Summary

A study investigating neural drive and co-activation of the deltoid muscle segments in healthy individuals at the motor unit level.

Abstract

Introduction

Outcomes following total reverse shoulder replacements vary. Poor recovery is associated with inefficient shoulder control including co-activation of deltoid portions during tasks in which it is not required. However: outcomes from studies that examine deltoid muscle control in patients using classical surface EMG are limited to the muscle-level. New works provide evidence that complex movements are controlled at the level of individual or grouped motor units, not at the muscle-level; and the level of common drive to motor units varies between muscles, activities, and people. As such, it is reasonable to hypothesise that the level of common drive to motor units across the deltoid may differ between muscle-heads, activities, and people. The goal of our work is to understand how the central nervous system controls shoulder muscles in health and disease, and its impact on recovery from surgery. The aim of this study was to determine the organisation of neural drive to the deltoid muscle at the single motor unit level in healthy individuals.

Methods

Three high-density EMG grids were used to record activity of the deltoid portions during force-matched tasks with different mechanical constraints. Participants [n=11, 1 female] were seated, with their shoulder positioned in 45-degrees abduction. Participants produced isometric force for abduction (A), abduction with flexion (AF), and abduction with extension (AE). Participants matched a force target with 4 repetitions of 5s ramp up, 20s hold, 5s ramp down. Signals were decomposed into motor unit spike trains using blind source separation, and manually edited. Common drive within and between deltoid heads was determined using coherence spectral analysis in the 0-5Hz band. Coherence values were Z-transformed; values >1.65 were considered significant. Linear mixed models tested if common drive was affected by force direction, deltoid portion, or pairs of deltoid portions, and their interactions.

Results

Common drive was observed within deltoid portions in the 3 force directions. A significant muscle x force direction (p=0.01) was observed; common drive within the middle deltoid was higher in A than AF (p=0.05), and was lower in the posterior than middle deltoid during A (p=0.03). Between deltoid portions: Differences in common drive were dependent on pairing of deltoid portions (p<0.001) but were not force direction dependent (p=0.45). Most participants exhibited significant common drive between lateral-posterior, and lateral-anterior deltoid portions, compared to low, insignificant common drive between the posterior-anterior deltoid portions.

Discussion

Common drive within the middle deltoid depended on force direction, suggesting task-dependent control. Because the moment arm of posterior deltoid is small or even negative at 45-degree shoulder abduction, lower common drive to the posterior than middle deltoid portion could be interpreted as being efficient. High common drive between lateral-posterior, and lateral-anterior portions suggests a neuromechanical functional unit enabling force production in the AE and AF directions. However, this common drive was independent of force direction, suggesting a relatively fixed common drive to these deltoid portions. Strong common drive within deltoids portions and low common drive between posterior and anterior deltoid portions would allow for flexible control between these portions.