Investigating motor unit firing rates during arm cycling compared with intensity-matched isometric contractions in humans.

Studies in humans have not assessed motor unit firing rates (MUFRs) recorded from intramuscular electromyography (EMG) during a rhythmic locomotor output. Using arm cycling as a model of locomotor-generated activity, the purpose was to determine whether MUFRs differed during arm cycling compared with intensity-matched isometric contractions. We hypothesized that MUFRs would be greater during arm cycling than isometric contractions, assessed at various working intensities. Young males (n = 10) and females (n = 4) completed arm cycling bouts and isometric contractions of the elbow flexors. Indwelling fine-wire electrodes were inserted into the biceps brachii to record MUFRs during arm cycling with combinations of two power outputs (25 W and 50 W) and cadences (30 rpm and 60 rpm), and subsequently compared with intensity-matched isometric contractions. Motor unit recordings were analyzed during the flexion phase of arm cycling when biceps brachii EMG activity was high, and with the forearms in a neutral grip position. Results indicated that MUFRs were significantly higher during arm cycling compared with isometric contractions (P = 0.003), and MUFRs increased with greater cycling intensity (P < 0.001). Higher MUFRs demonstrated during arm cycling were likely influenced by greater descending drive and/or enhanced spinal motoneuron excitability, facilitated through central pattern generator (CPG)-mediated changes to intrinsic motoneuron properties. Thus, different neural control strategies are used during rhythmic locomotor output compared with isometric contractions in humans.NEW & NOTEWORTHY Motor unit firing rates (MUFRs) during an upper-limb locomotor task have yet to be recorded in humans, and behavior of the active motor units during dynamic central pattern generator (CPG)-mediated locomotor activity is currently unknown. The present study used indwelling fine-wire electrodes to record MUFRs during arm cycling and compared the firing rates to intensity-matched isometric contractions. Results demonstrated that MUFRs were significantly higher during arm cycling, indicating different neural control strategies between locomotor outputs and isometric contractions.