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Development of strength in adulthood

This is an excerpt from Life Span Motor Development 7th Edition With HKPropel Access by Kathleen M. Haywood & Nancy Getchell.

As a result of differential growth of muscle mass during adolescence the average adult man is stronger than the average adult woman. Women can produce only 60% to 80% of the force that men can exert, although most of these differences can be attributed to differences in arm and shoulder strength rather than in trunk or leg strength (Asmussen, 1973). As noted in chapter 9, sex differences in muscle mass are more pronounced in the arms and shoulders than in the trunk and legs.

However, the average difference in body or muscle size accounts for only half the difference in strength between men and women. Cultural norms probably play a role when the sexes are measured for strength. These norms, of course, begin to exert their influence very early in life. For example, Shephard (1982) noted the effect of repeating strength measures on boys and girls who have never been tested for strength. Whereas the boys showed no tendency to improve over three visits, the girls improved on each subsequent visit in almost every case and improved significantly on two of the eight strength measures (figure 11.5). It is possible that the task gained acceptability to the girls as they became more familiar with it. The boys may have been more used to all-out demonstrations of strength. Girls at that time were probably not encouraged to go all out and might even have been discouraged from doing so, which would have limited their experience in exerting strength. Neither should motivation be discounted as a major factor in strength measurement. Certainly, if Shephard had recorded only the first set of scores, he would have concluded that the sex differences in strength were much greater than what he found after comparing the third set of scores. Clearly, cultural norms can be influenced by exposure, in this case experience with activities. Media can be influential in changing cultural norms. So, even women who did not participate in strength training as girls can begin training in young, middle, and older adulthood as they see strength training gain acceptability and participation increase among their peers. If measured for strength, these women might be more likely to feel comfortable giving a maximal effort.

If you were a coach or personal trainer of young girls, how might you work to overcome any stigmas associated with weight training in order to help them improve their strength?

Some research has hinted that sex differences exist in muscle fiber composition—that is, that men and women do not have the same proportions of type I (slow-twitch) and type II (fast-twitch) muscle fibers. If so, part of the sex differences in strength might be attributed to muscle fiber composition. Animal studies indicate that muscle composition is related to isometric strength (see Komi, 1984, for a review). On the other hand, Davies, White, and Young (1983) could find no relationship between strength and muscle fiber composition in boys and girls 11 to 14 years of age. Much more research is needed on this topic with individuals of all ages.

Strength levels generally are maintained throughout the 20s and 30s. For the average adult, both isometric and isotonic strength decline thereafter. McKay et al. (2017) established the normative reference values for strength and flexibility of 1,000 healthy children and adults stratified into age groups of 3 to 9, 10 to 19, 20 to 59, and 60+ years. The norms for isometric strength in 12 muscle groups increased with age in childhood and adolescence. Norms decreased for some muscle groups in adulthood but for all muscle groups in older adulthood. From 10 years of age on, the norms were higher for males than females.

We see several trends in the overall decline of strength with aging; table 11.1, from Spirduso (1995), summarizes these trends. On the left are the better-maintained aspects of strength and on the right are the aspects that decline more in the general population.

These losses are what we would expect from the loss of muscle mass in older adulthood, yet the loss of strength might be larger than the loss of muscle mass. Young, Stokes, and Crowe (1985) found a 39% loss of strength but only a 25% loss in cross-sectional area in the quadriceps muscles of older men compared with younger men. Aniansson, Hedberg, Henning, and Grimby (1986) documented a 10% to 22% loss of strength (figure 11.6) but a 6% loss of muscle mass in the same muscle group over a 7-year span.

Thus, loss of muscle mass does not parallel loss of strength in older adulthood. Spirduso, Francis, and MacRae (2005) identified a number of factors, in addition to muscle atrophy, that can contribute to loss of strength with aging (figure 11.7). As illustrated, a decrease in activity, poor nutrition, and an increase in the likelihood of disease contribute to loss of strength either directly or through changes in the body systems.

The nervous system might be involved because of a loss of motor neurons in the spinal cord with aging, resulting in a loss of motor units (Green, 1986; Grimby, 1988). Other units reinnervate some of the fibers of the lost motor neurons, such that the number of fibers per motor neuron increases (Campbell, McComas, and Petito, 1973; Fitts, 1981). The result would be a loss of muscular coordination, especially fine motor coordination. The vascular system also might be involved in the loss of strength. The number of capillaries per muscle fiber seems to decline with aging, but this is almost assuredly related to a trend toward inactivity (Cartee, 1994). In older adults who undertake aerobic exercise, the number of capillaries actually increases and muscle blood flow improves.

KEY POINT In the absence of training, adults lose strength at a greater rate than would be expected based strictly on loss of muscle mass.

As with so many other aspects of aging, it is difficult to distinguish whether loss of muscle mass and strength in older adults is related to aging of tissues or disuse. We know that strength is better maintained in frequently used muscles than in infrequently used muscles (Kauffman, 1985; Wilmore, 1991). Kallman, Plato, and Tobin (1990) demonstrated how variable the loss of strength is among older adults. They observed young, middle-aged, and older adults over a 10-year period. Many of the older adults lost less strength than middle-aged and young adults lost during the 10 years, and some lost no strength at all. This variability most likely reflects extrinsic factors among these adults, especially exercise and activity levels, reminding us that significant loss of strength with aging is not a foregone conclusion.

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