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Activation of fast-twitch fibers assessed with twitch potentiation



Introduction: 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. Muscle Nerve 46: 218–227, 2012