The link queen posts again!!
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Shoulders extended. You can see the humeral head push forward. If the posterior shoulder is stiff or short and/or the anterior capsule is loose or the subscapularis fatigues or fails to restrict the forward movement of the humeral head, impingement will follow. Many folks have trouble with this because of stiff/short pec minor or thoracic kyphosis.
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Here's what it should look like. The humerus stops moving when the scapula stop. this prevents excessive shoulder extension and wear n tear on the anterior structures.
Bill