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Shoulder Muscle Imbalance: Cause or Consequence of Impingement?

Physical therapists often treat injured athletes so they can get back on the playing field. Therapists know that training a single muscle in one arm or leg isn't enough. Muscles on both sides of a joint are often affected by an injury to one side of the joint.

This was shown clearly in a recent study of overhand athletes. These players are involved in racquet sports, volleyball, swimming, and field events. Shoulder pain from impingement is common. Impingement in the shoulder means "pinching" of some structure inside or outside of the shoulder joint.

It's well known that the scapula (shoulder blade) plays an important part in normal shoulder motion. When the shoulder is injured, movement of the scapula changes. What researchers don't know is which comes first: muscle imbalance or impingement?

This study measured speed and timing of muscles around the scapula. Two groups were tested: one group with shoulder impingement, and one without. The researchers found the biggest differences occur in the timing of shoulder muscle activity. They were surprised to find delays in muscle activity of both the injured and uninjured shoulders in the patient group.

The researchers conclude that treatment of sports injuries must correct the timing and coordination of muscle activity around the shoulder. This must be done on both sides. For now it looks like muscle imbalance occurs first, and then impingement.

References: Ann M. Cools, PT, et al. Scapular Muscle Recruitment Patterns: Trapezius Muscle Latency with and without Impingement Symptoms. In The American Journal of Sports Medicine. July/August 2003. Vol. 31. No. 4. Pp. 542-549.

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