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Overview of Exercise

by Brian D. Johnston

Exercise stimulates tissue change and adaptation (eg, increase in muscle mass and strength, cardiovascular endurance), whereas rest and recovery allow such change and adaptation to occur. Recovery from exercise is as important as the exercise stimulus. Regular physical activity reduces the likelihood of medical illness, decreases the incidence of the major causes of death, and improves the overall health and quality of life for patients with most medical conditions. By increasing muscle mass and strength and fostering cardiovascular endurance, exercise improves functional status for sports and activities of daily living and protects against injury. Specific exercise programs are also commonly prescribed to rehabilitate patients after MI, major surgery, and musculoskeletal injury. Preoperative exercise regimens are prescribed before many elective surgical procedures to enhance postoperative recovery. Regardless of indication, recommendations for exercise should be based on 2 main principles:

  • Goals for activity should be specific to the patient, accounting for motivation, needs, physical ability, and psychology, to maximize the likelihood of patient participation and desired outcome.

  • Activity should be prescribed in a proper dose to achieve a desired effect. An exercise stimulus should be sufficient for the body to adapt to a higher state of function but not so great that it causes injury or nonadherence. More exercise or higher-intensity activity is not always better; too little or too much activity may prevent achievement of desired outcomes.

A prescription for exercise should specify intensity (level of exertion), volume (amount of activity in a session), frequency (number of exercise sessions), and progressive overload (either the amount of increase in one or more of these elements per workout or the actual load). The balance of these elements depends on individual tolerance and physiologic principles (ie, as intensity increases, volume and frequency may need to decrease, whereas as volume increases, intensity may need to decrease). Intensity, volume, and frequency can be increased concurrently, but increases are limited because human tolerance to strain is finite. The objective is to discover the appropriate amount of exercise for optimal benefit in the context of the patient’s goals, health status, and current fitness level. Fixed and traditional generic recommendations (eg, 3 sets of 10 to 12 repetitions, running 30 min 3 times/wk) may be suboptimal because they do not address a person’s specific requirements or capability (ie, people with marked deconditioning require a different program than people with the ability to train at higher intensity levels).

Achieving long-term adherence is important and challenging. People differ greatly in their motivation and ability to sustain what they may perceive as arduous activity. To improve adherence, training programs typically start at low intensity levels and gradually increase to the target level. Some people require individually supervised exercise (eg,by a personal trainer), others benefit from the support of organized group activity (eg, an exercise class, group bike ride), and some are able to engage in solitary exercise long-term. For people to sustain motivation over the long term, exercise prescriptions should take into account their needs (eg, leg strengthening exercises for someone wheelchair dependent), what is actually required for them to achieve a particular goal (ie, how realistic the goal is), and preferences (the type of fitness program).

Exercise programs should encompass multiple dimensions of fitness, including

  • Stretching and flexibility

  • Aerobic capacity (cardiovascular endurance)

  • Strength (including muscular endurance and muscle size or structure)

  • Balance

Pre-exercise medical evaluation

Before beginning a sports or vigorous exercise program, children and adults be screened with a history and physical examination (for full discussion, see Screening for Sports Participation) with emphasis on detecting cardiovascular risks. Testing is done only if disorders are clinically suspected.

Stretching and flexibility

Flexibility is important for safe, comfortable performance of physical activities. Stretching may be beneficial in strength training to improve range of motion and help relax muscles. These exercises can be done before or after other forms of training or as a regimen itself, as occurs in yoga and Pilates sessions. Although stretching before exercise enhances mental preparedness, there is no evidence that stretching decreases risk of injury. However, there is no need to discourage preactivity stretching if people enjoy it. General warming-up (eg, with low-intensity simulation of the exercise to be done, jogging on the spot, calisthenics, or other light activities that increase core temperature) seems to be more effective than stretching for facilitating safe exercise. Stretching after exercise is generally preferred because tissues stretch more effectively when warmed.

Specific flexibility exercises involve slowly and steadily stretching muscle groups without jerking or bouncing. To improve flexibility, a stretch should be held for at least 20 sec and not for more than 60 sec (there are no adverse effects from holding a stretch > 60 sec but there is no added benefit). Each stretch is repeated 2 to 3 times, and each time the stretch is held progressively further. Some mild discomfort is to be expected, but high pain levels should be avoided as pain can be a signal of unintended minor tissue tearing. For many muscles, flexibility increases sufficiently with a properly designed strength training program because muscles both stretch and work through the complete range of motion.

