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Single muscle fiber contractile properties of young competitive distance runners

Human Performance Laboratory, Ball State University, Muncie, Indiana

Submitted 19 September 2007 ; accepted in final form 29 May 2008

The purpose of this investigation was to characterize the contractile properties of individual slow- and fast-twitch myofibers from highly trained distance runners. Muscle biopsies were obtained from the gastrocnemius of eight competitive runners (Run) and eight recreationally active individuals (Rec). Slow-twitch [myosin heavy chain (MHC) I] and fast-twitch (MHC IIa) myofibers were isolated and analyzed for diameter (µm), peak force (P_o; mN), unloaded contraction velocity (V_o; fiber lengths/s), and power. Maximum oxygen uptake was higher (P < 0.05) in Run (71 ± 1 vs. 47 ± 2 ml·kg^–1·min^–1). Diameter of MHC I and MHC IIa fibers from Run subjects was 20% greater (P < 0.05) than Rec. Peak force of the MHC IIa fibers was 31% higher (P < 0.05) in Run, whereas P_o of MHC I fibers was not different between groups. No differences for specific tension (P_o/cross-sectional area) were present between groups for either fiber type. V_o was higher (P < 0.05) in MHC I (+70%) and MHC IIa (+18%) fibers from Run subjects. In vitro peak absolute power (µN·s^–1) of both fiber types was greater (P < 0.05) in Run (131 and 85% for MHC I and MHC IIa, respectively). Additionally, normalized power (W/l) of the MHC I fibers was 64% higher in Run, whereas no differences were noted for normalized power of MHC IIa fibers. These data indicate that highly trained endurance runners have elevated contraction velocity in both slow- and fast-twitch myofibers. These characteristics of the fast-twitch muscle fibers have not been previously reported in competitive endurance athletes and may contribute to the high level of running performance in these athletes.

myosin heavy chain; exercise; endurance

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