Myosin Heavy Chain Isoform Distribution Following Eight Weeks Of Sprint Training

Abstract

2328 The plasticity of human skeletal muscle is demonstrated by alterations in MHC isoform composition in response to training. This is generally displayed by an increase in muscle fibers expressing a single MHC isoform with a concomitant reduction in hybrid fibers. PURPOSE: The present investigation examined single fiber MHC composition changes in response to high intensity, short duration sprint cycle training. METHODS: Ten untrained college-age male subjects participated in eight weeks of a progressive sprint cycle training program. Training involved 15 sec maximal sprints separated by 5 min rest. Subjects completed 4 sprints × 2 days in week 1 and increased to 6 sprints × 3 days at week eight. Muscle samples from the vastus lateralis were obtained before and after training. A 30 sec Wingate test was used to evaluate performance prior to and after training. RESULTS: For the 30 sec sprint mean power and total work increased from pre to post. Single fiber analyses revealed a reduction in the MHC IIx isoform (2.0 ± 1.0 to 0.2 ± 0.1%, pre to post) while there was no change in hybrid fiber composition (total hybrids = 24%). CONCLUSION: A total sprint time of as little as 32 min over eight weeks is sufficient to elicit significant high intensity sprint performance changes. While these performance changes were accompanied by modest alterations in single fiber composition high intensity sprint training over eight weeks did not influence hybrid fiber distribution.