedentary individuals, active persons are more likely to be better adjusted, to perform better on tests of cognitive functioning, to exhibit reduced cardiovascular responses to stress, and to report fewer symptoms of anxiety and depression.

In one report, persons who increased their activity levels from 1965-1974 were at no greater risk for depression than those individuals who were active all along; however, persons who were active and became inactive were 1.5 times as likely to become depressed by 1983 compared with those who maintained an active lifestyle. Longitudinal studies have also documented significant improvement in psychologic functioning. Exercise training reduces depression in healthy older men and in persons with cardiac disease or major depression.

Exercise also improves self-confidence and self-esteem, attenuates cardiovascular and neurohumoral responses to mental stress, and reduces some type A behaviors. Although exercise training generally has not been found to improve cognitive performance, short bouts of exercise may have short-term facilitative effects.

Despite the positive physical and mental health benefits of exercise, long-term adherence to exercise programs remains problematic. Overall physical activity levels decrease with aging, in minority populations, in females, in disabled persons, and in those with chronic disease. Only an estimated 50% of all persons who initiate an exercise program continue the habit for more than 6 months. The issue of nonadherence is particularly important because exercise is only beneficial if it is maintained for extended periods. Thus, developing strategies to improve exercise initiation and adherence, especially for persons who are among the least active (eg, 75% of black women; less educated, obese, elderly persons), is important.[19, 20, 21, 22, 23, 24]

PreviousNextDifferent Types of Exercise

Exercise has been defined as an activity for the express purpose of improving fitness or health. Physical activity includes all forms of activity (eg, occupational, recreational, sports-related) that are performed without the specific purpose of fitness or health. Different types of exercise are as follows:

