The term ‘muscle contraction’ refers to the development of tension within the muscle. The term is a little confusing, as contraction means becoming smaller in much English usage. Some sport and exercise scientists
would prefer the term ‘action’ to be used instead, but this has yet to be widely adopted. There are three types of muscle contraction:
• isometric (static) contraction, in which the muscle develops tension with no change in overall muscle length, as when holding a dumbbell stationary in a biceps curl;
• concentric contraction, in which the muscle shortens as tension is developed, as when the dumbbell is raised in a biceps curl;
• eccentric contraction, in which the muscle develops tension while it lengthens, as in the lowering movement in a biceps curl.
Both concentric and eccentric contractions can, theoretically, be isotonic (constant tension) or isokinetic (constant speed). They normally occur in a manner in which neither tension nor speed is constant.
The following axioms relating to muscular contraction should be noted (Rasch and Burke, 1978).
• A muscle fibre can only develop tension within itself.
• When a fibre or muscle develops tension both ends tend to move; whether these movements occur depends on resistance and the activity of other muscles.
• When a muscle develops tension, it tends to perform all of its possible actions at all joints it crosses.
The above axioms suggest that, to bring about the movements of the human body, muscles act together rather than individually with each playing a specific role—this is one important feature of coordinated movement. In group action, muscles are classified according to their role.
Agonists are the muscles that directly bring about a movement by contracting concentrically. The group is sometimes divided into prime movers, which always contract to cause the movement, and assistant movers, which only contract against resistance or at high speed. Brachialis and biceps brachii are prime movers for elbow flexion while brachioradialis is generally considered to be an assistant mover.
Antagonists are muscles that cause the opposite movement from that of specified agonists. Their normal role in group action is to relax when the agonists contract, although there are many exceptions to this. At the elbow, the triceps brachii is antagonistic to brachialis and biceps brachii.
Stabilizers contract statically to fix one bone against the pull of the agonist(s) so that the bone at the other end can move effectively. Muscles that contract statically to prevent movements caused by gravity are called supporting muscles, such as the abdominal muscles in push ups.
Neutralizers prevent undesired actions of the agonists when the agonists have more than one function. They may do this by acting in pairs, as mutual neutralizers, when they enhance the required action and cancel the undesired ones. For example, the flexor carpi radialis flexes and abducts the wrist while the flexor carpi ulnaris flexes and adducts the wrist: acting together they produce only flexion. Such muscles are also called helping synergists. Neutralizers may also contract statically to prevent an undesired action of agonist(s) that cross more than one joint (multijoint muscles). The flexion of the fingers while the wrist remains in its anatomical position involves static action of the wrist extensors to prevent the finger flexors from flexing the wrist. Such muscles are also called true synergists.
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