The time course of the contractile force measured during a twitch under fixed sarcomere length.

A sarcomere length-controlled feedback system was constructed utilizing the laser diffraction technique of Haugen & Sten-Knudsen (1976) to detect sarcomere length changes. The system allowed the sarcomere length to be kept constant within 0.02% during an isometric twitch. The contractile force developed approximates closely to the force exerted by the crossbridges when their translatory movements are prevented. Thus, the force developed under this condition should correspond to the intensity of the active state as defined by A. V. Hill (1949). The time course of the twitch under constant sarcomere length differs substantially from that of the active state curves obtained using quick stretches and quick releases. Thus, the force does not rise quickly to its maximum but rather resembles the fixed-end twitch curve by leading it only slightly (5 ms at 5 degrees C). Its peak value does not reach the level of the tetanic plateau, but is only 9% higher than the maximum fixed-end twitch tension. The force remains above the curve of the fixed-end twitch during its entire course. It is shown that the quick-stretch procedure which results in active state curves as those obtained by A.V. Hill (1949) led to a considerable elongation of the sarcomeres. It is concluded that the slow rise of the contractile force under ordinary isometric conditions is due to properties inherent in the contractile machinery other than those resulting from the extension of series elastic components.