Aerobic (eg, walking, swimming)Anaerobic (eg, sprinting)Isotonic (eg, lifting weights)Resistance training: This involves providing some form of resistance to the contracting muscles to stimulate the body to increase strength. Multiple types of equipment are used for resistance training, including hand weights; cam machines; pulleys; and hydraulic, elastic, rubber, fiberglass, and magnetic equipment. Strength or resistance training is very important to improve functionality and reduce the risk of injury. As people age, the lean tissue (ie, muscle) declines more from lack of use than from aging itself. Regularly performing some type of resistance training is imperative. Because the demand on the heart is generally less during strength training than while walking at a moderate pace, resistance training is regarded as safe for patients with many heart conditions. Patients should never strain or hold their breath while attempting to lift something; straining can adversely affect blood flow to the heart. PreviousNextGeneral GuidelinesResistance and repetitionsResistance: The appropriate resistance may be provided by hand weights, elastic resistance, calisthenics, or machines and should be no more than what one can lift for approximately 15-20 repetitions. Perceived effort should only be moderate or somewhat hard. Repetition: A set is a group of repetitions, such as 2 or 3 sets of 15 repetitions. The number of sets depends on several factors, including time constraints, motivation, and personal goals. One to 3 sets are adequate for strength development. Add 1 set per week, increasing up to 3 sets. Progress: Progression can be made as one finds that the weight being used can be lifted more than 20-25 times. One should then increase the resistance slightly (eg, add 1-5 lb) and resume the training. As one reaches muscle fatigue, more stimulation of the muscle tissue results in protein being added to the muscle groups. Significant strength changes generally occur within 6 weeks. Other: Stretching should also be part of the exercise plan.Type of exercise (mode)Intensity: This should range from low to moderate for healthy individuals.Duration: Continuous aerobic activity for 20-60 minutes is recommended.Frequency Individuals with a less than a 3-MET capacity should engage in multiple short sessions each day.Individuals with a 3- to 5-MET capacity should engage in 1-2 sessions per day.Individuals with a greater than 5-MET capacity should engage in 3-5 sessions per week.Energy (caloric) expenditurePer-minute calculation: To calculate kilocalories per minute (kcal/min), multiply the METs times 3.5 times body weight in kilograms (kg) and divide by 200 (ie, kcal/min = [METs × 3.5 × kg body weight]/200). For example, the energy (caloric) expenditure of a 70-kg individual at a prescribed 6-MET capacity with a weekly goal of 1000 per week is calculated as (6 × 3.5 × 70 kg)/200, which equals 7.35 kcal/min (30.87 kJ/min). To convert kilocalories to kilojoules, note that 1 kcal = 4.2 kJ. Per-week calculation: This determines the exercise duration per week. Using the numbers from the example above, divide 1000 kcal (4200 kJ) by 7.35 kcal/min (30.87 kJ/min), which equals 136 min/wk or approximately 20-30 minutes, 6 d/wk. Lifetime activitiesVary the type of activity. Pick an activity that is enjoyable. The activity can be any type that uses most muscles, elevates the heart rate, and may be sustained for 20 minutes or longer. For example, one may find stationary cycling boring but enjoy playing tennis or racquetball. Vary the duration and intensity within the guidelines. Some days, decrease the intensity of the activity but increase the duration. On other days, warm up and then increase the intensity to the upper range of the guidelines but decrease the duration. Contract with a friend (buddy system) or participate in group classes.Use music for rhythm. If an activity is boring, either change it or find another one that is enjoyable. For example, if one is stationary cycling, videos that show outdoor scenery are available. Also, new saddles are available that make cycling much more comfortable. Make exercising enjoyable by selecting at least 2 activities that are enjoyable.Conditioning may be realized from many activities if applied correctly.Walk daily, whether one has a dog or not.Selecting the right physical activitiesSelect physical activities that are enjoyable, use most of the muscles, are rhythmic, and may be sustained for several minutes to an hour. Plan to exercise every other day until more adequately adapted to the activity.Think of the frequency, intensity, time, and type (ie, FITT) plan. Frequency: This is how often per week one will perform the exercise. Plan on most days of the week.Intensity: This is how hard one exercises. Moderate effort is appropriate.Time: This is the duration of each session. Start off with as little as needed (10 min if necessary).Type: This is the choice of physical activity, which can include recreational activities and domestic or occupational activities. A short list of each follows: Recreational activities Participating in aerobic activity classes; performing calisthenics, gymnastics, low-impact aerobics, martial artsBackpacking, climbing hills, stair climbing, walking, hiking, orienteering, runningPlaying badminton, baseball, basketball, catch (eg, flying discs), cricket, handball, racquetball, lacrosse, rugby, shuffleboard, table tennis, tennis, volleyball, water polo Body building, bowling, boxing, cycling, dancing, fencing, gardening, golfing, horseback riding, hunting, in-line skating, skating, rope skipping, skiing, snow shoeing, weight lifting, windsurfing Canoeing, sailing, scuba diving, swimming, fishing, participating in water activitiesDomestic or occupational activities – Cleaning windows, doing housework, mowing, packing and unpacking, plowing, sanding, sawing, sweeping, stocking shelves, pushing a wheelbarrow, performing yard work, etc Set goals, which may include those regarding health, improving physical capacity or performance.Motivation may be helpful for compliance. See the following tips: Join a class or facility, or contract with a friend (buddy system).Listen to one's body (eg, slowing down or skipping if tired or ill). Start at the present level to prevent soreness.Exercise at the same time each day.Make sure to have good-quality nutrition.Make exercising a priority; scheduling a time benefits the individual.Get advice if help is needed.PreviousNextExercise Prescription for Special PopulationsAdvanced ageMaximum ventilatory perfusion (VQ) drops 5-15% per decade in individuals aged 20-80 years. A lifetime of dynamic exercise maintains the individual's VQ at a level higher than that expected for any given age. The rate of decline in VQ is directly related to maintenance of the physical activity level, which emphasizes the importance of physical activity. Developing and maintaining aerobic endurance, joint flexibility, and muscle strength is important in a comprehensive exercise program, especially as people age. Elderly women and men show comparable improvement in exercise training, and adherence to training in elderly individuals is high. Resistance training exercise alone has only a modest effect on risk factors compared with aerobic endurance training, but resistance training