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1 Laboratorio di Ingegneria del Sistema Neuromuscolare (LISiN), Politecnico di Torino, Torino, Italy; Center for Sensory-Motor Interaction, Aalborg University, Aalborg, Denmark
2 Department of Applied Physiology, University of Strathclyde, Glasgow, United Kingdom
* To whom correspondence should be addressed. E-mail: dario.farina{at}polito.it.
Muscle fiber conduction velocity (MFCV) provides indications on motor unit recruitment strategies due to the relation between conduction velocity and fiber diameter. The aim of this study was to investigate MFCV of thigh muscles during cycling at varying power outputs, pedal rates, and external forces. Twelve healthy male participants aged between 19 and 30 years cycled on an electronically braked ergometer at 45, 60, 90, and 120 revolutions per minute (rpm). For each pedal rate, the subjects performed two exercise intensities, one at an external power output corresponding to the previously determined lactate threshold (100% LT) and the other at half of this power output (50% LT). Surface EMG signals were detected during cycling from vastus lateralis and medialis muscles with linear adhesive arrays of 8 electrodes. In both muscles, MFCV was higher at 100% LT compared to 50% LT for all the average pedal rates except 120 rpm (mean ± SE, 4.98 ± 0.19 m/s vs 4.49 ± 0.18 m/s; P<0.001). In all conditions, MFVC increased with increasing instantaneous knee angular speed (from 4.14 ± 0.16 m/s to 5.08 ± 0.13 m/s in the range of instantaneous angular speeds investigated; P<0.001). When comparing MFCV at the same external force production (i.e., 90 rpm/100% LT vs 45 rpm/50% LT and 120 rpm/100% LT vs 60 rpm/50% LT), MFCV was higher at the faster pedal rate (5.02 ± 0.17 m/s vs 4.64 ± 0.12 m/s and 4.92 ± 0.19 m/s vs 4.49 ± 0.11 m/s, respectively; P < 0.05), due to the increase in inertial power required to accelerate the limbs. It was concluded that, during repetitive dynamic movements, MFCV increases with the external force developed, instantaneous knee angular speed, and average pedal rate, indicating progressive recruitment of large, high conduction velocity motor units with increasing muscle force.
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