Issue link: https://www.massageandbodyworkdigital.com/i/943527
Because the coracobrachialis medially rotates a laterally rotated arm back to anatomic position, and laterally rotates a medially rotated arm back to anatomic position, it can be called a de-rotator. It eliminates the position of arm rotation regardless of whether the arm is laterally or medially rotated. So, is the coracobrachialis a medial rotator, a lateral rotator, or not a rotator at all? The answer is yes, yes, and yes, depending on the position the humerus is in when the motion starts. This is an important concept to understand, because our clients do not always begin motions from anatomic position. So, when we try to understand if the coracobrachialis of the client is being used, and perhaps overused and injured during a motion of their body, we have to consider the fact that the coracobrachialis might have engaged for medial rotation or lateral rotation, even though our textbook might not state that it can rotate the arm. And certainly, this concept can be widened to all muscles that can change their joint action when there is a change in the posture of the body. So, let's look at a few more examples. Anterior view of the right coracobrachialis. 1A: In anatomic position, the coracobrachialis has no rotation capability at the glenohumeral joint. 1B: In a position of lateral rotation of the arm, the coracobrachialis can medially rotate the arm back to anatomic position. 1C: In a position of medial rotation of the arm, the coracobrachialis can laterally rotate the arm back to anatomic position. Illustration by Giovanni Rimasti. Permission Joseph E. Muscolino. 1A 1B 1C CORACOBRACHIALIS—A DE-ROTATOR The coracobrachialis attaches from the coracoid process of the scapula to the medial side of the shaft of the humerus (Image 1A). Because it crosses the glenohumeral (GH) joint anteriorly and medially, it flexes and adducts the arm at the GH joint. But because it attaches on the medial side of the humeral shaft, it does not have the ability to rotate the humerus at the GH joint. Or at least, it does not have the ability to create rotation when the body is in anatomic position, which is the position we usually use to state a muscle's joint action(s). However, if the humerus is first laterally rotated, then the coracobrachialis wraps around the shaft of the humerus, and its humeral attachment ends up instead being located more anterolaterally relative to the trunk. If the coracobrachialis contracts and shortens from this position, it will pull on the humerus, rotating it medially back to anatomic position (Image 1B). So, the coracobrachialis is a medial rotator of the arm at the GH joint, but only if the arm is first in a position of lateral rotation. If, instead, the humerus is first medially rotated relative to anatomic position, then the coracobrachialis has to wrap around the shaft of the humerus in the opposite direction, and its humeral attachment ends up being located more posterolaterally relative to the trunk. If the coracobrachialis contracts and shortens from this position, it will pull on the humerus, again rotating it back to anatomic position. But this motion is lateral rotation of the humerus at the GH joint (Image 1C). So, the coracobrachialis can also be a lateral rotator of the arm at the GH joint, but only if the arm is first in a position of medial rotation.