does aid carbohydrate metabolism through the development or maintenance of muscle mass and effects on basal metabolism. Furthermore, resistance training is recommended by most health promotion organizations for its effects on maintenance of strength, muscle mass, bone mineral density, functional capacity, and prevention and/or rehabilitation of musculoskeletal problems (eg, low back pain). In elderly individuals, resistance training is both safe and beneficial in improving flexibility and quality of life. Persons with cardiovascular disease are usually asked to refrain from heavy lifting and forceful isometric exercises, but moderate-intensity dynamic strength training is safe and beneficial in persons at low risk. Pulmonary diseaseIndividuals with pulmonary disease should engage in low workloads of short duration.Patients should exercise in frequent intervals rather than a longer duration and fewer intervals.Supplemental oxygen is needed.DiabetesExercise should be of short duration with a gradual progression to longer durations, as tolerated.Exercises include aquatic aerobics, swimming, walking, jogging, or bicycling, among others.Monitor blood sugar because hypoglycemia or hyperglycemia can occur in diabetic persons during exercise. Before exercise: Consume a meal 1-3 hours before exercising, administer insulin at least 1 hour before exercise, and, if the blood glucose level is greater than 250 mg/dL, check urine for ketones. During exercise: Supplement energy (caloric) intake every 30 minutes and maintain adequate fluid replacement.After exercise: Monitor the glucose level, increase energy (caloric) intake for 12-24 hours following activity, and expect postexercise hypoglycemia; thus, appropriately adjust insulin dosing. ObesityIndividuals who are morbidly obese should be cautious of orthopedic stresses.They may start with non–weight-bearing exercises (eg, swimming, water aerobics, floor exercises).These individuals should avoid high-impact aerobic activities.Emphasize duration as tolerated and exercise frequency (sessions per week).Emphasize premeal exercise, especially exercise in the morning before eating breakfast to mobilize fats (lipolysis).MaternityResearch on exercise during pregnancy continues to demonstrate marked benefits for the mother and fetus.[25, 26, 27, 28, 29] The type, intensity, frequency, and duration of the exercise seem to be important determinants of its beneficial effects. Maternal benefits include improved cardiovascular function, limited weight gain and fat retention, easier and less complicated labor, quick recovery, and improved fitness. A small study involving Hispanic women, the largest minority group in the United States with the highest birth rates, evaluated physical activity before and in early pregnancy. The results corroborated data from previous studies that suggest physical activity in early pregnancy decreases the risk of pregnancy-related hypertensive disorders.[30] Fetal benefits may include decreased growth of the fat organ, improved stress tolerance, and advanced neurobehavioral maturation.Offspring are leaner at age 5 years and have a slightly better neurodevelopmental outcome. Postpubertal effects are still unknown. In the absence of medical contraindications, women should be encouraged to maintain their prepregnancy activity level. In general, activity should be individualized (exercise prescription), depending on previous activity levels Available outcome data suggest that a healthy woman may begin or maintain a regular exercise regimen during pregnancy, with benefit and without adversely affecting the course and outcome of the pregnancy. Data also suggest that the clear difference between theoretic concern and observed outcome is best explained by the hypothesis that the physiologic adaptations to exercise and to pregnancy are complementary and fetoprotective. Although an upper level of safe performance is not established, otherwise healthy mothers-to-be appear to obtain the benefits of a regular exercise regimen without undue risk to the embryo and fetus. The exact regimen can be flexible and individualized (exercise prescription), provided that both the exercise and the pregnancy are monitored. OsteoporosisExercise is an essential part of treatment for patients with osteoporosis. Just as regular workouts build muscle, these activities also maintain and may even increase bone strength. By strengthening the muscles and bones and improving balance, exercise can reduce the risk of falls and resulting fractures. Exercise works well with medications that increase bone density and strength.Exercise, medication, and proper diet, including an absorbable source of calcium, phosphorus, boron, and other bone-building minerals, are more effective in combating osteoporosis than any one treatment alone. Weight-bearing exercises and resistance training benefit bones and muscles as well as help improve general health. Weight-bearing exercise For most people who have osteoporosis, brisk walking is ideal.Walking can be performed anywhere, requires no special equipment, and carries minimal risk of injury.If walking is too difficult or painful, workouts on a stationary exercise cycle are a good alternative.Resistance training Lifting weights or using strength-training machines strengthens bones, especially if one exercises all of the major muscle groups in the legs, arms, and trunk. A qualified trainer, exercise specialist, or therapist is important for instructing and guiding resistance-training programs.Joining a gym or fitness facility is a good way to begin because these facilities typically provide access to trainers who can advise on proper techniques. Strength training is a slow process, so it should be started at a low level and should be gradually built up over several months. For each exercise, select weights or set the machine so the muscle being trained becomes fatigued after 10-15 repetitions. As muscles strengthen, gradually add more weight. The weight should not be increased more than 10% per week because larger increases can increase the risk of injury. Tips for trouble-free exerciseLift and lower weights slowly to maximize muscle strength and to minimize the risk of injury.Perform resistance workouts on any given muscle group every second or third day. This gives your body a chance to recover.Avoid exercise that puts excessive stress on the bones, such as running or high-impact aerobics. Rowing is appropriate if proper form is used and the rowing machine provides a way to maintain continuous inertia with the use of a flywheel. Stiffness is normal the morning after exercise. If pain continues for most of the following day, joints become swollen, or a limp develops, stop the program until comfortable again and reduce the weight and number of repetitions by 25-50%. If bone, joint, or muscle pain is severe, call the doctor. If a particular area of the body feels sore right after exercise, apply ice for 10-15 minutes. Wrap ice in a towel or plastic bag or just hold a cold canned or bottled beverage on the spot. Vary the routine to make it more interesting. For example, if the strength-building program involves 12 separate exercises, complete 6 in one session and the other 6 in the next session. PreviousNextExercise Prescription for Individuals With Coronary Artery Disease

