GROUP ACTION OF MUSCLES
Muscles do not work in isolation. They must, for smooth co-ordinated movement to occur, operate in one of the following roles.
Prime Movers or Agonists
In this case they are those muscles which initiate and perform movement.
Antagonists
These can produce the opposite movement to that produced by the agonists. When the agonists work the antagonists must relax reciprocally, i.e. exactly an equal amount. The tension of the agonist contraction is equalled by the relaxation of the opposing muscles in order to allow smooth movement to occur.
Synergists
These are muscles which contract in order to bring about a joint position to make the action of the agonists stronger. They most frequently may be observed in action when the agonists are bi- or multi-axial muscles, e.g. the wrist extensors. Synergists also contract to prevent extra or additional movements that the agonists might otherwise perform. They operate from unconscious levels.
Fixators
These muscles also operate from unconscious level to fix the attachments of the agonists, antagonists and synergists. This does not mean that they fix a component of the body and keep it there throughout the whole of one particular muscle action; rather their role is dynamic as is that of the synergists. Fixator muscle work probably constitutes about 75% of normal daily muscle action. Their role is not isometric except for very short periods; it becomes isotonic in alternating patterns so that movement is smooth. In the example quoted above the fixator muscle work would be those around the elbow to fix the forearm and hand, the shoulder to fix the arm and shoulder girdle and of the remainder of the body to fix such parts as are not totally supported.
The fixator muscle work of an action such as threading a needle will be very different from that in throwing a heavy ball, both in quantity and quality. In the former case the starting position may be sitting and therefore the fixator work will be confined to those muscles which maintain the sitting position and the shoulder girdle and arm muscles involved in a fine pincer grasp and approximation of thread to needle. In the case of throwing a heavy ball, the fixator work will change rapidly as the body prepares for and carries out the throw followed by a braking action to prevent loss of position or balance.
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