Single muscle fiber contractile properties with run training
The purpose of this investigation was to examine the contractile properties of individual slow and fast myofibers in response to various training periods throughout a collegiate cross-country season in male runners. Muscle biopsies were obtained from the gastrocnemius of five runners at three time points during the season; after a summer aerobic training phase (T1); after an 8 week period of intense training (T2) and after a 4 week taper period (T3). Absolute (4.6±0.3 L/min) and relative (71±0.7 mI/kg/min) maximal oxygen consumption were unchanged during the study duration. Run performance (8 km) improved 3% from T1 to T2 (27:26 to 26:38 min:sec) and 1% from T2 to T3 (26:21 min:sec). A total of 328 individual myofibers were isolated and studied at 15°C. MHC I diameter was 3% lower (P < 0.05) at T2 compared to T1 and 4% smaller at T3 compared to T2. MHC I and Ila fibers were 18 and 11 % stronger (P < 0.05) respectively, from T1 to T2. MHC I fibers produced 9% less force (P < 0.05) after the taper (T2-T3) while MHC Ila fibers were 9% stronger (P < 0.05). Specific tension (Po/CSA) increased (P < 0.05) 38 and 26% for MHC I and Ila fibers, respectively from T1 to T2 and was unchanged with the taper. Maximal shortening velocity (Vo) of the MHC I fibers decreased (P < 0.05) 23% from T1 to T2 and 17% from T2 to T3. MHC I peak power decreased (P < 0.05) 20% from T1 to T2 and 25% from T2 to T3. Power corrected for cell size decreased (P < 0.05) 15% from T2 to T3 and was 24% lower at T3 compared to T1 for the MHC I fibers only. MHC Ila diameter, Vo, peak and normalized power were unaltered during the study duration. These data suggest that myocellular function is sensitive to changes in run training and it appears that MHC I fibers are moreaffected than MHC IIa fibers. Furthermore, the increase in interval training with the taper may have offset the modest reduction in training volume and prevented any positive adaptations at the cellular level.