Aerobic exercise

Aerobic (cardiovascular) exercise is continuous, rhythmic physical activity. Exertion occurs at a level that can be supported by aerobic metabolism (although brief periods of more intense exertion triggering anaerobic metabolism may be interspersed) continuously for at least 5 min as a starting point and is increased slowly over time. Aerobic conditioning increases maximal O 2 uptake and cardiac output (mainly an increase in stroke volume), decreases resting heart rate, and reduces cardiac and all-cause mortality; however, too much activity causes excessive wear on the body and increases cellular oxidation. Examples of aerobic exercise include running, jogging, fast walking, swimming, bicycling, rowing, kayaking, skating, cross-country skiing, and using aerobic exercise machines (eg, treadmill, stair-climbing, or elliptical machines). Certain team sports such as basketball and soccer can provide vigorous aerobic exercise but may stress knees and other joints and be counterproductive in the long term. Recommendations should be based on patient preferences and abilities.

Aerobic metabolism starts within 2 min of beginning activity, but more sustained effort is needed to achieve health benefits. The usual recommendation is to exercise 30 min/day at least 3 times/wk with a 5-min warm-up and a 5-min cool-down period, but this recommendation is based as much on convenience as it is on evidence. Optimal aerobic conditioning can occur with as little as 10 to 15 min of activity per session 2 to 3 times/wk if interval cycling is implemented. In interval cycling, short periods of moderate activity are alternated with intense exertion. In one regimen, about 90 sec of moderate activity (60 to 80% maximum heart rate [HR max ]) is alternated with about 20 to 30 sec of intense sprint-type activity (85 to 95% HR max or as hard as the person can exert for that time while maintaining proper body mechanics). This regimen is more stressful on joints and tissues and so should be done infrequently or alternated with more conventional low- to moderate-intensity training.

Resistance training machines or free weights can be used for aerobic exercise provided that a sufficient number of repetitions are done per set, rest between sets is minimal (near zero to 60 sec), and intensity of effort is relatively high. In circuit training, the large muscles (of the legs, hips, back, and chest) are exercised followed by the smaller muscles (of the shoulders, arms, abdomen, and neck). Circuit training for only 15 to 20 min can benefit the cardiovascular system more than jogging or using aerobic exercise machines for the same amount of time because the more intense workout results in a greater increase in heart rate. This combined aerobic and resistance training enhances muscular endurance of all the involved muscles (ie, not just the heart).

Volume of aerobic exercise is graded simply by duration. Intensity is guided by heart rate. Target heart rate for appropriate intensity is 60 to 85% of a person’s HR max (the heart rate at peak O 2 consumption [VO 2peak ], or the rate beyond which aerobic metabolism can no longer be sustained because O 2 is lacking and anaerobic metabolism begins). HR max can be directly measured, or calculated as


Alternatively, the Karvonen formula can be used to calculate target heart rate:


These formulas are based on the general population and may not provide accurate targets for people at the extremes of physical fitness (ie, highly trained athletes or physically deconditioned patients). In such people, metabolic or VO 2 testing may provide more accurate information.

Chronologic age should be distinguished from biologic age. People of any age who are less accustomed to aerobic exercise (less conditioned) will reach their target heart rate much sooner and with less effort, necessitating briefer exercise periods, at least initially. Obese people may be deconditioned and must move a larger body weight, thus causing the heart rate to increase much faster and to a greater extent with less vigorous activity than it does in thinner people. Patients with medical disorders or who are taking certain drugs (eg, β-blockers) may also have a modified relationship between age and heart rate. A safe starting point for these patients may be 50 to 60% of the age-based target heart rate. These targets can be increased based on patient tolerance and progress.

Strength training

Strength (resistance) training involves forceful muscular contraction against a load—typically provided by free or machine weights, cable weights, or sometimes body weight (eg, push-ups, abdominal crunches, chin-ups). Such training increases muscle strength, muscle endurance, and muscle size. Strength training also improves functional ability and, depending on the pace of the program, aerobic performance. Cardiovascular endurance and flexibility increase concurrently.

