Print version ISSN 1517-8692
Rev Bras Med Esporte vol.3 no.3 Niterói July/Sept. 1997
ARTIGO DE OPINIÃO
Walter R. Frontera
It is well known that strength training in younger people and athletes results in significant physiological adaptations and performance gains. For many years, however, this type of exercise training was considered dangerous for older men and women. Endurance training, on the other hand, became an accepted form of physical activity in the elderly due to its positive effects on cardiovascular function and health. The myth that strength training is not useful and/or safe in older adults has started to disappear. During the last ten years, many scientific studies have shown that older adults can be safely trained with strengthening exercises. Furthermore, many investigations have demonstrated that strength training results in significant physiological, functional, and psychological benefits.
WHY RESEARCH IN AGING?
In the context of human evolution, aging appears to be a recent phenomenon. During the 20th century a dramatic increase in life expectancy and in the number and percentage of people in the older age groups has been reported. In 1990, approximately 9% of the population of the world was 60 years or older and it has been estimated that by the year 2030 the proportion of people in this age group will increase to approximately 16%. Despite the variability in demographics among countries (in some countries approximately 20% of the population already belongs to this age group), the trend towards an older society seems to be universal. Thus, it has been estimated that each month, the world sees a net gain of 800,000 people over 65.
A FUNCTIONAL APPROACH
The problem, however, is not the statistical or mathematical significance of the population changes. Our main concern should be the functional implications of advanced adult age. Aging is associated with loss of muscle strength, muscle mass, motor units, aerobic capacity, hormonal reserve, and many other physiological changes. Together, these losses result in a reduction in maximum gait speed, ability to perform activities of daily living, power to climb stairs or rise from a chair, and other impairments and disabilities. As a result of those physiological losses, an 80-year old person cannot do those things that he/she considered easy at age 20. The force required to complete certain tasks may represent, in the elderly, a maximal or supramaximal effort when compared to the same task performed by younger (and stronger) persons. The end result of the reduction in physiological capacity is deconditioning, loss of independence, and increased use of medical services, all of which place a significant burden on all individual governments and society in general.
AGING OR AGE-RELATED LOSSES?
Aging is difficult to define. Many statistical reports use a specific age limit (usually 60 or 65 years) to classify the population but it is clear that chronological age cannot be considered a good index of physiological age. According to the cell biologist Leonard Hayflick, age changes begin in different parts of the body at different times and the rate of annual change varies among various cells, tissues, and organs, as well as from person to person. On the other hand, the physiologist Paola Timiras defines aging as the sum of all changes that occur with the passage of time. Thus, aging does not seem to be neither a simple nor a uniform process easy to define or study.
Of particular interest are observations that relate aging to changes that usually characterize inactivity and/or malnutrition. The sequence of events starting with physical and biochemical changes in muscle and ending in functional impairment and disability appear to be very similar in these processes. Further, we know that elderly men and women have a reduced level of habitual physical activity and reduce their food intake. Perhaps, some of the changes usually ascribed to aging are associated processes and not the end result of, or consequences of aging.
STRENGTH TRAINING IN THE ELDERLY
If the assumption that some of the physiological losses with age can be explained by associated processes is true, it can be suggested that properly designed corrective interventions, such as exercise training programs, may prevent some of the losses and help with the recovery of functional capacity in the elderly.
Interest in the strength of the aging skeletal muscle is evident when we consider that the number of published scientific studies on the topic has increased from 3 in the 1966-1974 period to 132 in the years 1994 to1997. Since 1988, at least 25 studies have explored the physiological adaptations and functional benefits of strength training of the muscles of the lower limbs. An additional 6 studies have examined the effects of strength training on muscles of the upper limbs.
In general these studies have included men and women in the 60- to-90 yr-old age range and the training has been based on the progressive resistance exercise model which requires a progressive change (i.e., every week) in the training load to adjust for the gains made during the preceeding sessions. The purpose of this approach is to maintain the training stimulus constant. An example of the exercise prescription used in several studies is included in the following table:
Programs similar to the one described above have resulted in strength gains ranging from 15 to 175% of the initial (pre-training) strength. A 10 to 15% increase in muscle mass has been consistently reported based on measurements using sophisticated imaging techniques (CT scan or MRI). Individual muscle fibers (both type I and II) also show significant hypertrophy (10 to 30%) as seen in cross-sections of biopsy specimens stained with histochemical methods. It seems that the process that leads to strength gains and hypertrophy includes a dynamic turnover of muscle proteins and that skeletal muscle in old age is capable of responding to the stimulus provided by exercise with the synthesis of new myofilaments. Finally, strength training has been shown to preserve bone density while improving muscle mass, strength, and balance in postmenopausal women. These observations have particular clinical relevance given the high incidence of falls in the elderly and the associated morbidity and mortality.
As reported in the younger population, the larger relative increases in strength compared to the changes in muscle size suggest a significant effect on the nervous components of the neuromuscular system. The nature of the so-called neural adaptations is not clear but adaptations in the nerve conduction velocity, spinal reflexes, motor unit activation and synchronization, and central cognitive-learning processes may all play a role.
At least two studies have reported peripheral adaptations in the oxygen transport chain resulting in small but significant improvements in maximal aerobic power after strength training. Both, capillary supply and the activity of oxidative enzymes (citrate synthase) have been reported to increase suggesting the adaptations in VO2 max are peripheral in nature. Additional cardiovascular adaptations include an attenuation of the blood pressure response to exercise when subjects lift the same load after training. This adaptation could reduce the stress on the cardiovascular system during regular activities such as carrying objects at home or work. Despite these positive observations, strength training should not replace endurance training for developing aerobic power and capacity.
Strength training seems to be a safe type of exercise even for the frail elderly. The limits to strength training in the older population are not well understood. Both, men and women, respond to strength training and even nonagenarians seem to retain the ability to adapt to this type of exercise. Long-term studies (1-2 years) do not show a clear plateau in strength gains. In other words, after many months of training, volunteers continued to show improvements. Finally, functional adaptations to strength training include increased walking endurance, increased balance time, decreased stair climbing time, and a reduction in the risk of falls. All these have clear implications for achieving independence in life.
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Endereço para correspondência:
Walter R. Frontera, MD, PhD
Associate Professor and Chairman
Department of Physical Medicine and Rehabilitation
Harvard Medical School
Spaulding Rehabilitation Hospital
125 Nashua St.
Boston, Massachusetts, USA 02114
Endereço para correspondência: