Influence of internal shortening on time course of active state in rat papillary muscle.

In order to investigate internal shortening effects upon time course of active state, a study on rat papillary muscle has been carried out. Internal (or CE) shortening was evaluated according to a mechanical muscle model in which parallel elastic (PE), series elastic (SE), contractile (CE) elements are represented. CE force-velocity-shortening-time relationships have been used to describe time course of muscle active state in isometric, completely isotonic, afterloaded, and stopped isotonic contractions. The results obtained suggest that, regardless of type of contraction utilized, the parameters of muscle active state are strongly influenced by the amount of CE shortening.