Volume typically is categorized in terms of amount of weight lifted, the number of sets, and the number of repetitions per set. However, an equally important parameter is tension time, which is the total duration of lifting and lowering the weight during one set. To achieve moderate conditioning (developing both muscle mass and strength), appropriate tension time may be about 60 sec. A tension time of 90 to 120 sec is appropriate for injury rehabilitation and improving muscular endurance. When the goal is increasing strength, tension time is more important than number of repetitions, because the number of repititions can vary within tension time due to differences in technique and set duration. When a person can achieve at least a 60-sec tension time with good technique, resistance (weight) can be increased so that a tension time of at least 60 sec is tolerable at the next weight level. Number of sets is determined by intensity of the training; more intense training necessitates fewer sets.

Intensity is essentially a subjective measure of perceived effort and how close a person comes to muscular fatigue in a given set (or exhaustion in a workout). Intensity may also be characterized objectively by the amount of weight lifted expressed as a percentage of the person’s maximum for one repetition (1 RM) of a given exercise; ie, for a person who can deadlift at most 100 kg one time, 75 kg is 75% RM. A general guideline is to exercise with a load at 70 to 85% RM. Heavier loads increase risk of injury and are usually appropriate only for competitive strength athletes. Lifting < 30 to 40% RM provides minimal strength gain, although aerobic conditioning and muscular endurance may occur with sufficient tension time and effort. During strength training, the stimulus of tissue change is governed primarily by the quality and effort of training. For example, a person lifting 85% RM once (where 6 repetitions could be done with maximal effort) would have less stimulus for tissue change than if lifting 75% to 80% RM multiple times (close to or at muscular fatigue).

Intensity is limited by motivation and tolerance. For many patients undergoing rehabilitation, discomfort, pain, and exercise inexperience result in less effort than may be possible or tolerated. As a result, more sets are required to derive desired benefit. People should vary the intensity of workouts regularly to provide both a mental and a physical break. Exercise should be done at the highest intensity level during no more than half of the sets in a given workout. People should incorporate breaks from high-intensity training (eg, 1 wk every 3 mo, perhaps coordinated with holidays or vacations) into their fitness planning to allow for sufficient recovery. Continual high-intensity training is counterproductive, even for trained athletes. Symptoms such as fatigue or muscle heaviness when not exercising, lack of motivation to exercise, reduced exercise performance, joint and tendon pain, and increased resting heart rate suggest that exercise has been too intense.

Proper body mechanics are important for personal safety and effective strength training. People should strive for smooth body mechanics and avoid jerking or dropping weights, which can cause minor tissue injury due to sudden force. It is equally important to encourage controlled breathing, which prevents dizziness (and in extreme cases, fainting) that can occur with the Valsalva maneuver. People should exhale while lifting a weight and inhale while lowering a weight. If a movement is slow, such as lowering a weight for 5 sec, people may need to breathe in and out more than once, but breathing should still be coordinated so that a final breath is taken in just before the lifting phase and released during lifting. BP increases during resistance training and tends to be highest when gripping excessively (common during the leg press exercise when working the large lower body muscles and clenching the machine's hand grips very tightly). However, BP returns to normal quickly after exercise; the increase is minimal when breathing technique is correct, regardless of exertion.

Balance training

Balance training involves challenging the center of gravity by undertaking exercises in unstable environments, such as standing on one leg or using balance or wobble boards. Basic strength training improves balance because it increases muscle size and strength around the joints, thus improving stability indirectly. Balance training can help some people with impaired proprioception and is often used in an attempt to prevent falls in the elderly (see Exercise in the Elderly).


Proper hydration is important, particularly when exertion is prolonged or occurs in a hot environment. People should be well hydrated before activity, drink fluids regularly during extended exertion, and replace any deficit remaining after activity. During exertion, about 120 to 240 mL (½ to 1 cup) of fluid every 15 to 20 min is reasonable depending on heat and exertion level; however, overhydration, which can cause hyponatremia and consequent seizures, is to be avoided.

Pearls & Pitfalls

  • Avoid overhydration during exercise because it can cause hyponatremia sometimes severe enough to cause seizures.

Fluid deficit after exertion is calculated by comparing preexercise and postexercise body weight. Fluid deficit is replaced on a one-for-one basis (ie, 1 L for each kg lost, or 2 cups/lb). In most cases, plain water is acceptable. Electrolyte-containing sports drinks may be preferred. However, fluids with a carbohydrate content of > 8% (8 g/100 mL, or 20 g in a typical 250-mL serving) decrease gastric emptying and slow fluid absorption. Mixing plain water with sports drinks at a 50:50 ratio allows faster absorption of the glucose and electrolytes. Patients with findings suggesting heat illness (see Heat Illness) or dehydration may require oral or IV fluid and electrolyte replacement immediately.

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