Does the amplitude of biceps brachii M waves increase similarly in both limbs during staircase, electrically elicited contractions?

Objective: Humans usually tend to control more finely muscle force production in dominant than non-dominant upper limbs. It is well established that motor unit recruitment is a key mechanism by which muscle force is controlled, and we hypothesized that a relatively smaller number of motor units may be recruited in muscles of dominant than non-dominant limbs for any given increase in synaptic input. Hence, we investigated peripheral properties of dominant and non-dominant biceps brachii through the analysis of M-wave responses to incremental electrical stimulation. Approach: Current pulses at progressively greater intensities were applied in the proximal region of biceps brachii of 16 subjects while surface electromyograms were recorded with a grid of electrodes in the distal region. M-wave amplitude was averaged across channels and normalized with respect to the maximum amplitude value, separately for each stimulation intensity and limb. Amplitude-current intensity curves were interpolated to provide an equal number of stimulation levels between limbs. Differences between dominant and non-dominant arms were assessed through the average increase in M-wave amplitude for consecutive stimulation intensities (increments). Main results: Wilcoxon's signed-rank test showed that increments in the M-wave amplitude were significantly smaller (p = 0.017) in dominant than non-dominant biceps brachii. Significance: The results suggest that there was a more gradual recruitment of motor units in biceps brachii of dominant than non-dominant arms. This is in agreement with the hypothesis of a broader spectrum of motor unit recruitment thresholds in the dominant arm, which may contribute to a finer regulation of force production.