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Exercise stimulates tissue change and adaptation, whereas rest and recovery allow such change and adaptation to occur. Recovery from exercise is as important as the exercise stimulus. Regular physical activity decreases the incidence of the major causes of death, 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. Regardless of indication, recommendations for exercise should be based on 2 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. More 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 over time, 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 only to a point 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. Fixed and traditional 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.

Exercise programs should encompass multiple dimensions of fitness:

• Stretching and flexibility
• Aerobic capacity
• Strength
• Balance

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. Specific flexibility exercises involve slowly and steadily stretching muscle groups without jerking or bouncing. 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 patients 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 may be preferred because tissues stretch more effectively when warmed.

Aerobic exercise: Aerobic 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 increased slowly over time. Aerobic conditioning increases maximal O₂ 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).

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 on convenience as much as 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 used. 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 all-out sprint-type work (85 to 95% HR_max or as hard as the person can exert for that time). 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 as long as a sufficient number of repetitions are done per set, rest between sets is minimal (20 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 workout is often more intense and heart rate increases more as a result.

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 O₂ consumption [VO_2peak], or the rate beyond which aerobic metabolism can no longer be sustained because O₂ 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:

However, the more athletic or deconditioned the person is compared to average, the less accurate these formulas are, thus making metabolic or VO₂ testing more valuable.

Chronologic age should be distinguished from biologic age. People of any age who are less accustomed to aerobic exercise (less conditioned) reach the 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, causing heart rate to increase much faster and to a greater extent with less vigorous activity than in thinner people. Disorders and some drugs (eg, β-blockers) also modify the relationship between age and heart rate. For people who have a disorder or take certain drugs, a target of 50 to 60% of HR_max is probably sufficient, at least initially.

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

Volume typically is categorized in terms of amount of weight lifted and number of sets and number of repetitions per set. However, an equally important parameter is tension time, which is the total duration of lifting and lowering the weight in one set. Appropriate tension time may be about 60 sec for moderate conditioning (a good balance in developing muscle mass and strength) and 90 to 120 sec for injury rehabilitation and muscular endurance. For increasing strength, tension time is more important than number of repetitions, which can vary within tension time by 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.

Intensity is generally 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 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 appropriate mainly 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.

Intensity is limited by motivation and tolerance; for many people undergoing rehabilitation, discomfort, pain, and exercise inexperience result in less effort than may be possible or tolerated, so that more sets are required to derive equal benefit. Intensity should vary on a regular basis to provide both a mental and physical hiatus. If exercise is done at the highest intensity level, it should occur no more often than in about half of the sets and workouts should be avoided for 1 to 2 wk every 3 mo. 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 pains, and increased resting heart rate suggest that exercise has been too intense; exercise should be avoided for 1.5 to 2 wk.

Good technique is important for safety and involves avoidance of jerking or dropping weights, which can cause minor tissue injury due to sudden force, and 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 with the leg press exercise when working the large lower body muscles and clenching the machines hand grips very tightly). However, BP returns to normal quickly after exercise; the increase is minimal when breathing technique is correct, no matter how hard a person exerts.

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. Balance training can help some people with impaired proprioception and is often used in an attempt to prevent falls in the elderly (see Exercise and Sports Injury: Exercise in the Elderly).

Hydration: 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) q 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.

Fluid deficit after exertion is calculated by comparing pre- and post-exercise body weight and 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. Often it is best to mix plain water with sports drinks at a 50:50 ratio to allow faster absorption of the glucose and electrolytes. People with findings suggesting heat illness (see Heat Illness) or dehydration may require oral or IV electrolyte replacement immediately.

Last full review/revision April 2009 by Brian D. Johnston; Paul L. Liebert, MD

Content last modified April 2009