Measurements Made After Stretching the Contracting Muscle

Abstract

This chapter faces the problem to explain the origin of the additional amount of positive work done by a contracting muscle when it shortens immediately after being stretched, a condition that most often occurs during legged terrestrial locomotion. This task implied experiments made on the isolated muscle, on a single muscular fiber and on a tendon-free segment of the fiber. Initial experiments made on the whole muscle were unable to explain the whole amount of the extra work done after stretching with the release of elastic energy and suggested that the contractile component itself was in some way ‘enhanced’ by previous stretching. An enhancement of the contractile component was in fact subsequently suggested by a shift of muscle’s force-velocity relation and of the stress-strain relation of the undamped ‘elastic’ elements. Experiments made on a single fiber showed that after stretching: (i) the rate constant and the effect of temperature on the fast phase of stress relaxation are consistent with an energy transfer from the undamped to the damped structure within the sarcomeres, and (ii) shortening against the maximal isometric force takes place in four distinct phases similar to those measured after release from a state of isometric contraction. Experiments made on a tendon-free segment of the fiber showed that: (i) the above conclusions are not due to stress relaxation of the tendons, (ii) an energy transfer does occur after stretching between undamped and damped sarcomere structures, and (iii) muscle enhancement by stretch is not entirely due to sarcomere length inhomogeneity.