Final answer:
The triceps brachii muscle originates from the scapula and the posterior surface of the humerus, and it inserts on the olecranon process of the ulna. Its primary action is to extend the forearm at the elbow joint, which is essential for push-up motions and pushing objects away from the body.
Step-by-step explanation:
The triceps brachii is a three-headed muscle located at the back of the upper arm. Its primary function is to extend the forearm. The origins of the triceps brachii are the scapula at the infraglenoid tubercle and the posterior surface of the humerus. Specifically, it has two points of origin on the humerus: the upper part of the posterior humerus and just below the greater and lesser tubercle. The insertion point of the triceps brachii is at the olecranon process of the ulna, which is part of the forearm.
When the triceps brachii contracts, it pulls on the ulna, causing the elbow to extend, or straighten. This action opposes the biceps brachii, which flexes the elbow. The triceps brachii allows for the forceful extension necessary for actions such as pushing things away from the body or performing a push-up.