Raising the arm
to the side (shoulder-joint abduction) during the shoulder overhead
press - as in the behind neck press - involves the middle deltoid and
supraspinatus, the latter a relatively small rotator cuff muscle. The
main action of the supraspinatus is to raise the arm from alongside the
body to the level and overhead position, then to pull the humerus (upper
arm bone) up into the shoulder joint. If this muscle is weak, the
humerus may become depressed, especially if the pull of the latissimus
dorsi - which pulls the arm down - is very strong. When the humerus
isn't fully rotated in the shoulder joint, it can create problems when
raising the arm.
As the
supraspinatus and deltoid contract to raise the arm, the scapula rotates
upward via contraction of the upper and lower portions of the trapezius
and the serratus anterior. The serratus anterior pulls the lower outer
border of the scapula forward (sliding it around the ribcage and out to
the sides). The upper trapezius pulls the top of the scapula inward; the
lower trapezius pulls the inner border of the scapula downward. As a
result of these three forces, the scapula rotates around an axis through
its center. Synchronization of these muscles in rotating the scapula,
together with synchronized action of the deltoid and supraspinatus in
raising the arm, is critical for safe, full range shoulder overhead
press movements.
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