Physical inactivity is recognized as a risk factor for coronary artery disease. Regular aerobic physical activity increases exercise capacity and plays a role in both primary and secondary prevention of cardiovascular disease.[1, 2, 3, 4, 5, 6, 7, 12, 13, 14, 15, 16] The known benefits of regular aerobic exercise and recommendations for implementation of exercise programs are discussed.

Exercise training increases cardiovascular functional capacity and decreases myocardial oxygen demand at any level of physical activity in apparently healthy persons and in most individuals with cardiovascular disease. Regular physical activity is required to maintain these training effects. Myocardial work can be affected by caffeine intake, and caffeine intake has been shown to increase blood pressure response to exercise. The potential risks of physical activity can be reduced by receiving a medical evaluation, risk stratification, supervision, and education.

Exercise can help control blood lipid abnormalities, diabetes, and obesity. In addition, aerobic exercise adds an independent blood pressure–lowering effect in certain hypertensive patient groups, with a decrease of 8-10 mm Hg in both systolic and diastolic blood pressure measurements. A direct relationship exists between physical inactivity and cardiovascular mortality, and physical inactivity is an independent risk factor for the development of coronary artery disease. A dose response relationship exists between the amount of exercise performed (from approximately 700-2000 kcal/wk [2940-8400 kJ/wk] energy expenditure) and all-cause mortality and cardiovascular disease mortality in middle-aged and elderly populations.

The greatest potential for reduced mortality is in sedentary persons who become moderately active. Most beneficial effects of physical activity on cardiovascular disease mortality can be attained through moderate-intensity activity (40-60% of maximal VQ, depending on the participant's age). The activity can be accrued through formal training programs or leisure-time physical activities. A large, 10-year, case-controlled study including men in the Health Professionals Follow-up Study evaluated the effects of physical activity on the risk of myocardial infarction. The results noted that men who participated in vigorous-intensity activity for 3 h/wk reduced their risk of myocardial infarction by 22%.[31]

Although most supporting data are based on studies in men, relatively recent findings show similar results for women. Results of pooled studies reveal that persons who modify their behavior after myocardial infarction to include regular exercise have improved rates of survival.

Studies have revealed that intensive multiple interventions, such as smoking cessation, blood lipid reduction, weight control, and physical activity, significantly decreased the rate of progression—and, in some cases, lead to regression—in the severity of atherosclerotic lesions in persons with coronary disease.

In addition, limited data indicate that higher-intensity exercise, compared with lower-intensity exercise, improves left ventricular ejection fractions in persons with coronary artery disease. Current activity status (eg, persons remaining physically active or having been sedentary and becoming physically active) revealed the greatest decline in coronary artery disease risk. Persons who remain sedentary have the highest risk for cardiovascular disease mortality.

Exercise intensity should approximate 40-85% of VO2 reserve (VO2 R) or HR reserve (HRR), as determined by an exercise test. If a test is not performed initially, a reasonable estimate of 20-30 beats per minute (bpm) above HR rest is generally appropriate until testing is performed.

Activities can be prescribed according to the work intensity at which the training HR is achieved after 5-10 minutes at the same workload (steady state). This may be expressed as watts on an ergometer, speed on a treadmill, or METs. If an individual cannot assess intensity, HR counting (manually or with a pulse meter or cardiotachometer) is especially useful. HR counters are widely available and generally accurate for low- to moderate-intensity exercise.

