Subscapularis - The Armpit Muscle

Subscapularis

Origin:  Subscapular fossa (costal or anterior component of the scapula)

Insertion:  Lesser tuberosity of the humerus

Innervation:  Subscapular nerve (C5, C6)

Action:  Internal rotation of the humerus

Why do I care:  Subscapularis is one of the four rotator cuff muscles of your shoulder.  You can touch it if you stick your hand in the opposite arm pit and push on the front of your scapula, or shoulder blade.  This muscle can become shortened with chronic computer use or if placed in a sling for any length of time.  

So? So, if your internal rotators are shortened, you will lack appropriate external rotation of the humerus, which is necessary to get your arm completely overhead without pinching your other rotator cuff tendons in the shoulder joint, which will lead to bursitis, tendonitis and/or rotator cuff tears.