Paper C Dimensionless Time Traces of Mechanical Dynamics in Papillary Muscle Twitches

The mechanical dynamics of cardiac muscles are expressed as changes in muscle length and force. In isolated muscle, the length and force dynamics have been studied separately. Under auxotonic loading conditions, however, there are simultaneous changes in length and force. We wanted to study the differences in mechanical dynamics in twitches with different loading conditions. We represented the mechanical dynamics with two different expressions, defined as the sum of normalized stress and shortening. The first used normalized developed stress and the second used normalized instantaneous active stress as input. The resulting dimensionless time traces of mechanical dynamics demonstrated very similar shape of the contraction phase at the full range of loads from isotonic to isometric and in both auxotonic and afterloaded twitches. The same traces demonstrated wide variation during relaxation depending on load and twitch type. With the first expression, it was possible to obtain a rough estimate of peak isometric stress and peak isotonic shortening from two or more intermediate twitches. Both expressions visualized qualitative differences in load dependence between the contraction and relaxation phases, which may have implications on the clinical interpretation of systolic versus diastolic function.