The upper arm’s powerful and controlled movement is driven by the biceps and the triceps muscles. These two muscles are the primary movers responsible for the actions of the elbow joint. Their coordinated effort allows us to bend and straighten the arm, performing daily tasks with precision and strength.
Anatomy and Function of the Biceps Brachii
The biceps brachii muscle is situated on the anterior (front) side of the upper arm, between the shoulder and the elbow. Its name, derived from the Latin “biceps” meaning “two heads,” indicates its structure: a long head and a short head. Both heads originate on the scapula (shoulder blade) before merging to form a single muscle belly that attaches to the radius, an upper forearm bone.
The long head runs through the shoulder joint, allowing it to assist with minor movements at the shoulder. The short head originates near the shoulder’s coracoid process, helping stabilize the shoulder joint. The primary functions of the muscle are the flexion of the elbow (bending the arm) and the supination of the forearm (rotating the palm upward). The biceps is a powerful supinator due to the angle of its attachment on the radius.
Anatomy and Function of the Triceps Brachii
The triceps brachii muscle is located on the posterior (back) side of the upper arm, occupying the space between the shoulder and the elbow. This muscle is the sole occupant of the posterior compartment and is the counterpart to the biceps. Its Latin name, “triceps,” translates to “three heads,” describing its structure: the long head, the lateral head, and the medial head.
All three heads combine into a single tendon that inserts onto the ulna at the olecranon process. The long head originates on the scapula, meaning it crosses both the shoulder and elbow joints. This attachment allows the long head to contribute to the extension and adduction of the arm at the shoulder. The lateral and medial heads originate on the humerus, the upper arm bone. The function of the triceps is the extension of the forearm (straightening the elbow joint).
The Coordinated Antagonistic System
The biceps and triceps work together in a push-pull dynamic known as an antagonistic muscle pair, essential for smooth and controlled movement. In this system, one muscle contracts to generate movement, while the opposing muscle simultaneously relaxes. The muscle that is actively contracting and shortening to produce the movement is called the agonist, or prime mover.
When bending the arm, the biceps contracts (agonist) while the triceps relaxes and lengthens (antagonist). When straightening the arm, the roles reverse: the triceps contracts to become the agonist, and the biceps relaxes to become the antagonist. This reciprocal inhibition ensures the two muscles do not fight each other, allowing for efficient joint rotation and movement deceleration.