If an individual intends to walk on a level surface, activity can be prescribed as the treadmill step rate that generates the desirable HR. The step rate is the number of steps taken in 15 seconds while walking at the desired speed on the treadmill. Step rate can be counted easily because it requires less skill than counting HR. If this approach is used, caution individuals to avoid hills. Walking in shopping malls or gymnasiums allows individuals to avoid inclement weather and to exercise on a flat surface. Exercise should be supervised for the first few sessions to ensure that instructions are understood and the activity is well tolerated.

Individuals can also judge the intensity of exercise by the RPE, which can be equated with the desired HR during laboratory exercise and activities. The original scale is a 15-grade category scale that ranges from 6-20, with a verbal description at every odd number, beginning at 7 (very, very light) and progressing to 19 (very, very hard).

RPE values should be rated as follows:

Less than 12 – Perceived as fairly light (light intensity), 40-60% of HRmaxFrom 12-13 – Perceived as somewhat hard (moderate intensity), 60-75% of HRmaxFrom 14-16 – Perceived as hard (high intensity), 75-90% of HRmax

Activities can progress as tolerance is demonstrated. An appropriate initial intensity of training is 60-75% of HRmax (moderate) or an RPE of 12-13. However, many individuals may need to begin at 40-60% of HRmax (light). After safe activity levels have been established, duration is increased in 5-minute increments each week. Later, with increased strength and as the HR response to exercise decreases with conditioning, intensities can be increased to a frequency of 3-6 times per week. At this point, limited resistive exercises can be added, which have proved both safe and effective in secondary prevention.

Exercise prescription in the presence of ischemia or arrhythmias (moderate to high risk)

An exercise test and medical supervision are essential for this type of exercise prescription. The manifestations of arrhythmias or ischemia that require such precautions can vary but usually include the following:

Ventricular tachycardia (3-4 beats)Any arrhythmia that is symptomatic or causes hemodynamic instabilityChest discomfort that is believed to be anginaSignificant electrocardiograph (ECG) ST depressionInappropriate blood pressure responses such as significant hypertension or a decrease in systolic blood pressure of 20 mm Hg from baseline

Perform exercise testing in the usual fashion, but the conditioning work intensity is derived from the HR associated with the abnormality. If the exercise test continues to a high level of effort, the HR at 50-60% of maximum can be used if it falls at least 10 bpm below the abnormal level. Otherwise, the recommended peak training HR is 10 bpm less than that associated with the abnormality. These individuals are recommended to have medically supervised cardiac rehabilitation and reevaluation to restratify them to a lower risk. Repeat exercise testing at least yearly.

As the population ages and more elderly persons survive coronary events, increasing numbers need appropriate physical activity. Most of these persons initially demonstrate benefits from supervised exercise for a brief period. This is performed primarily to introduce the patient to exercise (which the individual may not have performed before) and to evaluate the patient for possible complications of exercise, such as arrhythmias, evidence of heart failure, anginal chest pain, or abnormal ECG ST segments. On the basis of the evaluation, the person can be categorized as low risk or moderate to high risk, and appropriate cardiac rehabilitation precautions can be taken.

Most individuals in secondary prevention can soon be restratified as low risk and can implement their exercise prescription at home or in a community program. In this setting, the previously mentioned primary prevention guidelines also apply. The intensity may be much less, and the frequency may be more, with appropriate changes in duration. Interval exercise testing is recommended at least yearly, and coronary risk factor modification should be aggressive.

In summary, implementation of physical activity strategies by physicians for both primary and secondary prevention should consider the dosing effect or expenditure of kilocalories or kilojoules over a unit of time (usually a week). The guidelines above ideally should entail 5-6 hours of various physical activities weekly if possible. The exercise routine must be individualized (exercise prescription) and should include both aerobic and resistance activities. The benefits of exercise are enhanced with good to excellent compliance with exercise and appropriate lifestyle modifications.

Postmyocardial infarction

As the safety of early ambulation was progressively demonstrated in patients after suffering myocardial infarction, other benefits were realized, such as the prevention of the deconditioning effects of bed rest, decrease of anxiety and depression, and improved functional status at discharge.

