Irreversible thermodynamics of an isometric twitch, time-dependent general formulation

We used the phenomenological laws of irreversible thermodynamics, to derive a system of coupled differential equations describing the time course of the mechanochemical coupling underlying an isometric twitch. Our general description is concerned only with the fundamental process of energy transduction taking place within an idealized unit element of muscle and does not include the description of associated processes such as the excitation-contraction coupling and the mechanical or external coupling that, in bulk muscle, lead to or mediate the fundamental process. The values of the initial conditions and of four parameters are required to solve for the time course of the process. Two of the parameters have a clear physical interpretation and, for the specific case of muscular contraction involving hydrolysis of ATP, their values can be readily estimated from available experimental data. The interpretation of the two other parameters is more speculative, but, from their strong interdependence with the other parameters, their values can be also approximated. Solution of the coupled differential equations yields results consistent with the typical experimental time course of an isometric twitch.