Isotonic Contractions

When a muscle is stimulated to contract, the contractile tissue begins to shorten and stretches the connective tissue so that muscle tension rises. When the force developed exceeds the effect of the load the muscle as a whole starts to shorten maintaining a constant tension (isotonic) and moves the load. Contraction alone is responsible for muscle shortening. This is the concentric (or positive) phase of the contraction. The work done by the muscle is the force times the distance moved.

The eccentric (negative) phase of the contraction is the controlled lowering of a load and the extension of the muscle to its original length from the contracted state. In an isotonic contraction, positive work is done when a load is moved over a distance and is proportional to the amount of muscle contraction. In muscle lengthening the work is negative as the distance over which the load is moved is reversed. In an isometric contraction, no work is done because the load is not moved. However, the muscle does require energy to contract up to, and be maintained at, the point where the load takes effect and maximum tension is generated. The muscle consumes more internal energy as the tension is increased and, depending on the neuromuscular efficiency, more muscle fibres can be recruited. This work may reach a constant as some fibres become fatigued but others take their place. It is the load upon the muscle which causes strengthening whether it be isotonic (positive or negative) or isometric in nature.