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Human Performance Laboratory, Ball State University, Muncie, Indiana 47306
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 (
= + 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 ( = 
2; YM and YW;
P < 0.05), IIa/IIx ( = 13 and 19 for YM
and YW, respectively; P < 0.05), and total hybrid
fiber proportion (I/IIa + I/IIa/IIx + IIa/IIx) decreased ( = 19 and 30 for YM and YW, respectively;
P < 0.05) with the training, as did the MHC IIx
distribution ( = 2; YW only; P < 0.05).
Alterations in the predominance of MHC isoforms within hybrid fibers
(decrease in MHC I-dominant I/IIa 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.
coexpressed fiber; aging; skeletal muscle; fiber type; progressive resistance training
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