Fiber recruitment affects oxidative recovery measurements of human muscle in vivo.
OBJECTIVE
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.


METHODS
We used (31)P 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.


RESULTS
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.


CONCLUSIONS
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.
Crowther
Gregory J
GJ
Department of Physiology, University of Washington, Seattle, USA. gcrowther@ups.edu
Gronka
Rodney K
RK
D000255:Adenosine Triphosphate / Q000378:metabolism*; D000328:Adult; D015331:Cohort Studies; D015444:Exercise / Q000502:physiology*; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction / Q000502:physiology; D018656:Muscle Fibers, Fast-Twitch / Q000201:enzymology*; D018657:Muscle Fibers, Slow-Twitch / Q000201:enzymology*; D018482:Muscle, Skeletal / Q000502:physiology*; D010100:Oxygen / Q000378:metabolism*; D010101:Oxygen Consumption; D011336:Probability; D012680:Sensitivity and Specificity
D000255:Adenosine Triphosphate; D010100:Oxygen