The Influence of Shortening History on Dynamic Muscle Function

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Previous investigators have reported that shortening history (e.g. velocity and distance of shortening) influenced the isometric force produced subsequent to muscle shortening. Few investigators, however, have reported the influence of shortening history on dynamic muscle function. PURPOSE: The purpose of this investigation was to compare the effect of shortening history on dynamic muscle function and isometric force following muscle shortening. METHODS: Cat soleus muscle (n=4) was isolated and subjected to a series of constant velocity and constant load contractions. During both of these protocols shortening started from three initial muscle lengths and terminated at a constant final muscle length, resulting in three shortening distances (50%, 75% and 100%). Furthermore, subsequent to end of the constant velocity shortening, stimulation remained on for one second allowing the muscle to perform a final isometric contraction. The muscular force produced during the constant velocity conditions, and the shortening velocity produced during the constant load conditions were recorded and analyzed at a consistent predetermined muscle length. Repeated measures analysis was used to determine if shortening history altered isometric force following shortening, force during constant velocity conditions and velocity during constant load conditions. RESULTS: These results indicated that shortening distance altered isometric force and dynamic muscle function unequally. More specifically, shortening distance influenced isometric force following shortening by only 4.6±2.7%. On the other hand, shortening distance altered force produced during constant velocity contractions and shortening velocity during constant load contractions by up to 30.3±9.3% and 63.3±9.7% respectively. In addition, power produced at specific velocities decreased with greater shortening distances resulting in an overall downward shift in the power velocity curve. CONCLUSION: Shortening history alters dynamic muscle function to a much greater extent than isometric force. This dynamic force and velocity depression are important when considering functional electrical stimulation, motor behavior models and sport performance.