The processes described in the preceding chapters culminate in the generation of mechanical activity by skeletal muscle. These mechanical properties of muscles are readily investigated using isolated muscle or nerve–muscle preparations such as the gastrocnemius or sartorius nerve–muscle preparations in the frog. Experiments on large mammalian muscles require an intact blood supply, in which case the experiments must be performed on an anaesthetized animal, with the nerve supply to the muscle cut and its tendon dissected free and attached to some recording device. The muscle is excited by applying a brief pulse of stimulating current to its nerve or directly to the muscle itself.

Isometric and isotonic contractions

When muscles contract they exert a force on their attachments (this force is equal to the tension in the muscle) and they shorten if they are permitted to do so. Hence we can measure two different variables during the contraction of a muscle: its length and its tension. Most often one of these two is maintained constant during the contraction. In isometric contractions the muscle is not allowed to shorten (its length is held constant) and the tension it produces is measured. In an isotonic contraction the load on the muscle (which is equal to the tension in the muscle) is maintained constant and its shortening is measured.

An isometric recording device has to be stiff, so that it does not in fact allow the muscle to shorten appreciably while the force is being measured.