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The mechanisms of force enhancement during constant velocity lengthening in tetanized single fibres of frog muscle.

The present paper deals with the mechanism responsible for the force enhancement in response to constant velocity lengthenings imposed at the plateau of isometric tetanic contractions. Experiments were performed at 3.5-5.6 degrees C on single fibres isolated from the tibialis anterior muscle of the frog. The resting sarcomere length was about 2.15 microns. The large variation observed in the characteristics of the force responses to lengthening and therefore in the shape of the force-lengthening velocity relations is mainly determined by the degree of longitudinal dishomogenities of muscle fibres. "Homogeneous" fibres and individual "tendon-free segments" selected along them exhibited superimposable force-lengthening velocity relations with negligible shape variation. Stiffness and force increased with lengthening, reaching, approximately steady values, respectively 10-15% and 60-70% greater than at the isometric tetanus plateau, at velocities of about 0.1 micron/s per half-sarcomere (hs). At higher lengthening velocities stiffness remained practically unvaried, whereas the steady component of the force responses continued to increase, reaching asymptotically a maximum value, about 100% greater than at the isometric tetanus plateau, at velocities between about 0.5 and 1 micron/s per hs. The amount of lengthening required to attain the peak value was smaller for stiffness (10 nm per hs) than for force (12-14 nm per hs). In terms of 1957 model of A. F. Huxley, the results indicate that lengthening (i) potentiates the mechanical output of muscle by increasing the number and the degree of extension of attached cross-bridges, (ii) enhances the speed of cross-bridge attachment and detachment.