Velocity Dependent Adaptations In Motor Unit Firing Rates During Bicep Curls

Abstract

Alterations in muscle control during changing contraction velocity are typically inferred from gross amplitude and timing analysis of the surface electromyographic (sEMG) signal [1]. However, more precise alterations can be directly measured from the firings of individual motor units (MUs) - basic units of neuromuscular control that comprise the sEMG signal. PURPOSE: Investigate velocity dependent adaptations in MU firing behavior in the Biceps Brachii during bicep curls using a non-invasive MU action potential recognition technology [2]. Adaptations were investigated separately for the concentric and eccentric phases of the exercise as they are known to present distinct muscle activation patterns [3]. METHODS: 5 healthy participants (3 males, 2 females, 23-32 y.o.) performed bicep curls from full extension to 90̊ flexion with their dominant arm at velocities of 30 and 180 deg/sec while holding a 5 lbs dumbbell. We collected elbow joint angle (Biometrics Ltd.) and sEMG signals from the Biceps Brachii (Galileo sensor, Delsys Inc.). EMG data were sampled at 2000 Hz, filtered between 20-450 Hz, and decomposed into the constituent MU firings. Time-varying MU firing rates were obtained by convolving firing times with a 0.3 sec Hanning window. RESULTS: We observed higher average firing rates in the concentric phase (12.9 ± 2.4 and 16.2 ± 3.5 in pulses per second, or pps, for the slower and faster velocity, respectively) compared to the eccentric phase (12.0 ± 2.3 and 11.9 ± 2.1 pps), in agreement with previously published data [4]. Moreover, with increasing velocity MU firing rates increased consistently during the concentric phase but not during the eccentric phase of the curl. CONCLUSION: Phase specific differences in MU firing rates during bicep curls are magnified when contraction velocity is increased. These data provide novel insights on the neural mechanisms of muscle contraction at varying velocity that have the potential to inform training and rehabilitation. 1) Roberts & Gabaldón. Integr Comp Biol, 2008. 2) De Luca et al. J Neurophysiol, 2015. 3) Hody et al. Front Physiol, 2019. 4) Del Valle, Thomas C. Muscle Nerve, 2005. Support: Funding was provided in part by NINDS (R44HD094626, R44NS077526) and De Luca Foundation (MA).