Changes in muscle coordination during functional shoulder movement in breast cancer survivors

INTRODUCTION: Persistent pain located in the ventral and dorsal region around the shoulder is a common clinical issue among breast cancer survivors, which affects motor function of the shoulder girdle (1). Thus, studying the recruitment of muscles by means of surface electromyography (SEMG) can contribute to the understanding of the changes in muscle coordination occurring in breast cancer survivors suffering from persistent pain. In this case-control study, we investigated the changes in the coupling of muscle pairs during functional shoulder movement. METHODS: Eleven cases and 11 matched controls took part in the study. The functional shoulder movements consisted of dynamic arm flexion (0-90o), abduction (0-90o) and a shoulder shrug composed of 2-s concentric, 2-s isometric and 2-s eccentric contractions repeated five times. The SEMG of ventral muscles, i.e., pectoralis major (PM), deltoideus anterior (DA), deltoideus medialis (DM) and dorsal muscles, i.e., upper trapezius (UT), middle trapezius (MT) and latissimus dorsi (LD) were collected. Normalized mutual information (NMI), a measure of functional connectivity between muscle pairs (2) was computed over 0.5-s epochs during the three investigated dynamic shoulder movement after detecting onset and offset of the five repetitions used to define concentric (1st tertile), isometric (2nd tertile) and eccentric (3rd tertile). A three-way analysis of variance was applied on averaged NMI values with group (case/control), contraction type (concentric/isometric/eccentric) and muscle pairs of the body region as independent factors. RESULTS: The NMI values were between 36-93% lower for cases compared with controls in the dorsal region for all movements and 32% higher in the ventral region during shoulder shrug (P ≤ 0.05). The NMI values were also 44-116% higher for eccentric compared with isometric or concentric contraction (P ≤ 0.05). Finally, the NMI values were higher for DM-DA/DM-PM and LD-MT compared with the other muscle pairs in the ventral and dorsal region, respectively (P ≤ 0.05). CONCLUSION: The present findings especially revealed enhanced coupling in the dorsal region but also reduced coupling between muscle pairs in breast cancer survivors with persistent pain during functional shoulder movement. This suggests the presence of altered coupling and decoupling strategies in muscle pairs (3). Moreover, computing NMI during dynamic contractions can be helpful to understand muscle pairs coordination during isometric, concentric and eccentric contractions.