Myosin heavy chain composition of single muscle fibers in male distance runners.

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.