Mechanical Properties of Muscle

Resting muscle is soft and freely extensible. On stimulation the muscle passes into a new physical state—it becomes hard, develops tension, resists stretching, lifts loads. This article deals with the problem of defining the new physical state in exact mechanical terms. Most of the research on this subject has been performed on the striated muscle of the frog, and for the sake of clarity the conclusions will be presented in concrete form, as the results of a set of experiments performed with the apparatus shown in fig. 1. The muscle, a frog's sartorius (a), lies on an array of stimulating electrodes. Its pelvic end is clamped rigidly and its free tibial end is attached to a light duralumin lever (b) pivoted on ball-bearings. The position of this lever, and thus the length of the muscle, is recorded by a photoelectric arrangement which is not shown. Forces may be applied to the muscle by loading it (c) (the load is applied near the fulcrum in order to reduce the effects of inertia); and the length of the resting muscle adjusted by moving the stop (d). For some purposes it is necessary to have an electro-magnetic stop (e) which can beremoved at any moment during the contraction. Instead of recording length changes in the muscle, one can record tension changes by means of the transducer (f). This is a small valve (RCA 5734) whose anode can be moved from outside, thus converting changes of tension into changes of voltage.