CROWTHER, G. J., and R. K. GRONKA. Fiber recruitment affects oxidative recovery measurements of human muscle in vivo. Med. Sci. Sports Exerc., Vol. 34, No. 11, pp. 1733–1737, 2002.
Fast-twitch and slow-twitch muscle fibers are known to have distinct metabolic properties. However, it has not been clearly established whether such heterogeneity within mixed-fiber muscles can influence measurements of energy metabolism in vivo. We therefore tested the hypothesis that differences in muscle fiber recruitment can cause differences in whole-muscle oxidative recovery from exercise.
We used 31P magnetic resonance spectroscopy to measure oxidative ATP synthesis in the ankle dorsiflexor muscles of eight healthy volunteers under a variety of recruitment conditions. Oxidative ATP synthesis after isometric exercise was quantified as the rate constant k PCr, the reciprocal of the time constant of PCr recovery.
k PCr was 37% higher after low-force ramp contractions (which primarily recruit slow-twitch fibers) than after ballistic contractions to the same peak force (which recruit both fast- and slow-twitch fibers). k PCr was also 24% higher after low-force ramp contractions than after high-force ramp contractions, presumably reflecting the recruitment of fast-twitch fibers at high forces.
Our results indicate that the muscle fibers recruited first in voluntary contractions have a higher oxidative capacity than those recruited last. Such metabolic differences among fibers can confound whole-muscle measurements and thus need to be taken into account when studying voluntary exercise.
Departments of Physiology & Biophysics and Radiology, University of Washington, Seattle, WA
Submitted for publication December 2001.
Accepted for publication July 2002.
Address for correspondence: Gregory J. Crowther, University of Puget Sound, Department of Biology, 1500 N. Warner Street #1088, Tacoma, WA 98416; E-mail: email@example.com.
We thank S. A. Jubrias and E. G. Shankland for technical assistance and K. E. Conley, M. J. Kushmerick, M. J. Lambeth, D. J. Marcinek, and an anonymous reviewer for useful advice.
This work was supported in part by NIH grants AR42928 and AR45184.