Early activity Walking is the recommended mode of activity unless the individual can attend supervised classes where other activities are provided. Begin limited walking and slowly continue, with a gradual increase in duration until 5-10 minutes of continuous movement has been achieved. Active but nonresistive range of motion of the upper extremities is also well tolerated early if the activities do not stress or impair healing of the sternal incision in persons who have had coronary bypass surgery. The emphasis of exercise in the first 2 weeks after myocardial infarction or coronary bypass surgery should be on offsetting the effects of bed rest or former periods of inactivity. Begin to increase activity when the individual's condition is stable, as measured by ECG tracings, vital signs, and symptomatic standards. Although the prescribed activity is usually well tolerated and safe, certain precautions are recommended, such as awareness of chest discomfort, faintness, and dyspnea. Supervise the initial activities and record symptoms, RPE, HR, and blood pressure. When safety and tolerance are documented, the activity can be performed without supervision. Late activity A symptom-limited exercise test is often performed after the individual's condition has stabilized (as early as 2-6 wk after the coronary event). In secondary prevention, such testing is essential in all patients before beginning a physical activity program. If more studies (eg, echocardiography, angiography) are not indicated, a regular conditioning program can be initiated with a careful prescription of activity based on results of the exercise test. For conditioning purposes, perform large muscle group activities for at least 20-30 minutes (preceded by a warm-up and followed by cool-down) at least 3-4 times per week. The exercise prescription should be based on the exercise test results. Supervised group sessions are recommended initially to enhance the exercise educational process, ensure that the participant is tolerating the program, confirm progress, and provide medical supervision in high-risk situations. Unsupervised home programs are acceptable for persons who are at low risk and who are motivated and understand the basic principles of exercise training. PreviousNextAdditional Research and Future Issues

The body of knowledge on exercise is large, but data on exercise and its effects on the cardiovascular system and long-term survival are still relatively limited. The responsibility for conducting research lies with government agencies, private health organizations, the insurance industry, employers, universities, and medical schools.

Basic knowledge of the anatomic, biochemical, and physiologic changes that result from various patterns of physical activity (short- and long-term, sustained and intermittent, isotonic and isometric, low and high intensity) in persons of different ages is needed, as is a determination of whether a certain minimum-intensity threshold of physical activity is required for benefit.

The biomedical and economic impact of participation in exercise programs on coronary artery disease, cerebrovascular and peripheral vascular disease, heart failure, and hypertension should also be evaluated. The psychosocial functioning of persons with coronary artery disease and the potential value of exercise in enhancing the quality of life for cardiac and other patients warrants further study. Future studies should include adequate numbers of women, ethnic groups, and elderly persons to better meet research objectives.

Furthermore, the presence and extent of coronary risk factors in disabled and disadvantaged individuals and in minority groups must be identified and better defined. Consequently, the effect of modifications (eg, increases in physical activity on members of these groups) should be explored. Large studies should also include a significant number of these persons.

Research should be continued to establish the cost-effectiveness of physical activity programs for the enhancement of cardiovascular health, with a focus on the type of promotional strategies required for initiating and maintaining physical activity (eg, insurance incentives, health personnel, public policy, media materials) and the social context of such activity (eg, industry and business, rural and urban settings, schools, churches, families). Research should also involve issues such as how physical activity can prevent (or decrease the duration of) the hospitalization of patients with chronic disease.

More information is also needed to identify societal, cultural, ethnic, and personal factors that affect development or maintenance of lifelong patterns of physical activity and incorporation of these into exercise promotion strategies. Research on better and more effective physical activity interventions that improve long-term compliance to a physically active lifestyle is urgently needed. Innovative nontraditional methods of increasing physical activity in the population must be developed, implemented, and evaluated.

In summary, future developments and studies should focus not only on the benefits of physical activity, but also on exercise adherence strategies and the methods used to facilitate dissemination of present and future knowledge to all members of society.

PreviousNextGlossary of Terms

Exercise intensity is generally expressed as a percentage of either HR or VO2. By definition, VO2 is the oxygen uptake by an individual at rest or during exertion, expressed commonly in milliliters of oxygen consumed per kilogram body weight per minute (mL/kg/min)

Heart rate reserve (HRR)  is defined as the maximal heart rate (HRmax) observed during a symptom-limited exercise stress test minus the resting heart rate (HR rest). A percentage of the HRR range is added to the HR rest to determine a target heart rate (THR) range to be used during exercise. This approach accounts for individual variability in the HR rest and better reflects the peak exercise oxygen consumption (VO2max). VO2max reflects the highest rate of oxygen consumption that one can achieve.

