Motor control of low-threshold motor units in the human trapezius muscle

The firing pattern of low-threshold motor units was examined in the human t rapezius and first dorsal interosseous (FDI) muscles during slowly augmenti ng, low-amplitude contractions that were intended to mimic contractile acti vity in postural muscles. The motor unit activity was detected with a speci al needle electrode and was analyzed with the assistance of computer algori thms. The surface electromyographic (EMG) signal was recorded. Its root-mea n-square (RMS) value was calculated and presented to the subject who used i t to regulate the muscle force level. In the trapezius, there was minimal, if any, firing rate modulation of early recruited motor units during slow c ontractions (less than or equal to1% EMG(max)/s), and later recruited motor units consistently presented higher peak firing rates. As the force rate o f the contraction increased (3% EMG(max)/s), the firing rates of the motor units in the trapezius approached an orderly hierarchical pattern with the earliest recruited motor units having the greatest firing rate. In contrast , and as reported previously, the firing rates of all motor units in the FD I always presented the previously reported hierarchical "onion-skin" patter n. We conclude that the low-threshold motor units in the postural trapezius muscle, that is the motor units that are most often called on to activate the muscle in postural activities, have different control features in slow and fast contractions. More detailed analysis revealed that, in the low for ce-rate contractions of the trapezius, recruitment of new motor units inhib ited the firing rate of active motor units, providing an explanation for th e depressed firing rate of the low-threshold motor units. We speculate that Renshaw cell inhibition contributes to the observed deviation of the low-t hreshold motor units from the hierarchical onion-skin pattern.