Isolated Activation of the Infraspinatus Muscle in Four Manual Muscle Testing Positions

Abstract

The most common way to generate a reference level for muscle activation to normalize infraspinatus electromyography (EMG) data is to record the maximum voluntary isometric contraction (MVIC) to identify the effects of exercise or intervention on infraspinatus muscle strengthening1–3. Unlike other normalization methods, including subMVIC and reference voluntary contraction (RVC), MVIC is advantageous because it has physiological meaning; derived data are expressed relative to the maximum4. For infraspinatus normalization, clinicians or researchers commonly take measurements while the subject is in the prone position5–8. However, the prone position can lead to over-activation of the posterior deltoid, and sub-optimal activation of the infraspinatus contributes to an increase in anterior humeral gliding in the glenoid9,10. From this point of view, increased translation of the instantaneous center of rotation of the humeral head may be associated with muscle imbalance between the rotator cuff (e.g., infraspinatus) and the scapulohumeral (e.g., posterior deltoid and middle trapezius) muscles8. Recent shoulder external rotation exercises focus on decreasing the contribution of the posterior deltoid to Isolated Activation of the Infraspinatus Muscle in Four Manual Muscle Testing Positions