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Exercise - The Exercise Prescription

aging physiological clinical training muscle aerobic balance

This section will outline the elements of a prescription designed to stimulate robust adaptation within the major physiologic domains that can be modified by exercise: strength, cardiovascular endurance, flexibility, and balance, as recommended by the American College of Sports Medicine and endorsed by most major medical consensus groups. These elements are discussed separately, because in most cases exercise training is quite specific in its effects, and little crossover will be seen. For example, balance training will not increase one's aerobic capacity or strength. Resistance training is unique in this regard; it has been shown to benefit all of these domains to some extent, with its most powerful effect in the realms of muscle strength and endurance.

Progressive resistance training. Progressive resistance training (PRT) is the process of challenging the skeletal muscle with an unaccustomed stimulus, or load, such that neural and muscle tissue adaptations take place, leading ultimately to increased strength and muscle mass. In this kind of exercise, the muscle is contracted slowly just a few times in each session against a relatively heavy load. Any muscle may be trained in this way, although usually six to twelve major muscle groups with clinical relevance are trained, for a balanced and functional outcome. The most important element of the PRT prescription is the intensity of the load used. It is evident from many years of research and clinical practice that muscle strength and size are increased significantly only when the muscle is loaded at a moderate or high intensity (60–100 percent of maximum).

The benefits of PRT are both metabolic and functional. It improves sensitivity to insulin and may therefore be important in both the prevention and the treatment of diabetes. It also increases bone formation and density, and has a role in the prevention and treatment of osteoporosis. It significantly improves muscle strength and is associated with muscle hypertrophy, and is therefore useful whenever muscle weakness or atrophy contributes to disease or dysfunction. Such disease or dysfunction includes falls, frailty, chronic heart failure, chronic lung disease, Parkinson's disease, neuromuscular disease, chronic renal failure, arthritis, and other chronic conditions associated with decreased activity levels and impaired mobility. In addition, PRT has marked psychological benefits, having been shown to improve major depression as well as insomnia, self-efficacy, and emotional well-being in older adults.

The potential risks of PRT are primarily musculoskeletal injury and rarely cardiovascular events (ischemia, arhythnias, hypertension). Musculoskeletal injury is almost entirely preventable with attention to the following points:

  • • Adherence to proper form
  • • Isolation of the targeted muscle group
  • • Slow velocity of lifting
  • • Limitation of range of motion to the pain-free arc of movement
  • • Avoidance of use of momentum and ballistic movements to complete a lift
  • • Use of machines or chairs with good back support
  • • Observation of rest periods between sets and rest days between sessions.

Cardiovascular endurance training. Cardiovascular endurance training refers to exercise in which large muscle groups contract many times (thousands of times at a single session) against little or no resistance other than that imposed by gravity. The purpose of this type of training is to increase the maximal amount of aerobic work that can be carried out, as well as to decrease the physiologic response and Table 3 Characteristics of Aerobics vs. Resistive Exercise SOURCE: Author perceived difficulty of submaximal aerobic workloads. Extensive adaptations in the cardiopulmonary system, peripheral skeletal muscle, circulation, and metabolism are responsible for these changes in exercise capacity and tolerance. Many different kinds of exercise fall into this category, including walking and its derivatives (hiking, running, dancing, stair climbing), as well as biking, swimming, ball sports, etc. The key distinguishing features between activities that are primarily aerobic versus resistive in nature are listed in Table 3. Obviously, there may be some overlap if aerobic activities are altered to increase the loading to muscle, as in resisted stationary cycling or stair-climbing machines. However, such activities are still primarily aerobic in nature, because they do not cause fatigue within a very few contractions, as PRT does, and therefore do not result in the kinds of adaptations in the nervous system and muscle that lead to marked strength gain and hypertrophy.

