Myosin Heavy Chain Composition of Single Muscle Fibers in Male Distance Runners

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Abstract
The purpose of this study was to characterize the myosin heavy chain (MHC) composition of single muscle fibers from the gastrocnemius of male collegiate distance (DIST; n = 7), middle-distance (MID; n = 6), and recreational runners (REC; n = 6). Additionally, mATPase histochemistry was used to serve as a comparison to previous studies and the single fiber MHC technique. SDS-PAGE of single muscle fibers revealed a higher proportion of MHC I in DIST compared to MID and REC (74.9 ± 4.3 vs 54.4 ± 2.8 vs 56.2 ± 2.9 %, respectively; p < 0.05), less MHC IIa/IIx in DIST compared to MID and REC (0.0 ± 0.0 vs 6.0 ± 2.4 vs 15.9 ± 4.2 %, respectively; p < 0.05), and more total hybrids (I/IIa+IIa/IIx+I/IIa/IIx) in REC than both run groups, DIST and MID (23.0 ± 3.3 vs 6.2 ± 1.1 vs 13.2 ± 2.6 %, respectively; p < 0.05). ATPase histochemistry (pH 4.54) revealed a higher percentage of type I fibers in DIST compared to MID and REC (71.1 ± 3.1 vs 56.3 ± 2.5 vs 59.8 ± 2.3 %, respectively; p < 0.05), a higher percentage of type IIa in MID compared to DIST and REC (43.3 ± 2.7 vs 28.5 ± 3.1 vs. 30.2 ± 3.1 %, p < 0.05), and a higher distribution of type IIb in REC than both run groups (10.0 ± 2.7 vs 0.4 ± 0.2 vs 0.4 ± 0.2 %, p < 0.05). These results suggest that distance running leads to an increase in MHC I expression, training for mid-distance events leads to a prevalence of MHC IIa, and run training leads to a decrease in hybrid fibers.

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