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December, 2011, Vol. 11, Issue 12

The Infraspinatus

By Ben Benjamin, PhD

Question: Which assessment test helps you differentiate an injury to the infraspinatus muscle-tendon unit from an injury to the rotator cuff?

Answer: Resisted Lateral Rotation

Resisted lateral rotation is the definitive test for verifying an injury to the infraspinatus muscle-tendon unit. For more refined assessment, and even when the injury is slight, the test can be performed at varying angles. This places more or less stress on the structure you are testing. The strength and flexibility of the infraspinatus muscle varies greatly from person to person. In the average individual, this muscle is fairly weak, but in a tennis player or weight lifter the infraspinatus can be very developed and strong.

You use the infraspinatus muscle-tendon unit fully in backhand motions in tennis, or when swinging a baseball bat or golf club. If it's injured, it often hurts when you do simple tasks such as write, open a door or reach behind you.

The Infraspinatus - Copyright – Stock Photo / Register Mark The infraspinatus muscle covers the lower portion of the scapula beneath the spine of the scapula, and its tendon attaches to the back of the greater tubercle of the humerus at the posterior upper arm. (Image 1) The infraspinatus muscle-tendon unit is responsible for lateral rotation of the shoulder. The infraspinatus tendon has a very large and broad tendon attachment, about a half-inch wide or more depending upon the size of the person, and a body range of one to two inches long.

Some people may have a minor strain of the infraspinatus tendon that remains barely noticed for years. They may experience slight discomfort when reaching up for something on a high shelf, or back for something toward the back seat of their car. However, when this minor strain does not heal properly, it can set the stage for a more severe injury later. If it starts to hurt when you pull your shirt off overhead, put on your socks, or when you lie on your side to go to sleep, then it means things are getting worse. Often, an infraspinatus tendon injury persists for years, defying all attempts at treatment. It can be a very painful and tenacious injury. If the strain is mild, it can be difficult to assess. But there is a special test that uncovers the injury, even if it is very mild.

Test 1 - Resisted Lateral Rotation (Images 2, 3 and 4)

Resisted Lateral Rotation - Copyright – Stock Photo / Register Mark

Have the person stand with their legs at least shoulder-width apart so they feel stable while doing the test. Place one hand on the upper arm, just above the elbow, and press the upper arm into the body to stabilize it. With the person's arm bent at a right angle in front of their body, place your other hand just above their wrist on the dorsal aspect of the client's lower arm. Now ask the client to push laterally toward you while you resist with equal and opposite force. This test is done at 90 degrees to the body.

If that test gives no positive result, you may need the more subtle test which is applied at 30 degrees. In this position, the client's wrist is situated about four inches from the person's navel. Putting the muscle into the stretch as you test it places much more stress on the structure and usually yields a positive result. A positive test means pain is felt, which indicates that the infraspinatus is injured

Click here for more information about Ben Benjamin, PhD.


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