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.

curriculum-key-fact
One way to remember which muscle is the agonist: it's the one that's in 'agony' when you are doing the movement, as it is the one that is doing all the work.

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.

Flexed arm. Biceps muscle is contracted. Triceps is relaxed

The biceps contracts and raises the forearm as the triceps relaxes

Antagonistic muscle pairs

The following groups of muscles are antagonistic pairs:

BicepsTriceps
HamstringsQuadriceps
Gluteus maximusHip flexors
GastrocnemiusTibialis anterior
Pectoralis majorLatissimus dorsi

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

The preparation and contact phase in football as footballers prepare to kick a ball
Preparation and contact phase in football

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.

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.

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