Long head: infraglenoid tubercle of scapula. Lat head: upper half post humerus (linear origin). Med head: lies deep on lower half post humerus inferomedial to spiral groove and both intermuscular septa
Action
Extends elbow. Long head stabilises shoulder jnt. Med head retracts capsule of elbow jnt on extension
Insertion
Post part of upper surface of olecranon process of ulna and post capsule
