Kohn, T. A., Essen-Gustavsson, B., & Myburgh, K. H. (2007). Exercise pattern influences skeletal muscle hybrid fibers of runners and nonrunners. Medicine and Science in Sports and Exercise, 39, 1977-1984.

This study determined whether relationships between skeletal muscle hybrid fiber composition in the vastus lateralis and whole-body exercise patterns help to elucidate their transitional capacity or a fine-tuning response to functional demands. Runners (N= 13) and nonrunners (N = 9) served as Ss. Related hybrid fiber occurrence and distribution of myosin heavy-chain isoforms (MHC) within hybrid fibers were observed for their response to training patterns.

Runners had more fibers expressing only MHC I, fewer expressing MHC IIx, and fewer IIa/IIx hybrid fibers than nonrunners. Hybrid IIa/IIx and I/IIa fibers were, respectively, negatively and positively related to training volume or average preferred racing distance in runners. The relationship between IIa/IIx hybrid fibers and preferred racing distance was more than linear. Only in nonrunners did IIa/IIx hybrid fibers correlate negatively with exercise hours. Nonrunners' IIa/IIx hybrid fibers had MHC IIa content ranging from 1 to 99%, with most between 41 and 60%. Runners favoring longer distances (>8 km or training >70 km/week) had no IIa/IIx hybrid fibers with the MHC IIa proportion >60%. In the 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.

Implication. Training volume influences both IIa/IIx and I/IIa hybrid fiber proportions in runners, but only IIa/IIx nonrunners. Hybrid IIa/IIx fiber proportions were related to racing distance. Distinctly different distributions of MHC isoforms within the hybrid fibers were seen in runners favoring longer distances versus those favoring shorter distances.

[This study shows just how specific exercise adaptation is within muscles. It is not the actual energy system that is stimulated by training and competing but a sub-group of fibers from the fibers usually associated with the aerobic energy system. This is further evidence that it is the specific skill and skill-associated fiber recruitment that is important when considering endurance training effects, not the general physiological factor.]

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