MECHANICAL-PROPERTIES OF CAT SOLEUS MUSCLE ELICITED BY SEQUENTIAL RAMP STRETCHES - IMPLICATIONS FOR CONTROL OF MUSCLE

1. Force changes in areflexive cat soleus muscle in decerebrate cats w ere recorded in response to two sequential constant velocity (ramp) st retches, separated by a variable time interval during which the length was held constant. Initial (i.e., prestretch) background force was ge nerated by activating the crossed-extension reflex, and stretch reflex es were eliminated by section of ipsilateral dorsal roots. 2. For the initial 400-900 mum of the first stretch, the muscle exhibited high st iffness, classically termed ''short-range stiffness.'' This high stiff ness region was followed by an abrupt reduction in stiffness, called m uscle ''yield,'' after which force remained at a relatively constant l evel, achieving a plateau in force. This plateau force level depended largely on stretch velocity, but this dependence was much less than pr oportional to the increase in stretch velocity, in that a 10-fold incr ease in velocity produced <2-fold increase in plateau force. 3. In exp eriments where the velocities of the two sequential ramp stretches wer e identical, the force plateau level was the same for each stretch, re gardless of the time elapsed before the second stretch (varied from 0 to 500 ms). In contrast, measures of stiffness during the initial port ion of the second stretch showed time-dependent magnitude reductions. However, stiffness recovered quickly after the first stretch was compl eted, returning to control values within 30-40 ms. 4. In one preparati on, in which the velocities of the two sequential ramp stretches were different, the force plateau elicited during the second stretch exhibi ted velocity dependence comparable with that recorded in the earlier s ingle velocity studies. Furthermore, muscle yield was still evident in the case where the force change was due solely to the change in veloc ity and where short-range stiffness had not yet recovered fully from t he initial stretch. On the basis of these findings, we argue that the classical descriptions of short-range stiffness and yield are inadequa te and that the change in force that has typically been called the mus cle 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 stret ches, a single pulse perturbation was superimposed at various times be fore, during, and subsequent to the constant velocity stretch. The for ce increment elicited in response to each pulse decreased relative to the initial isometric value, remained essentially constant until the e nd of the ramp, and then returned to its prestretch magnitude shortly (30-40 ms) after stretch termination. These findings indicate that len gthening muscle sustains a reduction in stiffness during a ramp stretc h. 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 t o a predictable and consistent mechanical state, a necessity if predic tive control mechanisms are used to preserve elastic behavior. Second, because the steady-state force generated by lengthening muscle is dep endent largely on the velocity of stretch (and not on prior perturbati on history) for a given initial force, the mechanical properties of mu scle in the ''post-yield'' phase are largely analogous to those of a v iscous damper. Third, it appears that stretch reflex compensation chan ges the basic form of the muscle's mechanical stiffness from one domin ated by viscouslike behavior to one dominated by elastic behavior.