Oxygen uptake reserve (VO2 R)  is the difference between resting and maximal VO2. Previous guidelines suggest exercise prescriptions should be based on the oxygen uptake reserve (VO2 R) rather than a direct percentage of the VO2max.[32] Exercise intensities based on VO2 R are approximately equal to the same percentage values for HRR; therefore, the use of HRR in determining appropriate exercise intensities is suitable in most cases. However, certain exceptions to using this approach may include patients with poor chronotropic responses, dysautonomia, pacemakers, or heart transplantation.

Target heart rate (THR)  for exercise is generally recommended from 50% to 85% HRR (or VO2 R). For deconditioned individuals, 40-50% HRR may be more appropriate for beginning exercise, whereas physically active individuals may require higher intensities to achieve improvements in their conditioning. As an illustration in determining THR, the example below uses a resting HR (RHR) of 70 and a HRmax of 180 bpm.

The HRR is 180 – 70, or 110 bpm. Using an average intensity of 60-80% HRR, the THR ranges are calculated (Karvonen approach) as follows:

THR = (HRR × 60%) + RHR;

THR = (110 × 0.60) + 70 = 136 bpm

to

(110 × 0.80) + 70 = 158 bpm

So, a THR range would be 136-158 bpm or a pulse count of 22-26 beats per 10 seconds.

Metabolic equivalents (METs)  are useful units when recommending exercise. By definition, 1 MET is the amount of oxygen consumed at rest or about 3.5 mL/kg/min. However, recent studies indicate that the average resting MET level in subjects with coronary heart disease is 23% to 36% lower than the 3.5 mL/kg/min standard value.[33] Nevertheless, most people walking 2 mph require 2 METs, and 3 mph require 3-4 METs. Published MET tables describe many activities in terms of the estimated MET requirements. For example, if an individual has a VO2max of 34 mL/kg/min, the VO2 R is 34 minus the resting VO2 of 3.5 mL/kg/min equals 30.5 mL/kg/min. Dividing this result by 3.5 yields 8.7 METs. Using 60-80% VO2 R, the recommended range of exercise METs may be determined by the following:

(8.7 × 0.60) + 1.0 (resting) = 6.2 METs

to

(8.7 × 0.80) + 1.0 (resting) = 8 METs

When one consults a common MET table, an exercise intensity of 6.2-8.0 METs is equivalent to a slow walk-jog combination exercise, hiking with a backpack, hill climbing, and numerous other moderately vigorous activities.

Exercise intensity may be customized to the individual (exercise prescription) based upon their metabolic response to progressive exercise if the VO2max and the anaerobic threshold (AT)  or ventilatory threshold (VT)  is determined. By definition, the VT may be described as the level of oxygen consumption (VO2) at which a significant increase in anaerobiosis occurs, as evidenced by an increase in blood lactate levels and respiratory responses to the increasing exercise workload. MET is a unit of energy or level of oxygen used at rest (1 MET = VO2 of 3.5 mL/kg/min).

Exertion below the VT can generally be sustained for long time periods; whereas, above the VT, the individual can only tolerate a limited amount of time such as several minutes. As a training principle, exercising at the VT may be optimal due to considerable stimulation with a minimal lactate accumulation. Many recreational as well as high-caliber endurance athletes train above the VT but below the respiratory compensation (RC)  point. The RC may be described as the point when the CO2 production is increased in relation to ventilation; typically when the individual no longer has voluntary control over the ventilation (ie, the ventilation is driven by metabolic factors).

Maximal voluntary ventilation (MVV) is defined as the maximal amount of ventilation per minute, generally determined by a 12-second hyperventilation procedure to maximal ventilation capabilities.

The respiratory exchange ratio (R or RER) or respiratory quotient (RQ)  is the ratio of the VO2 to the carbon dioxide produced in the body. At rest, the RER reflects the substrate's use of fuel sources (eg, carbohydrates, fats, proteins). The metabolism of fats or fatty acids yields a ratio of 0.7; of carbohydrates, 1; and of protein, 0.84. During exercise, the RER generally exceeds 1 because of the additional carbon dioxide produced as a byproduct of the bicarbonate system and because of lactate buffering.

Borg ratings of perceived exertion

The RPE scale is used widely in exercise science and sports medicine to monitor or prescribe levels of exercise intensity. The 95%-limits-of-agreement technique has been advocated as a better means of assessing within-subject (trial-to-trial) agreement.

The perception of exertion is a monitoring behavior that uses all sources of information to govern actions that can benefit or preserve health and partake of adaptive pursuits. How a person feels about exertion moderates his or her response to exercise and effort. The perception of what is happening in exercise, and its concomitant effect on physiologic function, must be known to further understand the nature of an exercise response. How a person feels modifies reactions to exercise stress and the mechanisms that underlie them.

