title: Single muscle fiber contractile properties during a competitive season in male runners.
abstract: 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 15 degrees C 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 (Vo) 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 Vo 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, Vo, and power of MHC I fibers and through increases in force per cross-sectional area of slow- and fast-twitch muscle fibers.
authors.affiliation: Human Performance Laboratory, Ball State University, Muncie, IN 47306, USA
authors.forename: M P
authors.initials: MP
authors.lastname: Harber
authors.forename: P M
authors.initials: PM
authors.lastname: Gallagher
authors.forename: A R
authors.initials: AR
authors.lastname: Creer
authors.forename: K M
authors.initials: KM
authors.lastname: Minchev
authors.forename: S W
authors.initials: SW
authors.lastname: Trappe
mesh_terms: D000328:Adult; D001706:Biopsy; D005080:Exercise Test; D006801:Humans; D007536:Isomerism; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018482:Muscle, Skeletal / Q000166:cytology* / Q000502:physiology*; D018995:Myosin Heavy Chains / Q000737:chemistry / Q000378:metabolism; D010807:Physical Endurance / Q000502:physiology; D010809:Physical Fitness / Q000502:physiology*; D012420:Running / Q000502:physiology*
chemical_list: D018995:Myosin Heavy Chains