Muscles never work alone. In order for coordinated movement to take place, the muscles must work as a group or team, with several muscles working at any one time.
The muscle directly responsible for initiating the desired joint movement is called the agonist or prime mover.
A muscle whose action is opposite to that of the agonist is known as the antagonist.
For example, flexing (bending) the arm at the elbow. The muscle responsible for this action is the biceps brachii, and it is therefore the agonist. Whilst the bicep shortens in order to pull on the radius and raise the lower arm, the triceps brachii (back of the arms) must lengthen in order for this action to take place. The triceps in this instance act as the antagonist.
The muscles work as a unit, requiring a high degree of coordination. This coordination is achieved by nervous control. When the prime mover is being stimulated, the nerve impulse to the antagonist is inhibited – this is known as reciprocal inhibition.
Fixator muscles are stabilizers which also work in the movement. Their role is to stabilise the origin so that the agonist can achieve maximum and effective contraction. The fixator muscle increases in tension but does not allow any movement to take place. In the above example, the trapezius and rhomboid muscles (upper back) contract to stabilise the scapula to create a rigid platform.
Synergist muscles or neutralisers prevent any unwanted movements which may occur, particularly at the shoulder when flexing the elbow (in the above example), as the bicep works over two joints.
This term can also be used to describe those muscles that assist the work of an agonist at a particular joint, making the movement more efficient. In this instance, the brachialis and brachioradialis muscles act as synergists.
The roles of each muscle are constantly changing as movements change. For example, when extending (straightening) the arm at the elbow, the roles are reversed, with the triceps shortening and acting as the agonist, while the biceps lengthen and act as the antagonist.
There is usually more than one agonist at a joint, and other muscles can assist the movement. The number of muscles involved depends on the type and amount of work being carried out.