Long Head (1b)
Origin: Infraglenoid Tubercle of the Scapula (3) 
Lateral Head (1a)
Origin: posterior and lateral Surface of the humerus (4)
Medial Head (1c)
Origin: posterior Surface of the humerus, below the Radial Groove (5)
Insertion: Olecranon Process of the Ulna (6)
Actions: forearm Extension, arm Adduction and Extension (shoulder Extension)
Helping Synergist Muscles:
- together with Anconeus (2) it acts to produce the forearm Extension
- together with Latissimus Dorsi, Teres Major, Deltoid (posterior fibers), Pectoralis Major, Subscapularis it acts to produce the arm Extension
Notes: the Medial Head (1c) can usually be visible closer to the elbow joint as it is mostly covered by the Long (1b) and Lateral (1a) Head