GLOSSARY

The infraspinatus is a thick, triangular muscle in the infraspinous fossa of the scapula and one of the rotator cuff muscles; it is the main external rotator of the shoulder.

Attachments and Innervation

• Origin: the infraspinous fossa of the scapula.
• Insertion: the middle facet of the greater tuberosity of the humerus.
• Nerve: the suprascapular nerve (C5 and C6).

Actions

• External rotation of the shoulder: the strongest external rotator, together with the teres minor.
• Posterior stability of the shoulder, preventing posterior displacement of the humeral head.
• Compressing the humeral head into the glenoid.

Relevance to Sport

• Decelerating the throw: in the follow-through phase of throwing, the arm rotates inwards at very high speed, and the infraspinatus slows it with an eccentric contraction.
• Infraspinatus atrophy in volleyball players: compression of the suprascapular nerve at the spinoglenoid notch, especially with the float serve, causes isolated and often painless atrophy of the infraspinatus in the hitting arm.
• Exercises: external rotation with a band or a dumbbell in side-lying, and eccentric external rotator training for throwers.

Clinical Application

The resisted external rotation test with the elbow at the side assesses infraspinatus strength; marked weakness may indicate a massive rotator cuff tear or suprascapular nerve injury.