Muscle Coordination and Action

The skeletal muscles can coordinate, oppose, or assist each other to facilitate voluntary body movements.

Depending on their function, a muscle or group of muscles are termed agonists, antagonists, synergists, or fixators.

An agonist muscle produces an action at a joint. For example, during elbow flexion, the biceps brachii is the agonist, pulling the radius towards the body.

In the same movement, the brachialis muscle acts as a synergist. It helps stabilize the elbow joint, allowing the biceps brachii to flex the arm efficiently.

The four rotator cuff muscles — the subscapularis, supraspinatus, infraspinatus, and teres minor — stabilize the humeral head in the glenoid cavity. They act as fixators and stabilize the origins of the biceps brachii, the agonist during arm flexion.

Lastly, antagonists are functionally opposite to the agonist, usually located on opposite sides of the bone. They reverse the movements facilitated by agonists.

For example, while the triceps brachii acts as an antagonist to the biceps brachii during elbow flexion, their role reverses during elbow extension.