Cumulative effects of H + and P i on peak power of skeletal muscle fibers from young and older adults.

The cellular mechanisms for the age-related loss in muscle power output and increased fatigability during dynamic contractions remain unresolved. We recently observed that the depressive effects of fatiguing levels of acidosis (H : pH 6.2) and inorganic phosphate (P : 30 mM) did not differ in muscle fibers from young compared with old men. However, the effects of these metabolites may have been saturated in the severe fatigue-mimicking condition, potentially masking any age differences in the sensitivity of the cross-bridge to these metabolites. Therefore, in this study we compared the contractile mechanics of myofibers (n = 327) from the vastus lateralis of 13 young (20-32 years, 7 women) and 12 older adults (70-90 years, 6 women) in conditions mimicking quiescent muscle and a range of fatiguing levels of acidosis (pH 6.8, 6.6, 6.2) and P (12, 20, 30 mM). The knee extensors of the older adults demonstrated hallmark signs of aging muscle including 19% lower thigh lean mass, 60% lower absolute power output, and ~1.8-fold greater fatigability compared with young. Single fiber force, shortening velocity, and power decreased progressively with increasing concentrations of H and P ; however, there were no age or sex differences in the depressive effects of these metabolites in any of the fatigue-mimicking conditions. The cross-sectional area of the fast fibers was 41% smaller in older compared with young adults, which corresponded with lower absolute fiber power. In contrast, there were no differences in fast fiber size-specific power, indicating the age-related decline in absolute fiber power was explained entirely by the differences in fiber size. Estimates of the amount of fast muscle in the thigh were associated with age-related differences in whole-muscle power output (r=0.78). These data suggest that the age-related loss in whole-muscle power output is primarily determined by the atrophy of fast fibers for both men and women, but that the age-related increase in fatigability cannot be explained by an increased sensitivity of the cross-bridge to H and P .