The upper arm is home to the biceps and the triceps, which are the primary muscles controlling movement between the shoulder and the elbow. These muscles are positioned on opposite sides of the humerus, the long bone of the upper arm. The biceps is located on the front of the arm, while the triceps is situated on the back. They work in a coordinated fashion to control the bending and straightening actions of the elbow.
The Biceps Brachii
The biceps brachii, meaning the “two-headed muscle of the arm,” is prominently located in the arm’s anterior compartment. This muscle involves two distinct heads that begin at the shoulder blade (scapula). The long head originates from the supraglenoid tubercle, while the short head starts at the coracoid process, a bony projection on the scapula.
These two muscle bellies merge in the middle of the upper arm to form a single, thick tendon that crosses the elbow joint. The tendon then inserts primarily onto the radial tuberosity, a bump on the radius bone of the forearm.
The biceps performs two main functions at the elbow joint. The first is elbow flexion, which involves bending the arm and bringing the forearm closer to the shoulder. The biceps is also a powerful supinator of the forearm, meaning it can rotate the forearm and palm upward, such as when turning a doorknob.
The Triceps Brachii
The triceps brachii is the large muscle covering the entire back of the upper arm, occupying the posterior compartment. Its name, meaning the “three-headed muscle of the arm,” reflects its structure, which includes the long head, the lateral head, and the medial head.
The long head is the only part of the muscle that originates on the shoulder blade, specifically at the infraglenoid tubercle of the scapula, which gives it a role in shoulder movement. The lateral and medial heads both originate on the posterior surface of the humerus.
All three heads converge into a single, strong tendon. This tendon inserts onto the olecranon process, the large, bony point at the back of the ulna. The triceps’ primary function is elbow extension, the action of straightening the arm, which is necessary for pushing movements.
Antagonistic Relationship and Coordinated Movement
The biceps and triceps muscles function as an antagonistic pair, a fundamental concept where two muscles produce opposite movements at a joint. When the biceps contracts to bend the elbow, the triceps must simultaneously relax to allow the movement to occur freely. Conversely, when the triceps contracts to straighten the arm, the biceps must lengthen and relax.
This coordinated action is managed by the nervous system through a mechanism called reciprocal inhibition. When the brain sends a signal for the agonist muscle, the primary mover, to contract, it simultaneously sends a signal to the antagonist muscle to inhibit its own contraction. This neurological process ensures smooth and efficient motion.
The antagonistic pairing allows complex movements, like lifting a glass of water or setting an object down gently, to appear fluid and controlled. Both muscles are constantly engaged; one provides the force and the other provides controlled resistance to decelerate the movement. This constant interplay prevents jerky motions and allows for the fine motor control required for daily tasks.