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Am J Physiol Regul Integr Comp Physiol (May 13, 2004). doi:10.1152/ajpregu.00686.2003
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Submitted on December 1, 2003
Accepted on May 7, 2004

Single Muscle Fiber Contractile Properties During a Competitive Season in Male Runners

Matthew P Harber1, Philip M Gallagher1, Andrew R Creer1, Kiril M Minchev1, and Scott W Trappe1*

1 Human Performance Laboratory, Ball State University, Muncie, IN, USA

* To whom correspondence should be addressed. E-mail: strappe{at}bsu.edu.

The purpose of this investigation was to examine the contractile properties of individual myofibers in response to periodized training periods throughout a collegiate cross-country season in male runners. Muscle biopsies of the gastrocnemius were taken following a summer base training phase (T1), an 8 week intense training period (T2), and a 4 week taper phase (T3). Five runners (n=5; age = 20±1 yr; wt = 65±4 kg; ht = 178±3 cm) completed all three time points. A total of 328 individual muscle fibers (MHC I = 66%; MHC IIa = 33%; hybrids = 1%) were isolated and studied at 15°C for their contractile properties. Diameter of MHC I fibers was 3% smaller (P < 0.05) at T2 compared to T1 and an additional 4% smaller (P < 0.05) after the taper. Cell size was unaltered in the MHC IIa fibers. MHC I and IIa fiber strength increased 18 and 11% (P < 0.05) respectively, from T1 to T2. MHC I fibers produced 9% less force (P < 0.05) after the taper while MHC IIa fibers were 9% stronger (P < 0.05). Specific tension (Po/CSA) increased 38 and 26% (P < 0.05) for MHC I and IIa fibers, respectively from T1 to T2 and was unchanged with the taper. Maximal shortening velocity (Vo) of the MHC I fibers decreased 23% (P < 0.05) from T1 to T2 and 17% (P < 0.05) from T2 to T3 while MHC IIa Vo was unchanged. MHC I peak power decreased 20% (P < 0.05) from T1 to T2 and 25% (P < 0.05) from T2 to T3 whereas MHC IIa peak power was unchanged. Power corrected for cell size decreased 15% (P < 0.05) from T2 to T3 and was 24% (P < 0.05) lower at T3 compared to T1 for the MHC I fibers only. These data suggest that changes in run training alter myocellular physiology via decreases in fiber size, Vo, and power of MHC I fibers and through increases in force per cross-sectional area of slow- and fast-twitch muscle fibers.




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