The subscapularis provides dynamic anterior support to the glenohumeral joint; it’s the largest rotator cuff muscle that is centring and stabilising the humeral head in the glenoid and is a key anatomical component of the long head of biceps tendon pulley system. 

 Nowadays, we all use our shoulder forward and internally rotated. This repetitive position firstly shortens pec minor (small pec muscle under the big guy), this will rotate scapula forward, downward. If you add that the humerus (upper arm bone) into internal rotation, through typing; the shoulder starts to be comfortable at the front lip of the glenohumeral joint. Till bicep tendon and Subscapularis needs more space while trying to reach up, or you want to pump the big guy on the bench, wanting to push-pull ups, swim or yoga. 

 Symptoms commonly know as shoulder impingement, which can lead to many other pathological issues, bicep tendon injury, for instance.   Furthermore, if the problem emerges with the ascending impacts within the kinetic chain,e.g. hip/feet; many other symptoms could occur globally.