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Received January 30, 2004
Revised February 25, 2004
Accepted after revision April 1, 2004
1 Ball State University
2 University of Arkansas for Medical Sciences
3 Karolinska Institute
4 Karolinksa Institute
* To whom correspondence should be addressed. E-mail: strappe{at}bsu.edu.
Muscle biopsies were obtained from the vastus lateralis before and after 84-d of bedrest from six control (BR) and six resistance-exercised (BRE) men to examine slow- and fast-twitch muscle fiber contractile function. BR did not exercise during bedrest and had a 17% and 40% decrease in whole muscle size and function, respectively. BRE performed 4 sets of 7 maximal concentric and eccentric supine squats 2-3 d/wk (every third day) that maintained whole muscle strength and size. Slow (MHC I) and fast (MHC IIa) muscle fibers were studied at 15°C for diameter, peak force (Po), contractile velocity (Vo) and force-power parameters. SDS-PAGE was performed on each single fiber after the functional experiments to determine MHC isoform composition. MHC I and IIa BR fibers were 15% and 8% smaller, 46% and 25% weaker (Po), 21% and 6% slower (Vo), and 54% and 24% less powerful after bedrest (p<0.05). BR MHC I and IIa Po and power normalized to cell size were lower (p<0.05). BRE MHC I fibers had no change in size or Vo; however, Po was 19% lower (p<0.05) resulting in a 20% and 30% decline (p<0.05) in normalized Po and power, respectively. BRE MHC IIa fibers had no change in size, Po and power, while Vo was elevated 13% (p<0.05). BRE MHC IIa normalized Po and power were 10% and 15% lower (p<0.05), respectively. MHC isoform composition shifted away from MHC I fibers resulting in an increase (p<0.05) in MHC I/IIa (BR and BRE) and MHC IIa/IIx (BR only) fibers. These data show that the contractile function of the MHC I fibers was more affected by bedrest and less influenced by the resistance exercise paradigm than the MHC IIa fibers. Considering the large differences in power of human MHC I and IIa muscle fibers (5-6 fold), the maintenance of whole muscle function with the RE program is likely explained by 1) the maintenance of MHC IIa power and 2) the slow to fast shift (MHC I ? MHC I/IIa) in single fiber MHC isoform composition.
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