Electromyographic activation patterns in CrossFit® athletes with unilateral subacromial shoulder pain: A cluster analysis approach

Abstract

Background

Subacromial shoulder pain (SASP) is a common condition affecting CrossFit® athletes during repetitive overhead movements. This study aimed to characterize within-subject muscle activation patterns of key scapular and shoulder muscles during overhead pressing in athletes with unilateral SASP, and to explore whether distinct activation strategies are influenced by the presence of symptoms.

Methods

A total of 20 male CrossFit® athletes (40 shoulders: 20 symptomatic and 20 asymptoamtic) were analyzed. Surface electromyography (sEMG) activity was recorded during concentric, isometric, and eccentric phases of a standardized shoulder press movement task using an 8-kg kettlebell. A repeated-measures ANOVA was performed to assess interaction effects. Additionally, hierarchical and k-means clustering were used to classify neuromuscular activation patterns.

Results

The ANOVA revealed significant interaction effects for lower trapezius (F = 36.71, p < 0.001, η²p = 0.491) and serratus anterior (F = 11.61, p < 0.001, η²p = 0.234), indicating differential activation across conditions and phases. Post-hoc comparisons revealed that symptomatic shoulders exhibited significantly lower activation in the isometric phase of the anterior serratus (mean difference = −10.68, p = 0.005) and greater activation in the concentric phase of the lower trapezius (mean difference = 13.94, p < 0.001). Cluster analysis identified two distinct activation profiles. However, no significant association between cluster membership and symptomatic condition (χ² = 0.114, p = 0.736).

Conclusion

CrossFit® athletes exhibit two distinct neuromuscular activation patterns, but these clusters were not conditioned by the presence of unilateral SASP. However, symptomatic shoulders showed specific neuromuscular alterations, particularly in the lower trapezius and serratus anterior during the concentric and isometric phases respectively.