Muscle movement

In the coordination of movement muscles work in pairs of groups. Muscles are classified by functions as agonists (prime movers), antagonists, synergists and fixators (stabilisers).

Although muscles are usually described as performing a particular action, they do not act alone. Any movement is the result of cooperation between a large number of muscles which is coordinated in the cerebellum in the brain for smooth efficient actions.


This is when two muscles or sets of muscles pull in opposite directions to each other. They don't actually work against one another but work in a reciprocal complementary way with one relaxing to allow the other to contract.

Agonists/prime movers

This is known as the main activating muscle. It is important to note that these terms are used in relation to a specific action. The roles are, therefore, relative to one another and are interchangeable. An example is the action of the biceps and triceps of the upper arm. Biceps are the agonist in flexion of the elbow joint, and triceps are the antagonist. In relation to extending or straightening the elbow the roles are reversed.


This term refers to muscles on the same side of a joint that work together to perform the same movement. An example of this is the flexing of the elbow. The biceps actually work synergistically with the brachialis muscle that lies underneath it.


These are muscles that stabilise a bone to give a steady base from which the agonist works. For the biceps and triceps to flex and extend the elbow joint, muscles around the shoulder and upper back control the position of the arm.

Biomechanically, muscles do one of two things - stretch or contract. Muscular contractions can be isometric or isotonic. Isotonic contractions may be further classified as either concentric or eccentric. The opposite of contracting is stretching which extends the muscles.

Isometric contraction

This is when the muscle works without actual movements (iso means same and metric means length). Postural muscles work by isometric contraction.

study tip

Learning muscles can be daunting. It is first helpful to break down the information into manageable chunks and learn a few muscles at a time.

The following may help you when studying muscles:

a Is there a clue in the name of the muscle as to where it is located in the body (for example, the tibialis anterior muscle is located alongside the tibia bone in the front of the lower leg)? b Try to visualise where the muscle is on your or a client's body. c Look for information that will help you remember its action (see key facts in the last column of the table below). d If you know where the muscle is located and attached you can work out its action by moving that body part and feeling the muscle contracting.

Isotonic contraction

This term refers to when the muscle force is considered to be constant (tonic meaning the same tone or tension) but the muscle length changes. There are two types of isotonic contraction:

Concentric contractions (towards the centre) - this type of contraction occurs when the muscle shortens to move the attachments closer such as when the biceps bends up the forearm.

Eccentric contractions (away from the centre) - this type of contraction occurs when a muscle is stretched as it tries to resist a force pulling the bones of attachment away from one another, such as when tensing the biceps and someone pulls your forearm straight.

During many everyday actions, both isometric and isotonic contractions occur simultaneously. As an example, in a standing position the quadriceps muscle straightens the knee when standing to keep you upright, thereby preventing your knee from bending (isometric contraction). If you sit down slowly, the muscle is stretched and an eccentric contraction controls the rate at which the knee bends to lower the body. If you then stand up, the muscle works concentrically to straighten the knee again.

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