RELATIONSHIP BETWEEN SHORT-RANGE STIFFNESS AND YIELDING IN TYPE-IDENTIFIED, CHEMICALLY SKINNED MUSCLE-FIBERS FROM THE CAT TRICEPS SURAE MUSCLES

1. Transient, stretch-evoked force responses of chemically skinned mus cle fibers from the cat hindlimb were investigated. The purpose of the se experiments was to determine the exent to which short-range stiffne ss, the apparent stiffness exerted by the fiber over the first 0.5% of length change, is higher in type I than type II muscle fibers. Fibers were obtained from soleus and vastus intermedius muscles, which conta in predominantly type I fibers, the LGm, a compartment of the lateral gastrocnemius muscle that contains predominantly type II fibers, and L G3, a compartment of mixed type. 2. Beyond a short range of similar to 1% of muscle length during a 0.5 muscle length/s (ML/s) stretch, most fibers exhibited an abrupt decrease in apparent stiffness or yield. F ibers from the muscles containing predominantly type S (slow twitch, o r type I) fibers, soleus and vastus intermedius, exhibited such a pron ounced decline in apparent stiffness that force declined as well durin g continued stretch. Most of the fibers from the LG3 compartment could be divided into two distinct groups depending upon whether or not the y showed a force yield at the stretch velocity of 0.5 ML/s. 3. The sho rt-range stiffness measured over the first 0.5% of stretch was greater for fibers showing force yield than for those that did not at matched initial forces and normalized stretch amplitudes. This result is cons istent with the hypothesis that the same mechanism that endows the fib er with high short-range stiffness is also responsible for a greater e xtent of yielding. 4. Fibers from soleus were found to exhibit a force yield over a 200-fold range of velocities (0.01-2 ML/s). In contrast, most fibers from the LGm compartment showed only an increase in exten t of yield with stretch velocity. Some of these fibers eventually yiel ded in force, but only when they were stretched at velocities >2 ML/s. The proposed relationship between high short-range stiffness and yiel ding was supported by the finding that short-range stiffness increased sharply in the range of velocities where the fiber showed the greates t increase in extent of yield. 5. After the physiological experiments, fibers were subjected to SDS gel electrophoresis. Two distinct patter ns of bands in the low molecular weight range were found to correspond to the two types that were identified on the basis of their dynamic m echanical properties. Fibers that did not yield at 0.5 ML/s showed a b and pattern very similar to that of rabbit psoas (type II) fibers. 6. These results support the hypothesis that type I fibers are specialize d in presenting a high short-range stiffness for effective postural co ntrol in advance of reflex mechanisms and that this property results f rom intrinsic properties of the fiber and is not due to differences in the dimensions of type I and II fibers. Yielding serves to protect th e fiber from damaging levels of force during lengthening contractions. The importance of these transient properties to the mechanical behavi or of muscle during ongoing movements is suggested by the observation that high stiffness followed by yielding is repeated with little alter ation during successive stretches.