The Effects of Parallel and Series Elastic Components on the Active Cat Soleus Force-Length Relationship

Typically, active muscle force is calculated by subtracting measured passive force from measured total force for corresponding whole muscle lengths (standard method). From a mechanical point of view, this requires a parallel elastic component (PEC) that is arranged in parallel to both the series elastic component (SEC) and the contractile component (CC). From a morphological point of view, however, the PEC should be rather in parallel to the CC, and both in series to the SEC (model [CC]). In this study, we investigated the differences in active muscle force estimated with these two different approaches and their impact on the interpretation of experiments. We measured passive forces without stimulation and total forces during supramaximal stimulation of six cat soleus muscles in end-held isometric contractions from lengths near active insufficiency to lengths close to inducing stretch damage. The active forces estimated with model [CC] reach about 10% higher maximum isometric forces and reveal about 10% longer optimal lengths of the CC compared to the standard method. Model choice affects the interpretation of the physiological working range and residual force enhancement. The active force-length relationships of the contractile component determined with model [CC] agree better with the theoretical sarcomere force-length relationship.