Respiratory and stress-induced activation of low-threshold motor units in the human trapezius muscle.

The study aimed to characterize trapezius motor unit firing pattern in low-amplitude contractions, with emphasis on respiratory modulated activity. Constant-amplitude contractions with shoulder elevation, controlled by feedback of the root mean square detected surface electromyographic (SEMG) signal, typing with arm movement and tasks with mental stress were performed. Single motor unit activity was recorded by a quadrifilar fine-wire electrode. A surface electrode simultaneously recorded SEMG activity. Contraction amplitudes ranged from 1 to 10% of the SEMG signal at maximum voluntary contraction (1-10% EMG(max)). The majority ( approximately 80%) of motor units recorded during constant-amplitude contractions showed firing rate modulation at the respiratory frequency. Respiratory firing rate modulation was clear for low amplitude contractions (< 3% EMG(max)), but was reduced at higher amplitudes (3-5.9% EMG(max)). Most motor units had peak firing rate at the transition from inspiration to expiration, but peak firing rate at the transition from expiration to inspiration or at the first harmonic frequency was also observed. The SEMG signal showed little or no respiratory modulation, possibly because respiratory phase varied between motor units. Respiratory modulation of firing rates was significantly reduced in experiments with mental stress and was rarely observed in typing experiments. Both central respiratory drive and peripheral afferent input may contribute to respiratory modulation of firing rates; however, animal studies indicate a central source of the respiratory modulated input. We speculate that the reduction in respiratory modulation of motor activity with mental stress is due to activation of alternative pathways providing excitatory input to trapezius motoneurons.