Effect of stretching on undamped elasticity in muscle fibres from Rana temporaria.

Muscle stiffness was measured from the undamped elastic recoil taking place when the force attained during ramp stretches of muscle fibres, tetanized on the plateau of the tension-length relation, was suddenly reduced to the isometric value developed before the stretch, T0. Sarcomere elastic recoil was measured on a tendon-free segment of the fibre by means of a striation follower. After small ramp stretches, stiffness increases to a value 1.33x greater than that measured during release from a state of isometric contraction to 0.9 T0. While the relative increase in stiffness is equal to that reported for fibres of Rana esculenta (Piazzesi et al., 1992), the absolute value of stiffness measured during release from isometric contraction is just over half. As stretch amplitude is increased, on the plateau of the force-length relation, stiffness decreases toward the isometric value. This finding shows that the decrease in stiffness with large stretches cannot be due to a decrease in myofilament overlap (as may be the case when stretching occurs on the descending limb of the tension-length relation, Sugi & Tsuchiya, 1988), but must be due to an effect of the ramp stretch per se. For a given stretch amplitude, the after-stretch transient shortening against T0 taking place after the elastic recoil (which is expression of the work enhancement induced by stretching, Cavagna et al., 1986, 1994) is similar in fibres with very different stiffness of their undamped elastic elements. This suggests that this work enhancement is not due to the recoil of damped elastic structures recruited during stretching because of sarcomere length inhomogenity, a condition which would result in a decrease in stiffness (Morgan et al., 1996).