Meeting Abstract
The mechanism of muscle contraction was first explained by the sliding filament theory. However, Force enhancement after stretch is a long know but poorly understood property of active muscles and can be poorly explained by this theory. Recently, a new theory of muscle contraction, the winding filament hypothesis (WFH) has been suggested. This hypothesis explains how titin acts in conjunction with sliding filament theory. Many theories have been proposed to account for force enhancement, including the idea that an elastic element forms in muscles upon activation. If activation of a passive elastic element is responsible for force enhancement, then shortening prior to stretch should reduce the extra force upon stretch. Previous research included experiments in which active muscles were shortened prior to stretch and observed no reduction in residual force enhancement (RFE) and it was observed when pre-shortening preceded stretch by ~1 second. The conclusion was that, if an elastic element is formed in muscle during activation, it is not slackened by shortening. The purpose of this study was to evaluate the effect of a delay between shortening and stretch on RFE in mouse soleus muscles. Muscles were placed initially on the descending limb of the force-length relationship. The muscles were first shortened and then stretched at a fixed amplitude (10% fiber length) and speed (40% fiber length/s), either immediately following shortening, or 100, 200, 300, 400 or 500 ms following shortening. As the interval between shortening and stretch increased, RFE increased up 300 ms, and declined thereafter. These observations are consistent with the idea that a passive structural elastic element, possibly titin, develops upon muscle activation in vertebrate skeletal muscle. These findings are consistent with the predictions of the WFH.
