Activation of fast-twitch fibers assessed with twitch potentiation.
BACKGROUND
The augmentation of twitch response following brief muscle activation, called twitch potentiation, has been shown to be much more pronounced in fast-twitch than in slow-twitch fibers. We thus explored the possibility of twitch potentiation as a noninvasive measure of fast-twitch fiber activation, by studying its dependence on the intensity of preceding contraction.


METHODS
Twitch contraction of plantar flexor muscles was evoked with supramaximal stimulation of the posterior tibial nerve, before and immediately after 6-s voluntary contractions at intensities of 10-100% of maximal voluntary contraction (MVC).


RESULTS
Except for low-intensity contractions (< 30%MVC), voluntary contraction induced twitch potentiation, the magnitude of which increased with increasing contraction intensity (P < 0.001). The shortened contractile process was associated with the potentiation.


CONCLUSIONS
These results are consistent with the concept of "hierarchical order of fiber activation", suggesting that the magnitude of twitch potentiation reflects the activation of fast-twitch fibers during a brief contraction.
Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Tokyo 153-8902, Japan. sasaki@idaten.c.u-tokyo.ac.jp
Kazushige
K
Sasaki
Yukie
Y
Tomioka
Naokata
N
Ishii
D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D015203:Reproducibility of Results; D019415:Torque