Lifting the Arms, Part 2: Rotate and Elevate the Shoulder Blades for Fuller, Safer Movement
To understand how to teach your students to lift their arms freely, it helps to know some basic shoulder anatomy. The shoulder blade, or scapula, is shaped roughly like a right triangle with its point facing downward, its inner (medial) edge running vertically alongside the spine (vertebral column), and its top edge running horizontally. The medial edge is called the vertebral border of the scapula. The upper-inner corner of the shoulder blade, at the top of the vertebral border, is called the superior angle. The lower tip, at the bottom of the vertebral border, is called the inferior angle. The most prominent feature of the top edge of the shoulder blade is a horizontal ridge of bone that runs along its length. This is the spine of the scapula, and it is palpable just under the skin if you reach one hand across your body to touch the top-rear part of your opposite shoulder. The outer end of this ridge, at the upper-outer corner of the scapula, is called the acromion process. Recessed under the acromion is the glenoid fossa, a slightly concave circle of bone the size of a small coin.
The shoulder blade is capable of several of movements. Abduction (also called protraction) is the movement of the scapula away from the midline of the body and around toward the front. Adduction (retraction) is the movement toward the midline. Elevation is the vertical lifting of the scapula. Depression is the downward push. Anterior tilt is the tipping of the top edge of the scapula forward and the inferior angle backward. Posterior tilt is the tipping the top edge backward and the inferior angle forward. Upward rotation is a more complex scapular movement. The inner edge of the scapula moves down while the outer edge moves up, so, when viewed from the rear, the whole bone turns either clockwise (left scapula) or counterclockwise (right scapula). Upward rotation is crucial to arm elevation. To understand why, let's consider the upper arm bone (humerus), and its relationship to the shoulder blade.
The top end of the humerus has a rounded head that sits under the acromion process of the scapula and abuts the glenoid fossa. The junction between the glenoid and the humeral head is the gleno-humeral joint. This joint allows most of the familiar arm movements at the shoulder, including abduction (reaching the arm out to the side), adduction (moving the arm across the body), flexion (bringing the arm forward), extension (bringing the arm backward), internal rotation (turning the arm in) and external rotation (turning the arm out). However, all of these movements can be enhanced by supportive movements of the scapula, and one arm movement, elevation (lifting the arm overhead), cannot be accomplished at all by movement at the gleno-humeral joint alone. It also requires strong upward rotation of the scapula.
When a student brings her arm from straight down at her side to straight up overhead, she lifts it through an arc of 180 degrees. However, even under the best of circumstances (that is, strong external rotation of the humerus), the gleno-humeral joint only allows about 120 degrees of arm lift. The remaining 60 degrees come from upward rotation of the scapula. Last month's column explained that it is important to turn the humerus outward while lifting the arm upward in order to help prevent pinching one of the rotator cuff tendons (the supraspinatus tendon) between the head of the humerus (below it) and the acromion process (above it). If the arm is not turned outward, it can only lift about 20 to 30 degrees before the bony outer side of the humeral head (called the greater tubercle) jams against the acromion and pinches the supraspinatus tendon. But even with maximal outward rotation of the arm, the greater tubercle starts jamming against the acromion (and pinching the supraspinatus tendon or nearby structures) at about 120 degrees of lift. The only reason the typical student can raise her arm all the way to 180 degrees is that she unconsciously rotates her scapula upward as her humerus tilts up. This tips her acromion up and out of the way of her humeral head so her arm can reach the vertical position without impingement.!--page-->
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