Series elasticity in frog sartorius muscle during release and stretch

When a stretch is applied to an isolated muscle during tetanic stimulation, the force developed is higher than the maximal isometric tension (Po). This force puts the series elastic component (SEC) under tension and in a domain which is not well defined in terms of tension-extension curve. In the present work, an attempt was made to determine the stiffness of the SEC for tensions greater than Po, using the sartorius muscle of the frog. For this purpose, rapid releases and stretches of different amplitudes were given during maximal isometric contractions. Plotting normalized tension (P/Po) against normalized length changes (negative or positive extensions, delta L/Lo.10(2] produced a tension-extension curve. The slopes of the linear part of each relationship on both sides of Po indicated an increase in SEC stiffness when the muscle was rapidly stretched. Furthermore, the transient character of the increase in stiffness was studied by measuring SEC stiffness during rapid releases applied at various time intervals after stretches: the muscle was found to be stiffer as the time interval was shorter. The results are discussed in terms of (i) non-linear behaviour of the passive and active parts of the SEC, (ii) enhancement of storage and release of potential energy.