Myburgh, K. H., Kohn, T. A., Essen-Gustavsson, B., & Andersen, J. L. (2003). Hybrid skeletal muscle fibers in competitive runners and in recreationally active non-runners. Medicine and Science in Sports and Exercise, 35(5), Supplement abstract 534.

The purpose of this study was to investigate the occurrence of pure and hybrid fibers in competitive runners specializing between 1.5 and 21.1 km and in recreationally active non-runners. Whether hybridicity was related to habitual exercise was to be determined. Single muscle fibers from the vastus lateralis of competitive runners (N = 7) and non-runners (N = 13) were analyzed.

Competitive runners covered ~81 km/wk and non-runners exercised ~3.5 hr/wk. VO2max was ~70 ml/kg in runners and ~40 ml/kg in non-runners. Both groups had a similar percentage of pure Type IIa fibers but runners had a significantly higher percentage of pure Type I fibers and lower percentage of pure Type IIx fibers. Training distance was unrelated to fiber type. The percentage of hybrid fibers was similar in both groups. Percentage of Type IIa/x fibers was exponentially related to specialist racing distance.

Implications. Distance running either increases the percentage of Type I fibers or people with a higher percentage of Type I fibers select to compete in distance races. The occurrence of hybridicity is similar in runners and recreationally active individuals. Hybridicity decreases as training volume increases.

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