Exercise is never a purely mechanistic physiologic reaction. The interpretation of the exercise experience governs the nature, quality, and extent of the exercise response. To fully understand and accurately assess the nature of an exercise behavior, measuring as many moderating variables as possible is necessary.

Physiologic measures can be used to grade the strain for each individual, but so can exertion estimates. In an exercise response, the underlying determining mechanism may not be the pure physiologic parameter being measured. Without knowledge of the psychologic moderator variables, the physiologic measurement alone is misleading; its value as a predictor variable is usually negligible. The particular circumstances in which the physiologic measurement is taken may be more important for predicting or analyzing a response than the variable itself. To exercise only according to HR is dangerous. The aches and strain that are felt may be very important indicators of the real degree of exertion. Rigid adherence to the objective measures of physiology may cause interpretive and prescriptive errors of great magnitude.

In many circumstances, the psychologic components of an exercise response are more reliable and relevant than the physiologic measures. This has been shown to be true in the assessment of long-term exercise strain for determining the early symptoms of overtraining or maladaptation.

Borg's original intention was to construct a category scale from 6-20 in which scale levels were roughly one tenth of the HR for equivalent scaled exercise levels. A score of 6 (no exertion at all) should exhibit an HR somewhere in the vicinity of 60 bpm for a young to middle-aged, mildly fit individual. This correspondence is generally reserved for middle-aged people exercising at moderate to high intensity levels. At best, this is a very rough estimation of the relationship; individual variability is significant. Also, the relationship between HR and RPE within an individual varies with different forms of activity. The RPE is best reserved for intraindividual comparisons for a specific form of exercise.

HRs are related linearly to the scale scores (r = 0.8-0.9). However, even with this relationship one cannot conclude that HR is a cause of the perceived exertion.

The perception of exertion integrates many more exercise factors than are considered with singular or isolated physiologic variables. This integration is a truer indication of an exercise response than is depicted by restricted variables such as HRs, lactate measurements, or blood measurements. To understand a certain RPE value, knowing (1) the age and other personal characteristics of the individual, (2) what type of activity was performed, and (3) the environmental conditions that existed at the time is important. The instructions for using the scale are to "estimate how hard and strenuous you feel the work to be." The perception should be general, rather than focusing on specific parts of the body (eg, "tired arms"). The perception of exertion should include as many contributory sensations as possible.

Once the verbal description is determined, the individual should choose an exact number that corresponds to the verbal descriptor. When a subject is unable to complete the highest workload, the rating should be of the work at the time of the final interruption. With athletes, the major problem with using RPE is their common tendency to underestimate the exertion level. Practice in using the scale is necessary. If the various categories can be aligned with other categories or levels of work response, an accurate level of discrimination can be developed between the categories. The RPE yields important additional data beyond those available through isolated physiologic variables. In conscientious and reliable individuals, its value exceeds that of singular parameters of performance.

Instructions for use:

During the exercise you are to rate your perception of exertion. Use this scale, where 6 means no exertion at all, and 20 means a totally maximum effort. The 13 on the scale is a somewhat heavy exercise but capable of being performed at steady state (ie, anaerobic threshold). When at a level of 17, the effort level requires you to push yourself hard even though it is possible to continue for some time. For many people, 19 is about as strenuous as exercise becomes because they often reserve a small amount of possible extra effort. Try to appraise the feeling of exertion as honestly as possible. Do not underestimate or overestimate it. It is of no value to underestimate the level to produce an impression of being brave or tough. Your own feeling of effort and exertion is all that is of interest. Look at the scale and wordings and decide on the word that best describes your effort level and the number alternative associated with that description.

The Borg scale is as follows:

6 – No exertion at all7-8 – Extremely light (very, very light)9-10 – Very light (A1 warm-up/recovery)11 – Light (A2 aerobic threshold)12-13 – Moderate (EN-1 anaerobic threshold)14-15 – Hard (EN-2 VO2max or 400-m swimming pace)16-17 – Very hard (AN-1 peak lactate or lactate tolerance, 200-m swimming pace)18-19 – Extremely hard (very, very hard [AN-2 anaerobic power, 25-50–m swimming pace])20 – Maximum all-out effort, with absolutely nothing being held in reservePrevious, Exercise Prescription

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