3. Measurements Made During or Starting from a State of Isometric Contraction

This chapter describes experiments aimed to detect: (i) the interaction between “motor” and passive elastic structures during muscular contraction, and (ii) the basic characteristics of the “motor” function. The interaction between motor and elastic structures explains the time course of the force exerted by muscle in response to a single or a series of stimuli (twitch, clonus and tetanus). The characteristics of the motor are evidenced by two fundamental relations: the force-length relation, i.e. the force exerted by muscle tetanically stimulated at different lengths, and the force-velocity relation, i.e. the velocity of muscle shortening and lengthening, at a given length, against different loads. The force-length relation is shown both when the maximal isometric force is measured as a function of the length of the whole muscle and as a function of the length of the sarcomere. The trend of the force-length relation is explained on the basis of the different overlap between actin and myosin at the different sarcomere lengths. The changes of the force-velocity relation with muscle length and time delay since the beginning of stimulation are explained. The functional consequences of both force-length and force-velocity relations are evidenced. The iso-velocity force-length diagram is described and it is shown how it depends on the elastic and the contractile components of muscle at different velocities of shortening. Quick release experiments on a single muscular fiber allow determining the existence within each half sarcomere of an undamped and a damped structure whose force-length relations are described over the half-sarcomere length.