How Does Our Body Muscle Function?

A fundamental property of muscle is that it has the capacity to shorten, or at least attempt to shorten, but is not capable of lengthening itself. As a result, a muscle functions to shorten the distance between its origin and insertion points on bone. In order for a muscle to then return to its lengthened state some external force must be applied. This external force may come in the form of gravity acting on a limb segment or another muscle group with an opposing (antagonistic) action.

Modes of muscular activity
In addition to shortening the distance between adjacent bones, muscles can also act to prevent or resist unwanted movements, or to slow down movements initiated by other muscles or some external force such as gravity. These various tasks are possible because muscle has three basic modes of activity.

1. During concentric contractions the creation of tension within the muscle overcomes any resistance to movement and causes that muscle to shorten. An example is the upward phase of the bicep curl.

2. During eccentric activity the generation of tension within a muscle acts to resist, but not stop, the lengthening of that muscle. Obviously then the tension within the muscle must be less than that required to halt the movement of the limb. Eccentric contractions are generally responsible for slowing down limb or trunk movements which have been initiated by other muscles or by the effect of gravity. For example, the biceps act eccentrically during the downward phase of the bicep curl in order to slowly and safely lower the forearm and thus prevent injury to the elbow joint.

3. Isometric actions involve the generation of tension within a muscle which exactly opposes the effect of any external forces acting on the limb. As a result the muscle generates tension but does not change length. Isometric contractions occur during the bicep curl if one deliberately holds the weight in a stationary position at some point during the movement. Isometric contractions also occur when one attempts to lift an immoveable object such as a car or truck.

During the upward phase of a bicep curl the biceps and brachialis (as well as other elbow flexors situated at the front of the joint) act concentrically in order to flex the elbow. These muscles then act in an eccentric manner in order to slowly lower the weights back to the starting position. Obviously gravity provides the motive force which lowers the weights but the elbow flexor muscles must act in order to slow the movement down.

This simple example highlights the fact that in the gym, the muscles involved in the upward phase of a lift are also active during the downward phase. A reasonably common mistake for students is to assume that some antagonistic muscle group acts to lower the resistance.