How muscles work together
Agonist, antagonist and fixator
Muscles can only pull, by contracting: they can't push. So they work in teams, each with a role:
- The is the muscle that contracts to produce the movement. It is also called the prime mover.
- The is the muscle that relaxes and lengthens to let the movement happen.
- A stabilises the joint or body part the agonist is pulling from, so the agonist can work well and there's no unwanted movement. Example: in a bicep curl, muscles around the shoulder, such as the trapezius, hold the shoulder steady.
Key terms
Antagonistic muscle action
The agonist and antagonist work as an : as one contracts, the other relaxes. When the movement reverses, they swap roles.
| Joint | Movement | Agonist | Antagonist |
|---|---|---|---|
| Elbow | Flexion | Biceps | Triceps |
| Elbow | Extension | Triceps | Biceps |
| Knee | Flexion | Hamstrings | Quadriceps |
| Knee | Extension | Quadriceps | Hamstrings |
In the exam, name the agonist from the movement at the joint: if the elbow is extending, the triceps is the agonist.
Common mistake
The agonist contracts and the antagonist relaxes. Saying the agonist relaxes, or that both work together, loses the mark.
Key terms
Worked examples: a kick and a bicep curl
A football kick:
- Backswing: the knee bends (flexion). The are the agonist and contract; the are the antagonist and relax.
- Kick: the knee straightens (extension). The roles swap: the quadriceps contract as the agonist and the hamstrings relax as the antagonist.
A bicep curl:
- Upward phase: the elbow flexes. = agonist, = antagonist.
- Downward phase: the elbow extends. Triceps = agonist, biceps = antagonist.
In both, a keeps the body steady: the shoulder in the curl, and the hip and trunk in the kick.