The physiology of aging: what you can do to slow or stop the loss of muscle mass - Research

Aging is not a passage of time, but rather an accumulation of biological events that occur over time. If we define aging as the loss of one's ability to adapt to a changing environment, biological and functional age becomes a measure of one's success for adaptation.

At the turn of the twentieth century, approximately 4 percent of the United States population was over the age of 65; today, it has climbed to 13 percent. Life expectancy has increased to approximately 76 years and is expected to reach 83 years by 2050. The number of older adults, currently about 36 million, has increased 11-fold. As individuals age, especially past 85 years old, there is a growing need for assistance with their everyday activities. Thus, we must determine the extent and mechanisms by which exercise and physical activity can improve health, capacity, quality of life and functional independence.

Sarcopenia

The most reported consequence of aging is the loss of skeletal muscle mass, known as sarcopenia. Researchers often agree that sarcopenia is not caused by a single factor but is primarily due to a complex interaction of muscle (myopathic) and nerve (neuropathic) alterations, in addition to a declining stimulus to these two physiological systems from decreased physical activity.

Possible reasons for the decrease in muscle mass involve changes in both the muscle fiber area and fiber number. Some studies report that Type I (slow twitch, aerobic) muscle fibers are resistant to age associated atrophy, at least until ages 60 to 70 years; while Type II (fast-twitch, anaerobic) muscle fibers appear to decline with age. The loss of muscle fibers and muscle atrophy have been reported for both males and females and corresponds to the age of 50 years.

One of the most obvious manifestations of muscle mass loss is the decreased ability to produce force. This decrease in the force producing capacity of skeletal muscle is not universal and depends on the type of contraction being performed and muscle group being examined. Research that examined isometric force production characteristics during the aging process reports that the earliest declines in strength occur, around age 40, in the forearm extensors and the muscles of the lower leg (dorsiflexors and plantar flexors) while the greatest overall loss of strength occurs in the two lower leg muscles (dorsiflexors and plantar flexors).

Muscular Endurance

Perhaps more important than the production of maximal strength is the ability to maintain muscular endurance. Some researchers note that as long as contractions are performed at the same relative percentage of their strength, older individuals can maintain voluntary isometric and dynamic muscular contractions for up to 60 seconds, when compared to younger individuals. Studies show that the decrease in the ability to maintain force production is not so much a function of age as it is related to muscle group location. However, the muscles of the lower extremity maintain force output better than the muscles of the upper extremity.

Factors associated with age-related muscular atrophy vary from person to person and different muscle groups. Changes observed in elderly skeletal muscle may also be caused by other factors, such as nutritional deficiencies, changes in endocrine status and lack of regular physical activity. If increased age causes the decline of muscle mass and function, can the appropriate resistance training program benefit an older individual? Recent studies show that elderly skeletal muscle is able to adapt to short term (12 weeks) training programs by increasing strength through muscle hypertrophy and improved functional performance.

Is Age Responsible?

It is also important to note whether the ultimate degree of muscle hypertrophy following training is age dependent. Some studies report that, when compared to older (62- to 72-year-old) individuals, younger (22- to 31-year-old) individuals have significantly greater increases in muscle after three months of strength training. Although there is some age effect on the responsiveness to exercise, this phenomena cannot be generalized to all muscle groups. However, there is little information available regarding the adaptability of elderly skeletal muscles to prolonged training periods.

To summarize, current research indicates elderly muscle can adapt to resistance exercise. Significant improvements in strength occurring from muscle hypertrophy may be the result of an increased sensitivity of androgen receptors and increased levels of circulating factors responsible for muscle growth, such as testosterone, IGF-I and growth hormone.

The American College of Sports Medicine (ACSM) publishes position stands regarding exercise for healthy adults as well as a position stand for older adults. They state that age should not be a limiting factor for beginning an exercise program (but medical approval should be obtained) and exercises should be progressive, individualized and involve all major muscle groups. Exercises should also be performed two to three times per week with at least one set of 8 to 15 repetitions. Multiple sets could provide more benefits and exercises that help with balance and posture should be practiced.

Note: The conclusions outlined here are those of the researchers only and should not be construed as an official statement of either the American College of Sports Medicine (ACSM) or the Aerobics and Fitness Association of America (AFAA).

Written by Michael G. Bemben, Ph.D., FACSM for the American College of Sports Medicine's (ACSM's) February 2001 Current Comment.

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