Overall, walking and its derivations surface as the most widely studied, feasible, safe, accessible, and economical mode of aerobic training for men and women of most ages and states of health. They do not require special equipment or locations, and do not need to be taught or supervised (except in the cognitively impaired, very frail, or medically unstable individual). Walking bears a natural relationship to ordinary activities of daily living, making it easier to integrate into lifestyle and functional tasks than any other mode of exercise. Therefore, it may be more likely to translate into improved functional independence and mobility than other modes of exercise.

The intensity of aerobic exercise refers to the amount of oxygen consumed (VO 2), or energy expended, per minute while performing the activity, which will vary from about 5 kcal/minute for light activities, to 7.5 kcal/minute for moderate activities, to 10–12 kcal/minute for very heavy activities. Energy expenditure increases with increasing body weight for weight-bearing aerobic activities, as well as with inclusion of larger muscle mass, and increased work (force x distance) and power output (work/time) demands of the activity. Therefore, the most intensive activities are those which involve the muscles of the arms, legs, and trunk simultaneously, necessitate moving the full body weight through space, and are done at a rapid pace (e.g.. cross-country skiing). Adding extra loading to the body weight (back-pack, weight belt, wrist weights) increases the force needed to move the body part through space, and therefore increases the aerobic intensity of the work performed. The rise in heart rate is directly proportional, in normal individuals, to the increasing oxygen consumption or aerobic workload. Thus, monitoring heart rate has traditionally been a primary means of both prescribing appropriate intensity levels and following training adaptations when direct measurements of oxygen consumption are not available. The relative heart rate reserve (HRR) is the most useful estimate of intensity based on heart rate. Training intensity is normally recommended at approximately 60 to 70 percent of the HRR. It is calculated as is shown below.

HRR = (Maximal heart rate - resting heart rate) + resting heart rate60–70% HRR =.6–.7(Max HR -resting HR) + resting HR

Therefore, a more easily obtainable and reliable estimate of aerobic intensity is to prescribe a level of "somewhat hard," or 12 to 14 on the Borg scale, which runs from 6 to 20. At this level, the exerciser should note increased pulse and respiratory rate, but still be able to talk. All of the major benefits of aerobic exercise (increased cardiovascular fitness, decreased mortality, decreased incidence of chronic diseases, improved insulin sensitivity, blood pressure, and cholesterol, for example) are attainable with this moderately intense level of aerobic training. As is the case with all other forms of exercise, in order to maintain the same relative training intensity over time, the absolute training load must be increased as fitness improves. The workloads should progress on the basis of ratings of effort at each training session. Once the perceived exertion slips below 12, the intensity of the regimen should be increased to maintain the physiologic stimulus for optimal rates of adaptation. As with PRT, the most common error in aerobic training is failure to progress, which results in an early plateau in cardiovascular and metabolic improvement.

Cardiovascular protection and risk factor reduction appear to require twenty to thirty minutes three days per week, as does improvement in aerobic capacity. Epidemiological studies of mortality, cardiovascular disease, diabetes and functional independence suggest that walking about one mile per day (presumably about twenty minutes at average pace) or expending about 2000 kcal/week in physical activities is protective, again pointing to the moderate levels that are needed for major health outcomes. It has been shown that exercise does not need to be carried out in a single session to provide training effects, and may be broken up into periods of ten minutes at a time.

The risks of exercise are summarized in Table 4. The risk of sudden death during physical activity appears to be limited primarily to those who do not exercise on a regular basis (at least one hour per week), which is another reason for advocating regular, moderate periods of exercise rather than periodic high-volume training.

Table 4 The Risks of Exercise in Older Adults SOURCE: Author

The benefits of aerobic exercise have been extensively studied since the 1960s (the most important of these for older adults are listed in Table 2). They include a broad range of physiological adaptations that are in general opposite to the effects of aging on most body systems, as well as major health-related clinical outcomes. The health conditions that are responsive to aerobic exercise include most of those of concern to older adults: osteoporosis, heart disease, stroke, breast cancer, diabetes, obesity, hypertension, arthritis, chronic lung disease, depression, and insomnia. These physiological and clinical benefits form the basis for the inclusion of aerobic exercise as an essential component of the overall physical activity prescription for healthy aging.

