The supraspinatus muscle in the shoulder is a quite small muscle and it may not be the only muscle that is tightened in the joint.
As with any muscular rigidity, we have to ask ourselves, what is the cause of involuntary tightened muscles in the body? They are induced by a reaction triggered by the autonomic nervous system (ANS), specifically the Sympathetic component of the nervous system.
There is no other mechanism for this condition in the body.
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