Force length time relations in heart muscle contracting under various mechanical loadings

In order to directly contribute to the understanding of ventricular contraction, we attempted to obtain a relatively simple description of force length time (FLT) relations in cat papillary muscle contracting under various modes. These modes include isometric contraction, isotonic contraction, combination of these two, and contraction against a spring having linear stiffness. To obtain these modes of contractions, an electromechanical servo system was constructed with length, force or both as feedback signals. The servo system controls the relation between force and length rather than either one of the two. The end systolic FL relation curves determined under all modes of contraction were quasilinear but did not superimpose on each other. The end systolic FL relation from isometric and isotonic contractions bounded the FL curves from the last 2 contracting modes.