To assess age and gender differences in muscle strength, isometric, concentric (Con), and eccentric (Ecc) peak torque was measured in the knee extensors at a slow (0.52 rad/s) and fast (3.14 rad/s) velocity in 654 subjects (346 men and 308 women, aged 20-93 yr) from the Baltimore Longitudinal Study of Aging. Regression analysis revealed significant (P < 0.001) age-related reductions in Con and Ecc peak torque for men and women at both velocities, but no differences were observed between the gender groups or velocities. Age explained losses in Con better than Ecc peak torque, accounting for 30% (Con) vs. 19% (Ecc) of the variance in men and 28% (Con) vs. 11% (Ecc) in women. To assess age and gender differences in the ability to store and utilize elastic energy, the stretch-shortening cycle was determined in a subset of subjects (n = 47). The older women (mean age = 70 yr) showed a significantly greater enhancement in the stretch-shortening cycle, compared with men of similar age (P < 0.01) and compared with younger men and women (each P < 0.05). Both men and women showed significant declines in muscle quality for Con peak torque (P < 0.01), but no gender differences were observed. Only the men showed a significant decline in muscle quality (P < 0.001) for Ecc peak torque. Thus both men and women experience age-related losses in isometric, Con, and Ecc knee extensor peak torque; however, age accounted for less of the variance in Ecc peak torque in women, and women tend to better preserve muscle quality with age for Ecc peak torque. In addition, older women have an enhanced capacity to store and utilize elastic energy compared with similarly aged men as well as with younger women and men.

The respiratory exchange ratio (RER) is lower during exercise of the same intensity in the trained compared with the untrained state, even though plasma free fatty acids (FFA) and glycerol levels are lower, suggesting reduced availability of plasma FFA. In this context, we evaluated the possibility that lipolysis of muscle triglycerides might be higher in the trained state. Nine adult male subjects performed a prolonged bout of exercise of the same absolute intensity before and after adapting to a strenuous 12-wk program of endurance exercise. The exercise test required 64% of maximum O2 uptake before training. Plasma FFA and glycerol concentrations and RER during the exercise test were lower in the trained than in the untrained state. The proportion of the caloric expenditure derived from fat, calculated from the RER, during the exercise test increased from 35% before training to 57% after training. Muscle glycogen utilization was 41% lower, whereas the decrease in quadriceps muscle triglyceride concentration was roughly twice as great (12.7 +/- 5.5 vs. 26.1 +/- 9.3 mmol/kg dry wt, P less than 0.001) in the trained state. These results suggest that the greater utilization of FFA in the trained state is fueled by increased lipolysis of muscle triglyceride.

This study was undertaken to determine whether carbohydrate feeding during exercise can delay the development of fatigue. Ten trained cyclists performed two bicycle ergometer exercise tests 1 wk apart. The initial work rate required 74 +/- 2% of maximum O2 consumption (VO2 max) (range 70-79% of VO2 max). The point of fatigue was defined as the time at which the exercise intensity the subjects could maintain decreased below their initial work rate by 10% of VO2 max. During one exercise test the subjects were fed a glucose polymer solution beginning 20 min after the onset of exercise; during the other they were given a placebo. Blood glucose concentration was 20-40% higher during the exercise after carbohydrate ingestion than during the exercise without carbohydrate feeding. The exercise-induced decrease in plasma insulin was prevented by carbohydrate feeding. The respiratory exchange ratio was unchanged by the glucose feeding. Fatigue was postponed by carbohydrate feeding in 7 of the 10 subjects. This effect appeared to be mediated by prevention of hypoglycemia in only two subjects. The exercise time to fatigue for the 10 subjects averaged 134 +/- 6 min (mean +/- SE) without and 157 +/- 5 min with carbohydrate feeding (P less than 0.01).

To determine the effects of strength training (ST) on muscle quality (MQ, strength/muscle volume of the trained muscle group), 12 healthy older men (69 +/- 3 yr, range 65-75 yr) and 11 healthy older women (68 +/- 3 yr, range 65-73 yr) were studied before and after a unilateral leg ST program. After a warm-up set, four sets of heavy-resistance knee extensor ST exercise were performed 3 days/wk for 9 wk on the Keiser K-300 leg extension machine. The men exhibited greater absolute increases in the knee extension one-repetition maximum (1-RM) strength test (75 +/- 2 and 94 +/- 3 kg before and after training, respectively) and in quadriceps muscle volume measured by magnetic resonance imaging (1,753 +/- 44 and 1, 955 +/- 43 cm3) than the women (42 +/- 2 and 55 +/- 3 kg for the 1-RM test and 1,125 +/- 53 vs. 1,261 +/- 65 cm3 for quadriceps muscle volume before and after training, respectively, in women; both P < 0.05). However, percent increases were similar for men and women in the 1-RM test (27 and 29% for men and women, respectively), muscle volume (12% for both), and MQ (14 and 16% for men and women, respectively). Significant increases in MQ were observed in both groups in the trained leg (both P < 0.05) and in the 1-RM test for the untrained leg (both P < 0.05), but no significant differences were observed between groups, suggesting neuromuscular adaptations in both gender groups. Thus, although older men appear to have a greater capacity for absolute strength and muscle mass gains than older women in response to ST, the relative contribution of neuromuscular and hypertrophic factors to the increase in strength appears to be similar between genders.

Aging does not affect the muscle mass response to either ST or detraining, whereas gender does, as men increased their muscle volume about twice as much in response to ST as did women and experienced larger losses in response to detraining than women. Young men were the only group that maintained muscle volume adaptation after 31 weeks of detraining. Although myostatin genotype may not explain the observed gender difference in the hypertrophic response to ST, a role for myostatin genotype may be indicated in this regard for women, but future studies are needed with larger subject numbers in each genotype group to confirm this observation.