Stability and error measurements in high-density electromyography of the middle deltoid in glenohumeral instability patients. Technical and practical implications of the experimental set-up

Glenohumeral instability is a highly prevalent injury characterized by the early appearance of physiological fatigue, which may affect muscle activation patterns. Understanding the variability of electromyographical activity may improve the comprehension of these changes. Therefore, this study aimed to determine the variability of high-density electromyographical activity in the middle deltoid of patients with glenohumeral instability. For this purpose, this study recruited 58 adults who had suffered at least one episode of shoulder dislocation during the year preceding enrollment. These patients had to perform a protocol of maximal (100% of maximum voluntary contraction (MVC) and submaximal (10%, 30%, 50%, and 70% of the MVC) isometric contractions in lateral abduction. To assess the neural control parameters, a grid of 64 electrodes was placed in the middle deltoid, recording the signal by using high-density electromyography. The results show a great variability in the number of identified motor units, with a progressive decrease across contraction levels, with significantly lower motor units at 50%, 70%, and 100% MVC compared with 10% and 30% MVC (one-way ANOVA, F(4,277) = 18.80; p < 0.001). In addition, the firing rate, the pulse rate, and the recruitment time demonstrated a direct relation to the MVC (r = 0.974, r = 0.990, r = 0.922). Moreover, the silhouette value was highly robust (0.85-0.90). Furthermore, this study suggested potential changes in motor unit recruitment behavior and spatial variability of electromyographical activity across the muscle. This study also proposed an identification of potential error sources and practical solutions to enhance this evaluation. In conclusion, high-density electromyography enabled the characterization of neuromuscular patterns in shoulder instability. However, while the findings support its feasibility for research applications, further research is needed to formally establish its validity.