1. Force changes in areflexive cat soleus muscle in decerebrate cats were recorded in response to two sequential constant velocity (ramp) stretches, separated by a variable time interval during which the length was held constant. Initial (i.e., prestretch) background force was generated by activating the crossed-extension reflex, and stretch reflexes were eliminated by section of ipsilateral dorsal roots. 2. For the initial 400-900 μm of the first stretch, the muscle exhibited high stiffness, classically termed 'short-range stiffness.' This high stiffness region was followed by an abrupt reduction in stiffness, called muscle 'yield,' after which force remained at a relatively constant level, achieving a plateau in force. This plateau force level depended largely on stretch velocity, but this dependence was much less than proportional to the increase in stretch velocity, in that a 10-fold increase in velocity produced <2-fold increase in plateau force. 3. In experiments where the velocities of the two sequential ramp stretches were identical, the force plateau level was the same for each stretch, regardless of the time elapsed before the second stretch (varied from 0 to 500 ms). In contrast, measures of stiffness during the initial portion of the second stretch showed time-dependent magnitude reductions. However, stiffness recovered quickly after the first stretch was completed, returning to control values within 30-40 ms. 4. In one preparation, in which the velocities of the two sequential ramp stretches were different, the force plateau elicited during the second stretch exhibited velocity dependence comparable with that recorded in the earlier single velocity studies. Furthermore, muscle yield was still evident in the case where the force change was due solely to the change in velocity and where short-range stiffness had not yet recovered fully from the initial stretch. On the basis of these findings, we argue that the classical descriptions of short-range stiffness and yield are inadequate and that the change in force that has typically been called the muscle yield reflects a transition between short-range, transient elastic behavior to steady-state, essentially viscous behavior. 5. To examine changes in the muscle's mechanical stiffness during single ramp stretches, a single pulse perturbation was superimposed at various times before, during, and subsequent to the constant velocity stretch. The force increment elicited in response to each pulse decreased relative to the initial isometric value, remained essentially constant until the end of the ramp, and then returned to its prestretch magnitude shortly (30-40 ms) after stretch termination. These findings indicate that lengthening muscle sustains a reduction in stiffness during a ramp stretch. 6. These results have important implications for the neural control of lengthening muscle. First, after stretch, areflexive soleus muscle quickly regains its initial stiffness, thereby resetting the muscle to a predictable and consistent mechanical state, a necessity if predictive control mechanisms are used to preserve elastic behavior. Second, because the steady-state force generated by lengthening muscle is dependent largely on the velocity of stretch (and not on prior perturbation history) for a given initial force, the mechanical properties of muscle in the 'post-yield' phase are largely analogous to those of a viscous damper. Third, it appears that stretch reflex compensation changes the basic form of the muscle's mechanical stiffness from one dominated by viscouslike behavior to one dominated by elastic behavior.
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