title: Exercise pattern influences skeletal muscle hybrid fibers of runners and nonrunners.
abstract: OBJECTIVE
  To determine whether relationships between skeletal muscle hybrid fiber composition and whole-body exercise patterns help to elucidate their transitional capacity or a fine-tuning response to functional demands.
  
  
  METHODS
  This study investigated hybrid fibers from vastus lateralis biopsies of runners (N= 13) and nonrunners (N = 9) and related hybrid fiber occurrence and distribution of myosin heavy-chain isoforms (MHC) within hybrid fibers to exercise patterns. MHC composition of single fibers was identified by SDS-PAGE.
  
  
  RESULTS
  Runners had more fibers expressing only MHC I, fewer expressing MHC IIx, and fewer IIa/IIx hybrid fibers (P < 0.05). Hybrid IIa/IIx and I/IIa fibers were, respectively, negatively and positively related to training volume or average preferred racing distance (PRDA) in runners (P < 0.05). The relationship between IIa/IIx hybrid fibers and PRDA was more exponential (R(2) = 0.88) than linear (R(2) = 0.69). Only IIa/IIx hybrid fibers correlated negatively with exercise hours in nonrunners (P < 0.05). Their IIa/IIx hybrid fibers had MHC IIa content ranging from 1 to 99%, with most between 41 and 60%. Runners favoring longer distances (PRDA > 8 km or training > 70 km x wk(-1)) had no IIa/IIx hybrid fibers with MHC IIa proportion > 60%. In these runners, MHC I within hybrid I/IIa fibers was skewed toward higher proportions (> 60%), whereas MHC I proportions were skewed oppositely in runners favoring shorter training or racing distances.
  
  
  CONCLUSIONS
  Training volume influences both IIa/IIx and I/IIa hybrid fiber proportions in runners, but only the former in nonrunners. Hybrid IIa/IIx fiber proportions were modulated by racing distance. Distinctly different distributions of MHC isoforms within the hybrid fibers were seen in runners favoring longer distances versus those favoring shorter distances.
authors.affiliation: Department of Physiological Sciences, University of Stellenbosch, South Africa.
authors.forename: Tertius A
authors.initials: TA
authors.lastname: Kohn
authors.forename: Birgitta
authors.initials: B
authors.lastname: Essen-Gustavsson
authors.forename: Kathryn H
authors.initials: KH
authors.lastname: Myburgh
mesh_terms: D000328:Adult; D004591:Electrophoresis, Polyacrylamide Gel; D015444:Exercise; D005260:Female; D006801:Humans; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000378:metabolism / Q000502:physiology*; D018995:Myosin Heavy Chains / Q000378:metabolism; D052097:Quadriceps Muscle / Q000378:metabolism / Q000502:physiology*; D012420:Running; D013019:South Africa
chemical_list: D018995:Myosin Heavy Chains