Signal in Human Motor Unsteadiness: Determining the Action and Activity of Muscles.

Abstract

When the human skeleton is moved by muscles, the resulting movement is inherently unsteady. This work introduces two new approaches for using motor unsteadiness as a window into central nervous system function.

The first approach, termed "Force Covariance Mapping" (FCM), demonstrates experimentally that there can be systematic differences in how forces exerted by limbs fluctuate depending on the direction of intended movement. Forces exerted in different directions by the human index finger were measured using a sensitive load cell. In certain directions of intended movement, forces were found to fluctuate in magnitude only, while in other directions of intended movement, forces were found to fluctuate in both direction and magnitude. Along with electromyographic (EMG) recordings and biomechanical estimates, force fluctuation data indicates that the central nervous system uses different muscular control strategies for different directions of intended movement: some movement directions are generated primarily by single muscles while others involve cooperation among multiple muscles.

The second approach, termed "EMG-weighted averaging" (EWA), couples measures of electrical activity with concurrent motor unsteadiness to estimate the direction of mechanical contribution (action) for a muscle of interest. EWA tracks how exerted forces fluctuate after EMG in a particular muscle increases. This approach allows the exploration of complex neuromechanical phenomena "in vivo". EWA was applied to forces exerted isometrically by the human index finger and EMG data from two muscles: the first dorsal interosseous (FDI) and extensor indicis proprius (EIP) muscles. EWA estimates for the action direction of these muscles were found to change depending on the intended movement direction. These changes could relate to several hypotheses of muscle action, including differential control of motor units within a muscle as well as nonlinear summation of force among muscles.

In addition, this work presents novel predictive equations describing spike-triggered averaging, a commonly-used neuroscience tool for understanding motor unit function that works by coupling motor unit electrical discharges with motor unsteadiness.

Studying human motor unsteadiness, in the detail presented in this work, holds great promise for increasing our understanding human motor function and pathology.