Using spike-triggered averaging to characterize motor unit twitch vectors in the first dorsal interosseous

Earlier studies in multifunctional muscles such as the first dorsal interosseous (FDI) have demonstrated that the selection and control of motor units (MUs) can vary as a function of generated force direction. While directionally dependent motor unit recruitment and rate properties imply that there may also be differential mechanical action, this has yet to be directly demonstrated. Our objective was to determine whether there exists a range of force vectors from different motor units in the FDI muscle within individual subjects. We utilized the spike-triggered averaging (STA) method to derive force twitch estimates from single motor units. We derived MU twitch direction from the ratio of individual twitch estimates recorded concurrently from the load cell. Fifteen units from 2 subjects were used to determine MU force vectors. We were able to estimate force twitch vectors from 7-8 different MUs in each subject. The results of our study suggest that there is varied mechanical action of motor units in the FDI. It is thus possible that differential activation of individual MUs in the FDI is a function of varied mechanical action.

I. Introduction

The selection and control of motor units (MUs) for a given motor task is assumed to depend primarily on the level of muscle force generation required for the task. Thus during isometric force production, motor units are believed to be recruited on a graded basis, generally according to motor unit size [1], which also corresponds to the magnitude of force output. However, there exist muscles that act to generate multi-directional forces across a joint. The system governing motor unit activation in such muscles remains incompletely understood.