Purpose: 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 (R2 = 0.88) than linear (R2 = 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/wk) 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.
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