Electromyographic Analysis of a Selective Exercise for the Serratus Anterior Muscle Among Patients With Frozen Shoulder and Subacromial Impingement Syndrome

Objective Patients with shoulder disorders show altered periscapular muscle activity (e.g., decreased serratus anterior and increased upper trapezius activities). We herein devised a novel method for strengthening serratus anterior without excessive upper trapezius activation, named squeezing ball exercise in which patients squeezed a ball between both elbows with maximum voluntary isometric contraction in the horizontal adduction direction with the arms elevated at a 45-degree angle. The present study aimed to investigate whether the squeezing ball exercise could produce high muscle activity in the serratus anterior in patients with frozen shoulder and subacromial impingement syndrome before the rehabilitation intervention. Design This is a proof-of-concept study. Serratus anterior and upper trapezius activities during squeezing ball exercise were evaluated using electromyography in 16 patients with frozen shoulders and subacromial impingement syndrome. Electromyography signals were normalized using maximal voluntary isometric contraction, and the muscle balance ratios (upper trapezius/serratus anterior) were calculated. Results The average serratus anterior and upper trapezius activity was 69.9% ± 30.8% and 10.2% ± 6.3% maximal voluntary isometric contraction during the squeezing ball exercise, respectively, whereas the upper trapezius–serratus anterior ratio of the affected side was 0.15 ± 0.07. Conclusions The high serratus anterior activation and low upper trapezius–serratus anterior ratio during squeezing ball exercise could be attributed to the limb position where shortened serratus anterior exerts itself without painful subacromial impingement. Squeezing ball exercise could be a novel rehabilitation tool for patients with frozen shoulders and subacromial impingement syndrome.