Shoulder Biomechanics, Part I: The Subscapularis Muscle

 

Hello Friends,

This is the first of a four-part series on the shoulder joint, focusing specifically on the rotator cuff and its biomechanical relationship with the deltoid muscle. Let's begin by looking at the muscles that comprise the rotator cuff, starting with the subscapularis. As figure 1 illustrates, the subscapularis occupies the space, or fossa, at the front of the scapula. From there it attaches to the lesser tuberosity, a knob-like structure on the humerus bone at the front of the shoulder. Concentrically contracting the subscapularis muscle (shortening the muscle on contraction) internally rotates the shoulder. The subscap also acts, in conjunction with the infraspinatus muscle, as a stabilizer of the humeral head in the socket (glenoid). We test strength and function of this muscle with the "belly press" test" 

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or the "bear hug" test. 

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Tightness in the subscapularis can limit external rotation of the shoulder

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Figure 1: The subscapularis muscle, illustrating the origin on the inside of the scapula and the insertion on the lesser tuberosity of the humerus.

Figure 2 illustrates one of the poses that stretch the subscapularis muscle, namely, Gomukhasana. The upper side humerus externally rotates in this pose, thus stretching the muscle as shown.

Figure 2: This illustrates the effect on the subscapularis muscle of the upper arm in Gomukhasana. External rotation of the humerus stretches the muscle.

Figure 3 illustrates engaging the subscapularis muscle in Ardha Baddha Padma paschimottanasana. Advanced practitioners can attempt to lift the hand off the back to engage the muscle in this pose. This also replicates the "lift off" test

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, which is used in orthopedics to test the function of the subscap muscle.

Figure 3: This image illustrates contraction of the subscapularis muscle to internally rotate the humerus.

Finally, we have the subscapularis as a stabilizer during a static position in a pose. In Warrior II, attempt to internally rotate the shoulders by imagining pressing the mound at the base of the index fingers down against an object. Resist this by externally rotating the shoulders at the same time. Co-contracting opposing muscles--like the subscap and infraspinatus--stabilizes the head of the humerus in the socket while the deltoid contracts to abduct the humerus. Click here to go into a bit more depth on the subject of stabilizing your shoulders in your Downward dog pose. 

Figure 4: Co-contracting the subscapularis and the infraspinatus stabilizes the humeral head in the socket while the deltoid muscle abducts the humerus.

Thanks for stopping by. Stay tuned for the next post when I'll go over the antagonist muscle for the subscapularis. By the end of this four-post series, you'll have a good understanding of the functional anatomy and biomechanics of the shoulder joint as applied to yoga. Click here to browse through the Bandha Yoga book series on anatomy, biomechanics and physiology for yoga.