Subscapularis, infra and supraspinatis – as well as teres – muscles join together to insert into the posterior shoulder.
When deactivated, the “cuff” union no longer helps stabilize the shoulder by counter-rotating the joint backward, and instead allows anterior attachments of other muscles to ratchet-up the forward torque.
This causes other muscles nearby to spasm, further exacerbating the problem – and even sometimes results in pinch-compressioning over nerve bundles, which causes nerve pain.
Candid Corvid Productions 