One of the hallmark features of aging is the loss of muscle mass and strength. Much of this loss can be explained by changes to the neuromuscular system such as a decreased number of motor neurons, decreased number of muscle fibers, and decreased muscle fiber size. But it is important to ask whether these changes are a consequence of aging or simply a result of an inactive lifestyle.
Age-Related Changes to Muscle
The age-related decline in muscle mass appears to occur in 2 phases. The first or slow phase of muscle loss, in which 10% of muscle mass is lost, occurs between the ages of 25 and 50. The majority of muscle loss occurs thereafter where an additional 40% is lost from the ages of 50 to 85. Overall, the human body loses 50% of its muscle mass by the age of 80. This muscle atrophy can be explained by significant decreases in both the total number of muscle fibers and muscle fiber size.
It has been shown that aging results in a loss of the power and speed producing fast-twitch fibers (particularly IIb) and an increase in the more aerobic slow-twitch fibers. This seems to make sense since movements that demand a high velocity of contraction (such as jumping and sprinting) tend to be less in the older years.
Mechanisms of Strength Loss
The problems with decreasing strength can be seen in its contribution to the osteoporotic decline in bone density, arthritic joint pain, and an overall reduced functional capacity.
With the loss in muscle mass evidently comes a decrease in muscular strength. However, as with muscle loss, most strength losses are not significant until the sixth decade. As briefly mentioned, this loss in strength can be attributed to a decrease in the number of motor units (nerve-muscle fiber complex), the decreased number of muscle fibers, and the reduction in muscle fiber size. It is also known that a decline in leg strength precedes upper extremity strength loss in the elderly. This is important due to the fact that strength, rather than cardiovascular function, is considered to be the most physically limiting factor in the elderly. This is apparent when considering strength-limiting activities faced by many seniors such as getting up from a seated position or walking upstairs.
Encouraging is the finding that aging does not seem to affect eccentric strength. This phase of contraction is an important consideration for the elderly due to the possible linkage between poor eccentric strength and the incidence of falls in the elderly.
The Importance of Active Living
Regular exercise is the most effective way to slow and counteract the effects of age-related muscle and strength loss. Comparisons between active and sedentary older adults suggest that much of the strength loss with aging is due to lifestyle factors. For example, individuals who continue to use certain muscles on a regular basis do not show the same age-related decreases in strength. In general, muscle atrophy, and thus strength loss, will occur any time the muscles are not required to work against a given load. The result will be a decrease in protein synthesis accompanied by an increase in protein breakdown. Overall, the muscle atrophies and loses much of its strength, characteristics are commonly seen in astronauts during space flight. Incorporating regular resistance training is the most effective means of attenuating this effect.
Studies have consistently shown that regular exercise can improve muscular endurance and strength in the elderly in a manner similar to that observed in young people. One of the largest studies in this field was done at McMaster University several years ago. The researchers looked at the effects of 2 years of twice/weekly strength training (80-85% 1RM) across 114 subjects between the ages of 60-80 years. The results indicated steady increases in strength in each of the muscle groups tested with no evidence of plateauing. There were also significant increases in muscle mass accompanying the gains in strength and, perhaps more importantly, there was evidence that these strength gains translated into an improved function (as measured by walking and stair climbing performance).
Although there are certain unavoidable changes that occur with aging, it is possible to delay or attenuate the losses of muscle mass and strength normally accompanying these changes. Since so many daily living activities such as walking, climbing stairs, and standing up from a chair are so dependent on strength it is imperative to minimize the age-related loss in strength as much as possible. The muscles in older adults maintain their ability to adapt; therefore, regular resistance training (2-3x/week) should be implemented into the lifestyle of such individuals. Moreover, a similar strength training protocol needs to be employed in younger adults as a means of prevention and staying healthy into the golden years!