Muscles are attached to bones by tendons. Muscles contract to move our bones by pulling on them.
However, muscles can only pull; they cannot push. This is why they usually work a joint in 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.
The following groups of muscles are antagonistic pairs:
|Joint||Antagonistic pair||Movements produced||Sport example|
|Elbow||Biceps; triceps||Flexion; extension||Chest pass in netball; badminton smash|
|Knee||Hamstrings; quadriceps||Flexion; extension||Jumping to block in volleyball; tuck jump in trampolining|
|Shoulder||Latissimus dorsi; deltoid||Adduction; abduction||Golf swing; breaststroke arms|
To allow antagonistic pairs to work efficiently, other muscles called fixators assist by supporting and stabilising the joint and the rest of the body.
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.
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.
The abdominals would be acting as fixators.
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 biceps 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 triceps 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.