Agonist and antagonist: muscle pairs

Agonist (AG-uh-nist) is the muscle that makes a movement happen. The agonist is the muscle doing the work, also called the prime mover. In a curl, biceps brachii is the agonist.

Memory hook: the agonist is in agony. It's the one doing the work.

Antagonist (an-TAG-uh-nist) is the muscle on the opposite side that relaxes to let a movement happen. The antagonist lengthens to allow the movement. In a curl, triceps brachii is the antagonist.

The word parts: ant = against, agonist = the one in action. Ant-agonist: against the agonist, on the other side of the joint.

Both muscles lie along the humerus, the upper arm bone.

• Biceps brachii (BY-seps BRAY-kee-eye): the two-headed muscle at the front of the upper arm. It bends the elbow and turns the palm up. The curl.
• Triceps brachii (TRY-seps BRAY-kee-eye): the three-headed muscle at the back of the upper arm. It straightens the elbow.

The lifting half of a curl is elbow flexion. Lowering a curl is extension.

Body Reps names agonist and antagonist roles only for the curl and the triceps press-down. Its lessons also give these actions, muscle by muscle, at the same body part:

Knee - Quadriceps: they straighten the knee. - Hamstrings: they bend the knee and extend the hip.

Spine - Rectus abdominis: it flexes the spine in a crunch and braces to stop it bending back. - Erector spinae: it extends the spine and holds it upright.

Foot - Tibialis anterior: it lifts the toes towards the shin. - Gastrocnemius: it points the foot and helps bend the knee.

• Anterior means towards the front. Tibialis anterior sits at the front of the shin.
• Posterior means towards the back. The hamstrings sit on the posterior thigh.