COMPARISON OF THE CARDIAC FORCE TIME INTEGRAL WITH ENERGETICS USING ACARDIAC-MUSCLE MODEL

Several investigators have found experimentally that the force-time in tegral varies non-linearly with energy expenditure over the course of a cardiac contraction. Also, recent research findings have indicated t hat the crossbridge cycle to ATP hydrolysis ratio in muscle fiber syst ems may not be coupled with a one-to-one ratio. In order to investigat e these findings, Huxley's sliding filament crossbridge muscle model c oupled with parallel and series elastic components was simulated to ex amine the behavior of the crossbridge energy utilization and force-tim e integral vs time. Crossbridge (CB) energy utilization was determined by considering the ATP hydrolysis for the crossbridge cycling, and th is CB energy was compared with the force-length energy in a contractio n. This CB energy was calculated in both isometric and isotonic contra ctions as a function of contraction time and compared to the force-tim e integral. Simulation results demonstrated that the ratio of the forc e-time integral to CB energy varies strongly throughout the cardiac cy cle for both isometric and isotonic cases, as has been observed experi mentally. Simulations also showed that using the force-length energy c omponent of energy vs the CB energy gave a better correlation between the total energetic predictions and the force-time integral, agreeing with recent finding that the crossbridge cycle to ATP hydrolysis ratio may not be coupled one-to-one, especially at lower force levels.