OXIDATIVE CAPACITY OF HUMAN MUSCLE-FIBER TYPES - EFFECTS OF AGE AND TRAINING STATUS

Morphometry and oxidative capacity of slow-twitch (type I) and fast-tw itch (type IIa and IIb) muscle fibers obtained from vastus lateralis n eedle biopsies were compared between younger (21-30 yr) and older (51- 62 yr) normal fit (maximal O-2 uptake = 47.0 vs. 32.3 ml . kg(-1) . mi n(-1)) and endurance-trained (66.3 vs. 52.7 ml . kg(-1) . min(-1)) men (n = 6/group). The older groups had smaller type IIa (31%) and IIb (4 0%) fiber areas and fewer capillaries surrounding these fibers than di d younger groups. The reduced type II fiber areas and capillary contac ts associated with aging were also observed in the older trained men. However, the capillary supply per unit type II fiber area was not affe cted by age but was enhanced by training. Additionally, on the basis o f quantitative histochemical analysis, succinate dehydrogenase activit ies of type IIa fibers in the older trained men [4.07 +/- 0.68 (SD) mm ol . min(-1) . l(-1)] were similar to those observed in younger traine d men (4.00 +/- 0.48 mmol . min(-1) . l(-1)) and twofold higher than i n older normal fit men (2.01 +/- 0.65 mmol . min(-1) . l(-1); age x fi tness interaction, P < 0.05). Type I muscle fibers were unaffected by age but were larger and had more capillary contacts and higher succina te dehydrogenase activities in the trained groups. The findings of thi s study suggest that aging results in a decrease in type II fiber size and oxidative capacity in healthy men and that this latter effect can be prevented by endurance training. Conclusions regarding the effects of age and training status on muscle capillarization depend largely o n how these data are expressed.