Stretch -shorten cycle work enhancement and connective tissue mechanical properties of the rat soleus during variable recruitment in situ.

摘要

A fundamental property of muscle function in vivo is its ability to vary force output in response to functional demand. This allows for movements at any joint to vary in their speed and intensity. It is not well understood, however, how varying force production by a muscle affects its interaction with the environment. This study applied ventral root stimulation to achieve a realistic model of in vivo recruitment. This technique allows the number of active fibers to be varied, rather than varying the force output of all muscle fibers simultaneously. This in vivo -like recruitment protocol was applied to two questions. First, are the stress-strain properties of the free tendon and aponeurosis dependent on the recruitment level of the muscle? In particular, do those properties differ at low recruitment levels when the active muscle fibers may not be uniformly distributed in the muscle? The second question addressed the interaction between muscle and connective tissue during dynamic. Specifically, is the improvement in muscle shortening work following an active pre-stretch dependent on the recruitment level of the muscle?;The stiffness of the tendon and four longitudinal regions of the aponeurosis did not vary with recruitment at the forces used in this study. However, the toe region of the force-strain relationship was very compliant under passive loading. This compliance caused contraction-induced strain to decrease as muscle length was increased, while total strain (passive + active) was constant at all lengths. This relationship indicates that internal muscle fiber shortening is reduced as muscle length increases.;The shortening work done by the soleus muscle was increased by all four types of stretch-shorten cycles used. The amount of enhancement was not influenced by recruitment. There was no difference in the enhancement following stretches that remained within or exceeded the elastic range of the cross-bridges, indicating that elastic energy storage in these elements is not an important factor in increasing work. Additionally, when different stretch lengths were used to attain the same peak strain in the connective tissue for different recruitment levels, more work was always done at the higher recruitment level.