Overview of Exercise

Before beginning a sports or vigorous exercise program, children and adults should undergo screening (ie, a history and physical examination), with emphasis on detecting cardiovascular risks. Testing is done only if disorders are clinically suspected.

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 or muscle soreness (1). 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 10 to 30 seconds and not for more than 60 seconds (there are no adverse effects from holding a stretch > 60 seconds but there is no added benefit) (2, 3). 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 or compression of nerves through joints (eg, excessive stretching of the hip joint on the sciatic nerve). For many muscles, flexibility increases sufficiently with a properly designed strength training program because muscles both stretch and work through an intended range of motion.

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 minutes as a starting point and is increased slowly over time. Aerobic conditioning increases maximal oxygen 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 (eg, cartilage wear contributing to osteoarthritis) and increases cellular oxidation and free radicals. 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 also provide vigorous aerobic exercise but may stress knees and other joints. Recommendations should be based on patient preferences and abilities.

Aerobic exercise starts within 2 minutes of beginning activity, but more sustained effort is needed to achieve health benefits. The usual recommendation is to exercise ≥ 30 minutes/day at least 3 times/week with a 5-minute warm-up and a 5-minute 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 minutes of activity per session 2 to 3 times/week if interval cycling is implemented. In interval cycling, short periods of moderate activity are alternated with intense exertion. In one regimen, about 90 seconds of moderate activity (60 to 80% maximum heart rate [HR_max]) is alternated with about 20 to 30 seconds 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, known as high-intensity interval training (HIIT), is more stressful on joints and tissues and so should be done infrequently (eg, initially once a week to allow the body to adapt to the strain, and then increase up to 2 or 3 times a week) or alternated with more conventional low- to moderate-intensity training. Although exercise decreases overall cardiovascular risks, risk of atrial fibrillation appears to be increased in endurance athletes, particularly those younger than age 55 (1, 2).

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 seconds), 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 minutes 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 and oxygen uptake. 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 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 approximated by direct measurement (3, 4), or calculated using the following formula:

Alternatively, the Karvonen formula can be used to calculate target heart rate (4):

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 VO₂ 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. People with obesity 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, beta-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 (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, deep knee bends). 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. Also, strengthened muscles around an injury (eg, thigh muscles with an injured knee) decrease pain, because strengthened muscles are more functional, and more functional muscles have less pain as they develop and restructure. Thus, a strength training program may help in injury rehabilitation.

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 seconds. A tension time of 90 to 120 seconds 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 repetitions can vary within tension time due to differences in technique, set duration, and how slowly each repetition is performed. When a person can achieve at least a 60-second tension time with good technique, resistance (weight) can be increased with the aim of a tension time of at least 60 seconds at the next weight level. Number of sets is determined by intensity of the training; more intense training may necessitate 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, effort, and volume 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, exercise inexperience, and/or limitation in range-of-motion (due to discomfort or pain) result in less effort than may be possible or tolerated. As a result, more sets are required to derive desired benefit (although the extent of adding more sets must take into consideration that doing too much activity can increase irritation and soreness to an injury). 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 or layoffs from high-intensity training (eg, 1 week every 3 months, perhaps coordinated with holidays or vacations) into their fitness planning to allow for sufficient recovery.

More frequent layoffs may be required for those who train with very high intensity frequently. Continual high-intensity training is counterproductive, even for trained athletes. In such people, symptoms such as fatigue or muscle heaviness when not exercising, lack of motivation to exercise, reduced exercise performance, joint and tendon pain (caused by inflammation), and increased resting heart rate suggest that exercise has been too intense for too long. Some people are conditioned mentally or physically to train very hard regularly (eg, athletes training at high-intensity), and so, in such people recovery breaks may be required (eg, 3 weeks on and 1 week off) to recover from such strenuous and regular activity.

Maximizing muscle hypertrophy requires good effort. However, exercising until the muscle can no longer do the repetition (muscle failure) is unnecessary and may be difficult to tolerate. The number of sets needed to maximize muscle hypertrophy varies based on a person's genetics—some people increase muscle hypertrophy with more sets, and others have better results with fewer sets. Regardless of the number of sets or exercises, the amount of improvement diminishes with each additional repetition or exercise. Doing too much exercise does not produce any better results and can result in loss of muscle and strength from overstimulation. Beginners should start with 1 set or exercise per muscle or muscle group and then slowly increase repetitions or exercises until reaching 3 to 5 sets of 1 exercise or 1 set each of 3 to 5 exercises, particularly if they do not train to muscle failure.

Variation helps by providing different stimuli; use of the same stimulus (eg, exercises in the same plane of movement) repeatedly eventually fails to elicit the desired effects because muscles adapt to the stimulus. Variation also helps prevent minor injuries caused by repetitive actions.

Proper body mechanics are important for personal safety and effective strength training. People should strive for proper body mechanics (eg, chest high, shoulders square and back, and stomach pulled in tight, while refraining from locking the joints) and exercise with smooth and continuous motions without jerking or lowering weights too quickly. 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 seconds, 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 when the lift begins. Blood pressure increases during resistance training (unrelated to atherosclerosis) 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, blood pressure returns to normal quickly after exercise; the increase is minimal when breathing technique is correct, regardless of exertion.

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 older people (see Exercise in Older Adults).

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 minutes is reasonable depending on heat and exertion level; however, overhydration, which can cause hyponatremia and consequent seizures, is to be avoided. Thirst is not an accurate indicator of fluid requirements. Drinking when feeling slightly or moderately parched but before feeling extremely thirsty helps maintain proper hydration and avoid electrolyte imbalance, illness, and possible seizures and/or death (eg, from hyponatremia due to excess consumption of free water).

Fluid deficit after exertion is calculated by comparing pre-exercise 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, particularly for exercise sessions that are longer and/or done in highly humid environments. 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 or volume depletion may require oral or IV fluid and electrolyte replacement immediately.