Ca 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), decreased Ca sensitivity, and reduced shortening velocity (V). However, changes in these cellular properties with aging remain unresolved, especially in older women, and the effect of submaximal Ca 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 and assessed the abundance of three proteins thought to influence Ca sensitivity. Fast fiber cross-sectional area was ~44% larger in young (6,479 ± 2,487 µm) compared with older adults (4,503 ± 2,071 µm, < 0.001), which corresponded with a greater absolute P (young = 1.12 ± 0.43 mN; old = 0.79 ± 0.33 mN, < 0.001). There were no differences in fast fiber size-specific P, indicating the age-related decline in force was explained by differences in fiber size. Except for fast fiber size and absolute P, no age or sex differences were observed in Ca sensitivity, rate of force development (k), or V in either slow or fast fibers. Submaximal Ca depressed k and V, 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.