Muscle activation patterns of the supraspinatus and deltoid muscle in abduction of the arm: Motion generator or GH (de)stabilizator?

It is generally alleged that mobility in the glenohumeral joint is generated by muscles with relative large force moment arms, while stability in the glenohumeral joint is predominantly guarantied for by muscles close to the joint with relatively small moment arms: the rotator cuff muscles. Consequently, there is a lack of glenohumeral stability in patients with rotator cuff tears, with subsequent relative cranial translation of the humerus and pain. For abduction of the arm, the deltoid would be the main moment generator, with the rotator cuff muscles providing glenohumeral stability. Nevertheless, these beliefs have never been assessed by biomechanical analyses.
Arm motion (e.g. arm muscle forces and moments) and scapular position directly correlate, but it is not clear whether scapulohumoral rhythm has an essential and active role in facilitating arm motions, or if it is simply the result of muscle activations and intended arm motions. The cooperation of muscle activation patterns and scapular position needs to be studied, because distorted scapulohumoral or scapulo-thoracal rhythm is related to subacromial pathologies.

We hypothesize that, with increased arm abduction moment loading with a constant exerted abduction force, primary arm moment generators (deltoid muscle) will have a greater increase in activation (EMG) as compared to primary arm/glenohumeral stabilizators (supraspinate muscle).

Intake and measurements at 1 timepoint.

Subjects will be seated, having their dominant arm fixed in a splint. Next, subjects will have to exert isometric forces in directions perpendicular to the long axis of the humerus, using visual feedback.

In a second session, the same approach will be applied, with the same exerted arm force. However, in the second session the exerted force will be applied at a point nearer to th GH-joint.

In this manner, subjects will participate in 2 sessions of measurements using the same exerted force. However, subjects will be subjected to 2 separate amounts of arm moment loading.

During the isometric force tasks, EMG will be recorded using surface electrodes.