Changes in muscle activity of the shoulder during maximum isometric contraction due to regional muscle weakness

We previously examined the order of the activity of scapular muscles during trials against an external load and during trials of isometric contractions in healthy participants. From these studies, we learned that scapular muscles can be divided into agonists and antagonists. An agonist muscle at the scapulothoracic joint (STJ) generates resistance against an external load just like the agonist at the glenohumeral joint (GHJ). In the present study, we investigated muscle activity during maximum isometric contraction in subjects with muscle weakness at the GHJ or STJ and analyzed the characteristics of the muscle activity. Two patients participated in this study. One patient (subject A) had muscle weakness at the GHJ, and the other patient (subject B) had muscle weakness at the STJ. We used electromyography to measure muscle activity during maximum isometric contractions of shoulder flexion, external rotation, and internal rotation and analyzed the changes in muscle activity as these muscles applied resistance against an external load. Subject A demonstrated lower muscle activity at both the GHJ and STJ than in the healthy shoulder and Subject B. The lower muscle activity observed at the STJ of subject A was possibly due to the absence of the transfer of energy to the STJ by the external load because of muscle weakness at the GHJ. This suggests that the normal STJ function depends on maintaining muscle strength at the GHJ at a constant level in isometric contraction.