Reduction in hybrid single muscle fiber proportions with resistance training in humans

David L. Williamson, IV, P. M. Gallagher, C. C. Carroll, U. Raue, S. W. Trappe

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The purpose of this investigation was to examine the effects of 12 wk of progressive resistance training (PRT) on single muscle fiber myosin heavy chain (MHC; I, I/IIa, I/IIa/IIx, IIa, IIa/IIx, IIx) isoform proportions in young individuals. Young, untrained men (YM; n = 6) and women (YW; n = 6) (age = 22 ± 1 and 25 ± 2 yr for YW and YM, respectively) received pre- and post-PRT muscle biopsies from the right vastus lateralis for single muscle fiber MHC distribution by electrophoretic analysis (192 ± 5 pre- and 183 ± 6 post-fibers/subject analyzed; 4,495 fibers total). Data are presented as percentages of the total fibers analyzed per subject. The PRT protocol elicited an increase in the pure MHC IIa (Δ = + 24 and + 27; YW and YM, respectively; P < 0.05) with no change in the pure MHC I distribution. The hybrid MHC distributions decreased I/IIa/ IIx (Δ = -2; YM and YW; P < 0.05), IIa/IIx (Δ = -13 and -19 for YM and YW, respectively; P < 0.05), and total hybrid fiber proportion (I/lIa + I/IIa/IIx + IIa/IIx) decreased (Δ = -19 and -30 for YM and YW, respectively; P < 0.05) with the training, as did the MHC IIx distribution (Δ = -2; YW only; P < 0.05). Alterations in the predominance of MHC isoforms within hybrid fibers (decrease in MHC I-dominant I/lIa and nondominant MHC IIa/IIx, increase in MHC IIa-dominant IIa/IIx; P < 0.05) appeared to contribute to the increase in the MHC IIa proportion. Electrophoresis of muscle cross sections revealed an ∼7% increase (P < 0.05) in MHC IIa proportion in both groups, whereas the MHC IIx decrease by 7.5 and 11.6% post-PRT in YW and YM, respectively. MHC I proportions increase in YM by 4.8% (P < 0.05) post-PRT. These findings further support previous resistance training data in young adults with respect to the increase in the MHC IIa proportions but demonstrate that a majority of the change can be attributed to the decrease in single-fiber hybrid proportions.

Original languageEnglish (US)
Pages (from-to)1955-1961
Number of pages7
JournalJournal of Applied Physiology
Issue number5
Publication statusPublished - 2001


All Science Journal Classification (ASJC) codes

  • Endocrinology
  • Physiology
  • Orthopedics and Sports Medicine
  • Physical Therapy, Sports Therapy and Rehabilitation

Cite this

Williamson, IV, D. L., Gallagher, P. M., Carroll, C. C., Raue, U., & Trappe, S. W. (2001). Reduction in hybrid single muscle fiber proportions with resistance training in humans. Journal of Applied Physiology, 91(5), 1955-1961.