The Deltoid Muscle: A Mover and a StabilizerEvery muscle in the body has a specific job. The deltoid muscle of the shoulder has two jobs. It's a mover and a stabilizer. It lifts the arm up, and it helps press the end of the upper arm bone (the humeral head) into the shoulder socket.
The deltoid muscle is triangular, with a wide top and a narrow bottom. It has three parts: anterior, middle, and posterior. The deltoid is an anterior stabilizer, which means it keeps the shoulder from dislocating forwards. One of the times this is important is when the arm is in an extreme pitching position for baseball.
Many studies have been done to look at the "mover" function of the deltoid. This is the first to report on the role of the deltoid as a stabilizing muscle. A three-dimensional tracking device was used to monitor the muscle. The effects of location and load on stability were measured.
All shoulders were tested with the arm in the cocked position, as if ready to pitch a ball. In this position, the arm is away from the body, the shoulder is outwardly rotated, and the elbow is bent in a 90-degree angle. In all the tests, there were no differences in work load among the three parts of the deltoid. Each part worked equally to stabilize the shoulder in this position.
The authors conclude that the deltoid muscle should be strengthened in any shoulder rehab program for anterior instability. Exercises for the deltoid muscle may be just as important as a program for other shoulder muscles, such as the rotator cuff and biceps.
Tadato Kido, MD, et al. Dynamic Stabilizing Function of the Deltoid Muscle in Shoulders with Anterior Instability. In The American Journal of Sports Medicine. May/June 2003. Vol. 31. No. 3. Pp. 399-403.
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