Flexibility training. Flexibility training includes movements or positions designed to increase range of motion across joints. Such range of motion is determined by both soft tissue factors (muscle strength, muscle and ligament length, scarring from surgery or trauma, joint and bursa fluid, synovial tissue thickness and inflammation, ligament laxity, tissue elasticity, degenerative changes of cartilage, temperature of tissues) and bony structure (deformities, arthritic and degenerative changes in bone, surgical devices). Obviously, only some of these abnormalities are amenable to exercise intervention, and these will be discussed below. In general, the effect of stretching the soft tissues around a joint slowly and consistently over time is to increase the pain-free range of motion for that joint.

Flexibility may be enhanced without the use of any specialized equipment. It is often helpful, however, to have a thin mat available for postures that are best done while stretched out on the floor.

The most effective technique for increasing flexibility is to extend a body part as fully as possible without pain, then hold this fully extended position for twenty to thirty seconds. The key requirement is to complete the movement slowly (without any bouncing or ballistic movements). Such bouncing does not increase efficacy and range of motion, but instead may cause muscle contraction that limits the range achievable. A technique known as proprioceptive neural facilitation (PNF) will maximize the stretching effectiveness. The technique is as follows. Once the body part has been stretched as far as possible, the muscle groups around the joint should then be completely relaxed, while maintaining the stretch. Next, an attempt is made to stretch a little further, which is usually possible. This final position is then held for about twenty to thirty seconds before returning to the initial position. PNF serves to counteract the involuntary resistance to overextension of a joint caused by a feedback loop of receptors within the muscle tissue that are activated by mechanical stretch.

Flexibility exercise is part of many other forms of exercise, such as ballet and modern dance, yoga, t'ai chi, and resistance training, because in all of these pursuits the muscle groups are slowly extended to their full range and held before relaxing, just as in PNF. It is not recommended to force a stretch beyond the point of pain, as this may result in injury to soft tissue structures and ultimately worsen function. As with all forms of exercise, as the range of motion increases over time, it is appropriate and necessary to extend the distance the joint is moved so that progress is maintained.

The physiologic benefit of flexibility exercise is increased range of motion across joints. There is some evidence that range of motion is related to functional independence in activities of daily living, posture, balance, and gait characteristics in older adults, as well as to pain and disability and quality of life in arthritis. Flexibility training itself does not result in improved strength or endurance, or marked improvements in balance. Therefore, it is best conceived of as an accessory to other forms of exercise that contributes to overall exercise and functional capacity. To the extent that pain, fear of falling, mobility, and function are improved, quality of life may improve as well. There is a need for much better quantitative research on effective doses and long-term benefits of this mode of exercise in the elderly.

Balance training. Any activity that increases one's ability to maintain balance in the face of stressors may be considered a balance-enhancing activity. Stressors include decreased base of support; perturbation of the ground support; decrease in proprioception, vision, or vestibular system input; increased compliance of the support surface; or movement of the center of mass of the body. Balance-enhancing activities impact on the central nervous system control of balance and coordination of movement, and/or augment the peripheral neuromuscular system response to signals that balance is threatened.

Intensity in balance training refers to the degree of difficulty of the postures, movements, or routines practiced. The appropriate level of difficulty or "intensity" for any balance-enhancing exercise is the highest level that can be tolerated without inducing a fall or near-fall. Progression in intensity is the key to improvement, as in other exercise domains, but mastery of the previous level before progression must be adhered to for safety.

Balance training has been shown to result in improved balance performance, decreased fear of falling, decreased incidence of falls, and increased ability to participate in activities of daily living that may have been limited by gait and balance difficulties. It is expected, although not proven, that such changes ultimately lead to improvements in functional independence, reduced hip fractures and other serious injuries, and improved overall quality of life.

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