Agonist and antagonist muscle pairs
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.
Image caption, The biceps contracts and raises the forearm as the triceps relaxes
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Antagonistic muscle pairs
The following groups of muscles are antagonistic pairs:
| Biceps | Triceps |
| Hamstrings | Quadriceps |
| Gluteals | Hip flexors |
| Gastrocnemius | Tibialis anterior |
| Pectorals | Latissimus dorsi |
| Biceps |
| Triceps |
| Hamstrings |
| Quadriceps |
| Gluteals |
| Hip flexors |
| Gastrocnemius |
| Tibialis anterior |
| Pectorals |
| Latissimus dorsi |
Antagonistic muscle pairs in action
In the preparation phase, when a footballer prepares 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.
Question
Describe how the antagonistic muscle pairs are working at the elbow during the downwards and upwards phase of a press up.
During the downwards phase, the triceps are the agonist and they contract eccentrically to control the flexion of the elbow so the body is lowered under control down towards the floor. The biceps are the antagonist.
During the upwards phase, the triceps are the agonist and contract concentrically to extend the elbow and the biceps are the antagonist.