Time behavior of series elasticity in cardiac muscle. Real-time measurement by controlled-length techniques.

Controlled-length quick releases were applied to cat papillary muscle at various times during an isometric contraction to study the series elasticity of cardiac muscle relative to time. Force and length were measured continuously during the course of constant-velocity releases. Series elastic compliance was evaluated from semilogarithmic plots with force referred to plateau levels. The force-extension curve of the series elastic element (SE) was exponential; the compliance of the SE varied with force independently of time. Series elastic compliance relative to time was not altered by changes in initial muscle length. However, when series elastic compliance was measured by extrapolating tension to the end of the release, it appeared to be greater earlier in the twitch. This finding was misleading; it resulted from inadequate release velocities in relation to varying rates of tension redevelopment at different times in the twitch. Force-extension curves of the SE were nearly identical at different initial muscle lengths. Yet, conventional comparisons of series elastic compliance in terms of shortening of the SE between peak tension and preload showed wide variation with initial length, because different portions of the force-extension curve were measured. This problem was avoided by expressing series elastic compliance in terms of the exponential length change (xf) necessary to reduce force by the factor 1/e = 0.368 anywhere on the force-extension curve. The average value of xe in 23 determinations was 1.87 ± 0.26% of initial muscle length; this value is about 25% smaller than that reported previously. Because of the exponential shape of the force-extension curve of the SE, the possibility of substantial series elasticity in myofilament cross-bridges cannot be excluded. However, until convincing evidence is presented to the contrary, it is suggested that the series elasticity of cardiac muscle be regarded as a passive element as it is in the traditional Hill model.