The influence of shoulder position during multi-joint exercises in the relative scapular muscles activity in symptomatic and asymptomatic conditions

BACKGROUND: Scapular muscles changes, as increased upper trapezius activity and decreased middle and lower trapezius and serratus anterior muscle activity, have been demonstrated in shoulder pain specific or non-specific conditions. Shoulder external rotation exercises have been recommended to improve scapular activity in shoulder pain. OBJECTIVE: To evaluate the relative scapular muscles activity during multi-joint exercises combining shoulder external rotation, trunk rotation and scapular squeeze. METHODS: Forty-one participants with and without shoulder pain were assessed in a cross-sectional study. They performed isometric multi-joint exercises at 0 degrees and 90 degrees of shoulder abduction with and without support. The relative activity of upper, middle, and lower trapezius and serratus anterior (upper/middle and lower portions) was measured through electromyography. The scapular muscular balance was assessed by the ratio between relative activity of the upper trapezius and the other muscles. RESULTS: Both groups presented similar results. The exercise at 90 degrees abduction led to increased relative muscle activity against maximal voluntary contraction in both groups for upper trapezius (with support: 4% MVIC, p = 0.001 or 15% MVIC, p < 0.0001; and without support: 11% MVIC, p < 0.0001 or 13%, p < 0.0001, for asymptomatic and symptomatic group, respectively) and lower trapezius (with support: 66% MVIC, p < 0.0001 or 62% MVIC, p < 0.0001, for asymptomatic and symptomatic group, respectively), for middle trapezius in the symptomatic group (with: 50% MVIC, p = 0.01; and without support: 35% MVIC, p = 0.014) and for lower trapezius (55% MVIC, p = 0.03) and serratus anterior lower portion (41% MVIC, p = 0.037) in the asymptomatic group (without support). In general, this position led to increased scapular muscle activity against upper trapezius activity. CONCLUSION: Multi-joint exercises performed at 90 degrees abduction present higher relative scapular muscle activity against MVIC but could be less suitable for common scapular muscle imbalance given the associated increased muscular ratio of scapular muscle activity against upper trapezius activity.