Single-muscle fiber contractile properties in lifelong aerobic exercising women.

The purpose of this study was to examine the effects of lifelong aerobic exercise on single-muscle fiber performance in trained women (LLE; = 7, 72 ± 2 yr) by comparing them to old healthy nonexercisers (OH; = 10, 75 ± 1 yr) and young exercisers (YE; = 10, 25 ± 1 yr). On average, LLE had exercised ~5 days/wk for ~7 h/wk over the past 48 ± 2 yr. Each subject had a vastus lateralis muscle biopsy to examine myosin heavy chain (MHC) I and IIa single-muscle fiber size and function (strength, speed, power). MHC I fiber size was similar across all three cohorts (YE = 5,178 ± 157, LLE = 4,983 ± 184, OH = 4,902 ± 159 µm). MHC IIa fiber size decreased ( < 0.05) 36% with aging (YE = 4,719 ± 164 vs. OH = 3,031 ± 153 µm), with LLE showing a similar 31% reduction (3,253 ± 189 µm). LLE had 17% more powerful ( < 0.05) MHC I fibers and offset the 18% decline in MHC IIa fiber power observed with aging ( < 0.05). The LLE contractile power was driven by greater strength (+11%, = 0.056) in MHC I fibers and elevated contractile speed (+12%, < 0.05) in MHC IIa fibers. These data indicate that lifelong exercise did not benefit MHC I or IIa muscle fiber size. However, LLE had contractile function adaptations that enhanced MHC I fiber power and preserved MHC IIa fiber power through different contractile mechanisms (strength vs. speed). The single-muscle fiber contractile properties observed with lifelong aerobic exercise are unique and provide new insights into aging skeletal muscle plasticity in women at the myocellular level. This is the first investigation to examine the effects of lifelong exercise on single-muscle fiber physiology in women. Nearly 50 yr of moderate to vigorous aerobic exercise training resulted in enhanced slow-twitch fiber power primarily by increasing force production, whereas fast-twitch fiber power was preserved primarily by increasing contractile speed. These unique muscle fiber power profiles helped offset the effects of fast-twitch fiber atrophy and highlight the benefits of lifelong aerobic exercise for myocellular health.