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Muscles transfer force to bones through tendons. They move our bones and associated body parts by pulling on them – this process is called muscle contraction.

However, muscle contraction cannot act to push the bone back into its original position, and because of this, muscles work in 'antagonistic muscle pairs'. One muscle of the pair contracts to move the body part, the other muscle in the pair then contracts to return the body part back to the original position. Muscles that work like this are called antagonistic pairs.

In an antagonistic muscle pair as one muscle contracts the other muscle relaxes or lengthens. The muscle that is contracting is called the agonist and the muscle that is relaxing or lengthening is called the antagonist.

For example, when you perform a bicep curl, the biceps will be the agonist as it contracts to produce the movement, while the triceps will be the antagonist as it relaxes to allow the movement to occur.

Antagonistic muscle pairs

The following groups of muscles are antagonistic pairs:

To allow antagonistic pairs to work efficiently, other muscles called fixators assist by supporting and stabilising the joint and the rest of the body. Some fixators also assist the agonist and act as a synergist.

The trapezius muscle can act as a fixator when the biceps is flexing the elbow joint.

The abdominals can act as fixators to stabilise the body for hip and knee movements.

Antagonistic muscle pairs in action

In the preparation phase, when footballers prepare to kick a football, their hamstrings contract to flex the knee while the quadriceps lengthens to allow the movement. The hamstrings are the agonist and the quadriceps are the antagonist.

In the contact and recovery phase, the quadriceps contract to extend the knee while the hamstrings lengthen to allow the movement. The quadriceps are the agonist and the hamstrings are now the antagonist.

The abdominals would be acting as fixators.