Single muscle fiber contractile properties of young competitive distance runners

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 (mu m), 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 similar to 20% greater (P < 0.05) than Rec. Peak force of the MHC IIa fibers was 31% higher (P < 0.05) in Run, whereas Po 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. Vo was higher (P < 0.05) in MHC I (+70%) and MHC IIa (+18%) fibers from Run subjects. In vitro peak absolute power (mu N center dot 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/1) 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.