Ca2+ dependency of limb muscle fiber contractile mechanics in young and older adults

Age-induced declines in skeletal muscle contractile function have been attributed to multiple cellular factors, including lower peak force (P o ), decreased Ca 2+ sensitivity, and reduced shortening velocity (V o ). However, changes in these cellular properties with aging remain unresolved, especially in older women, and the effect of submaximal Ca 2+ on contractile function is unknown. Thus, we compared contractile properties of muscle fibers from 19 young (24 ± 3 yr; 8 women) and 21 older adults (77 ± 7 yr; 7 women) under maximal and submaximal Ca 2+ and assessed the abundance of three proteins thought to influence Ca 2+ sensitivity. Fast fiber cross-sectional area was ~44% larger in young (6,479 ± 2,487 µm 2 ) compared with older adults (4,503 ± 2,071 µm 2 , P < 0.001), which corresponded with a greater absolute P o (young = 1.12 ± 0.43 mN; old = 0.79 ± 0.33 mN, P < 0.001). There were no differences in fast fiber size-specific P o , indicating the age-related decline in force was explained by differences in fiber size. Except for fast fiber size and absolute P o , no age or sex differences were observed in Ca 2+ sensitivity, rate of force development (k tr ), or V o in either slow or fast fibers. Submaximal Ca 2+ depressed k tr and V o , but the effects were not altered by age in either sex. Contrary to rodent studies, regulatory light chain (RLC) and myosin binding protein-C abundance and RLC phosphorylation were unaltered by age or sex. These data suggest the age-associated reductions in contractile function are primarily due to the atrophy of fast fibers and that caution is warranted when extending results from rodent studies to humans.