Single muscle fiber contractile properties during a competitive season in male runners

The purpose of this investigation was to examine the contractile properties of individual myofibers in response to periodized training periods throughout a collegiate cross-country season in male runners. Muscle biopsies of the gastrocnemius were taken after a summer base training phase (T1), an 8-wk intense training period (T2), and a 4-wk taper phase (T3). Five runners (n=5; age=20+/-1 yr; wt=65+/-4 kg; ht=178+/-3 cm) completed all three time points. A total of 328 individual muscle fibers [myosin heavy chain (MHC) I=66%; MHC IIa=33%; hybrids=1%] were isolated and studied at 15degreesC for their contractile properties. Diameter of MHC I fibers was 3% smaller (P<0.05) at T2 compared with T1 and an additional 4% smaller (P<0.05) after the taper. Cell size was unaltered in the MHC IIa fibers. MHC I and IIa fiber strength increased 18 and 11% (P<0.05), respectively, from T1 to T2. MHC I fibers produced 9% less force (P<0.05) after the taper, whereas MHC IIa fibers were 9% stronger (P<0.05). Specific tension increased 38 and 26% (P<0.05) for MHC I and IIa fibers, respectively, from T1 to T2 and was unchanged with the taper. Maximal shortening velocity (V-o) of the MHC I fibers decreased 23% (P<0.05) from T1 to T2 and 17% (P<0.05) from T2 to T3, whereas MHC IIa V-o was unchanged. MHC I peak power decreased 20% (P<0.05) from T1 to T2 and 25% (P<0.05) from T2 to T3, whereas MHC IIa peak power was unchanged. Power corrected for cell size decreased 15% (P<0.05) from T2 to T3 and was 24% (P<0.05) lower at T3 compared with T1 for the MHC I fibers only. These data suggest that changes in run training alter myocellular physiology via decreases in fiber size, V-o, and power of MHC I fibers and through increases in force per cross-sectional area of slow- and fast-twitch muscle fibers.