Decreases in serratus anterior (SA) muscle activity in
patients with shoulder pain and shoulder instability are associated
with abnormal scapular movements1)
A weakened SA induces excessive trapezius (upper fiber) (TU) muscle
activity due to the TU’s compensatory action2, 3)
, leading
to subacromial collisions through abnormal upward rotations
of the scapula1)
Those who have shoulder instability
showed 15% to 20% higher rotator cuff muscle activity
along with 8% to 10% lower deltoid (anterior fiber) (DA)
muscle activity compared with healthy persons4–6)
.