agonist, antagonist, and synergist
A muscle can only pull, never push — like a rope, it shortens to do work and then goes slack. So to make a clean, controlled movement the body has to organise its muscles into a team with roles, the way a rowing crew, a coxswain, and someone steadying the boat each do a different job toward one smooth stroke.
These roles are named by what each muscle does in a given movement. The agonist (or prime mover) is the muscle mainly causing the action — the biceps when you bend your elbow to lift a cup. The antagonist is the opposing muscle on the other side, which must relax and lengthen to allow the movement, and which can gently brake it — the triceps in that same elbow bend. Synergists are helper muscles that assist the agonist or fine-tune the direction, and stabilisers (fixators) hold a nearby joint still so the prime mover has a firm base to pull from — your shoulder muscles steadying the upper arm while the biceps lifts. Crucially, these are roles, not fixed identities: in the reverse movement the antagonist becomes the agonist. Whether you are picking up a cup gently or lowering it slowly, the same two muscles simply swap jobs.
Smooth movement depends on this teamwork being coordinated, so disordered teamwork is a core rehabilitation problem. After a stroke or spinal injury the timing can break down — antagonists may not relax (co-contraction), or muscles may only fire together in fixed, unhelpful patterns called synergies. Much of therapy is retraining who leads, who yields, and who stabilises, so that the prime mover is no longer fighting its own opposite.
Bending the elbow: the biceps is the agonist, the triceps the antagonist that relaxes to let it happen. Straighten the elbow against a weight and the labels flip — the triceps is now the agonist and the biceps the antagonist. Same muscles, opposite roles.
Agonist and antagonist are roles in a movement, not fixed identities — they swap when the movement reverses.
These are roles defined by the movement, not permanent labels on a muscle: in the opposite movement the same muscle takes the opposite role. Failure of the antagonist to relax (co-contraction) is a common reason movement looks stiff after brain or cord injury.