Aging, muscle fiber type, and contractile function in sprint-trained athletes. Biopsy samples were taken from the vastus lateralis of 18-to 84-yr-old male sprinters (n ϭ 91). Fiber-type distribution, cross-sectional area, and myosin heavy chain (MHC) isoform content were identified using ATPase histochemistry and SDS-PAGE. Specific tension and maximum shortening velocity (Vo) were determined in 144 single skinned fibers from younger (18 -33 yr, n ϭ 8) and older (53-77 yr, n ϭ 9) runners.

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... ime characteristics of the knee extensors were determined by using isometric contraction. The cross-sectional area of type I fibers was unchanged with age, whereas that of type II fibers was reduced (P Ͻ 0.001). With age there was an increased MHC I (P Ͻ 0.01) and reduced MHC IIx isoform content (P Ͻ 0.05) but no differences in MHC IIa. Specific tension of type I and IIa MHC fibers did not differ between younger and older subjects. Vo of fibers expressing type I MHC was lower (P Ͻ 0.05) in older than in younger subjects, but there was no difference in Vo of type IIa MHC fibers. An aging-related decline of maximal isometric force (P Ͻ 0.001) and normalized rate of force development (P Ͻ 0.05) of knee extensors was observed. Normalized rate of force development was positively associated with MHC II (P Ͻ 0.05). The sprint-trained athletes experienced the typical aging-related reduction in the size of fast fibers, a shift toward a slower MHC isoform profile, and a lower Vo of type I MHC fibers, which played a role in the decline in explosive force production. However, the muscle characteristics were preserved at a high level in the oldest runners, underlining the favorable impact of sprint exercise on aging muscle. exercise; myosin heavy chain; single-fiber contractile properties; muscle strength Values are means Ϯ SE. N, number of subjects; VL, vastus lateralis muscle. ANOVA P values and location of significant differences are shown. a,b,c,d,e Group is significantly (P Ͻ 0.05) different from the 18-to 33-, 40-to 49-, 50-to 59-, 60-to 69-, and 70-to 84-yr-old groups, respectively. NS, not significant. AGING SKELETAL MUSCLE AND SPRINT TRAINING