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Aging, muscle fiber type, and contractile function in sprint-trained athletes

¹Department of Health Sciences, University of Jyväskylä, Jyväskylä, Finland; ²Department of Clinical Neurophysiology, University of Uppsala, Uppsala, Sweden; ³Department of Biology of Physical Activity, University of Jyväskylä; ⁴The Finnish Centre for Interdisciplinary Gerontology; ⁵KIHU — Research Institute for Olympic Sports, Jyväskylä, Finland

Submitted 10 March 2006 ; accepted in final form 9 May 2006

Biopsy samples were taken from the vastus lateralis of 18- to 84-yr-old male sprinters (n = 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 (V_o) were determined in 144 single skinned fibers from younger (18–33 yr, n = 8) and older (53–77 yr, n = 9) runners. Force-time 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 < 0.001). With age there was an increased MHC I (P < 0.01) and reduced MHC IIx isoform content (P < 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. V_o of fibers expressing type I MHC was lower (P < 0.05) in older than in younger subjects, but there was no difference in V_o of type IIa MHC fibers. An aging-related decline of maximal isometric force (P < 0.001) and normalized rate of force development (P < 0.05) of knee extensors was observed. Normalized rate of force development was positively associated with MHC II (P < 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 V_o 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

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