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∘ and 90∘ 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∘ 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.