PURPOSE: 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. CONCLUSION: 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.
PURPOSE: 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. CONCLUSION: 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.
Authors: J P J Schmitz; W Groenendaal; B Wessels; R W Wiseman; P A J Hilbers; K Nicolay; J J Prompers; J A L Jeneson; N A W van Riel Journal: Am J Physiol Cell Physiol Date: 2012-10-31 Impact factor: 4.249
Authors: Sharon A Jubrias; Gregory J Crowther; Eric G Shankland; Rodney K Gronka; Kevin E Conley Journal: J Physiol Date: 2003-09-26 Impact factor: 5.182