The possibility of investigating the motor control strategies (i.e., motor unit (MU) recruitment and rate coding) from the myolelectric signal recorded non-invasively has been variously addressed with controversial results. Anyhow, by one consent, it was generally established that both amplitude (e.g., the root mean square (RMS)) and frequency (the median power spectral frequency (MDF)) content of the surface electromyogram (sEMG) reflect central and peripheral modifications of the neuromuscular system. More recently, the refinement of techniques for the estimation of the average muscle fibre conduction velocity (CV) together with the application of new non-linear approaches to sEMG time-series analysis provided further information on muscle dynamic. In the present work, three muscle efforts increasing with different slopes from 0% to 100% of the maximal voluntary contraction (MVC) were administered to the biceps brachii of 9 young healthy subjects. These ramps were followed by a 10 sec hold phase at the maximal contraction. In the present work we show that MDF, CV and RMS time-course were characterized by an increase up to a maximum, reached during the ramp or the hold phase depending on the speed of the ascending phase. After the maximum, these parameters decreased with different slopes. The improvement in terms of CV estimation allowed a more refined evaluation of the processes of fast twitch MU recruitment/derecruitment along the course of the experiment. Moreover, especially during faster ramp experiments, RMS showed a two step exponentially behaviour with a break point consistently positioned within the ramp phase. Concerning the non-linear analysis, %DET provided further important information on MU synchronization.
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