The amplitude of Mmax in human wrist flexors varies during different muscle contractions despite constant posture

The amplitude of the maximal direct motor response (Mmax) elicited by supramaximal peripheral nerve stimulation can vary with time and with changes in muscle length. We sought to investigate the variability in the amplitude of Mmax in the human wrist flexors (flexor carpi radialis, FCR) at a constant joint position during different functional tasks. The subjects performed isometric wrist extension, radial deviation and gripping contractions matching either 10% or 50% of the EMG activity recorded in the extensor carpi radialis brevis (ECRB) during a wrist extension maximal voluntary contraction (MVC). Three supramaximal stimuli were delivered to the median nerve near the elbow during each task with 2-3 s between stimuli. Considerable variation was observed in the Mmax amplitude between the six tasks for individual subjects (coefficient of variation (CV) range, 6.0-31.4%). There was significantly greater variability between tasks at 50% MVC than at 10% MVC (p = 0.017). However, there were no systematic differences in Mmax amplitude between the six tasks across the group (p > 0.05). These results suggest that the amplitude of Mmax cannot be assumed as constant during experiments involving voluntary contractions even when subjects maintain the same posture.