Single-fiber MHC polymorphic expression is unaffected by sprint cycle training

Purpose: The present investigation examined single-fiber MHC alterations in response to high-intensity, short-duration, sprint cycle training. Methods: Ten untrained college-age mate subjects participated in 8 wk of a progressive sprint cycle training program. Training involved 15-s maximal sprints separated by 5 min of rest beginning with four sprints X 2 d in week 1 and increasing to six sprints X 3 d at week 8. Muscle samples from the vastus lateralis were obtained before and after training. A 30-s sprint cycle test was used to evaluate performance before and after training. Results: For the 30-s sprint, mean power and total work increased from pre to Post. Single-fiber analyses revealed a reduction in the MHC IN isoform (2.0 +/- 1.0 to 0.2 +/- 0.1%, pre to post, P < 0.05) and an increase in MHC IIa (P = 0.08), whereas there was no change in hybrid fiber composition (total hybrids = 24%). Generally, MHC IIa content increased and MHC IN decreased (P < 0.05) as demonstrated by homogenate analyses of tissue samples. Conclusions: We report that as little as 32 min of high-intensity sprint cycle training over 8 wk is sufficient to improve sprinting performance. This training response is accompanied by an increase in MHC IIa and reduction in MHC IN content of the vastus lateralis. However, short-duration, high-intensity, sprint cycle training does not cause a reduction in hybrid muscle fiber content.