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1 Department of Clinical Neurophysiology, Institute of Neurology, University Medical Centre, Nijmegen, The Netherlands
* To whom correspondence should be addressed. E-mail: C.Houtman{at}neuro.umcn.nl.
To obtain more insight into the changes in mean muscle fiber conduction velocity (MFCV) during sustained isometric exercise at relatively low contraction levels, we performed an in-depth study of the human tibialis anterior (TA) muscle using multi channel surface electromyography (sEMG). The results show an increase in MFCV after an initial decrease of MFCV at 30% or 40% MVC in all of the five subjects studied. With a peak velocity (PV) analysis we calculated the distribution of conduction velocities of action potentials in the bipolar EMG signal. It shows two populations of PVs occurring simultaneously halfway through the exercise. The MFCV pattern implies the recruitment of two different populations of motor units. Due to the lowering of MFCV of the first activated population of motor units, the newly recruited second population of motor units becomes visible. It is most likely that the MFCV pattern can be ascribed to the fatiguing of already recruited predominantly type I motor units, followed by the recruitment of fresh, predominantly type II, motor units.
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