Abstract:
This investigation examined skeletal muscle fiber type specific differences in fiber
size, myonuclear content, satellite cell content, and capillarity with aging and lifelong
aerobic exercise. Healthy women (n=27) and men (n=41) were recruited and thoroughly
screened into three groups: young exercisers (YE women: n=10, 25±1 y, VO2max 44±2
ml/kg/min; YE men: n=10, 25±1, VO2max 53±3 ml/kg/min), lifelong exercisers (LLE
women: n=7, 72±2 y, VO2max 26±2 ml/kg/min; LLE men: n=21, 74±1 y, VO2max 34±1
ml/kg/min), and old healthy controls (OH women: n=10, 75±1 y, VO2max 18±1 ml/kg/min;
OH men: n=10, 75±1 y, VO2max 22±1 ml/kg/min). LLE men were further subdivided into
LLE-Performance (LLE-P: n=14, 74±1 y, VO2max 38±1 ml/kg/min) and LLE-Fitness (LLEF:
n=7, 75±2 y, VO2max 27±2 ml/kg/min), based on their training intensity and competitive
focus. LLE exercised on average ~5 days/wk for 7 h/wk for >50 y. Muscle biopsies were
obtained from the vastus lateralis and immunofluorescently stained and analyzed for fiber
type, size, myonuclear content, satellite cell content, and capillarity. On average, 683±22
and 558±20 fibers were analyzed for each female and male subject, respectively. Women
showed a few hierarchical trends (YE>LLE>OH) in composite fiber size (P<0.05 for YE
ii
vs. OH only) and capillarity (CCEF, P<0.05 for all). LLE women had similar fiber type
distribution as YE, and a trend toward greater MHC I fiber distribution (P=0.09) and lower
hybrid distribution (P=0.09) relative to OH. OH women had fewer MHC I fibers compared
to YE (P<0.05). LLE women had ~2x greater satellite cells than YE (P<0.05). In men, LLE
had a greater MHC I fiber distribution than OH (P<0.05), OH men had the highest MHC
IIA/IIX hybrid fiber distribution (P<0.05), and YE men trended toward greater MHC IIA
fiber distribution than LLE (P=0.08). LLE men had ~25% greater MHC I fiber size than the
other groups (P<0.05 vs. YE and OH) and greater MHC I fiber and all-fiber composite
capillarity compared to healthy aging (P<0.05 vs. OH). Increased training intensity (LLEP)
was associated with less MHC I/IIA hybrid fibers (P<0.05 vs. LLE-F). In summary,
these data suggest lifelong exercise provides myocellular benefits in fiber type
distribution, fiber size, satellite cell content, and capillarity when compared to old heathy
controls. The fiber type specific data suggest these benefits are distributed across slow
and fast fibers in the LLE women and primarily targeted to the slow fibers in LLE men.
This study provides novel insights into skeletal muscle profiles among lifelong exercising
individuals and suggests that women and men may benefit from aerobic exercise training
in distinct fiber type specific manners.
