Myocardial mechanics and the Fenn effect determined from a cardiac muscle crossbridge model

A three-element cardiac muscle fibre model, utilising Huxley's sliding filament theory for the contractile element and coupled with parallel and series elastic components, was simulated to see if it were possible to predict the cardiac Fenn effect. The force/length energy (FLE) was computed in both isometric and isotonic contractions, as a function of muscle fibre length (preload) in the isometric case and afterload in the isotonic contraction case. Simulation results demonstrated that isotonic contractions produced a greater FLE than isometric contractions at every corresponding afterload, with the difference being equal to the work produced in the isotonic case, which is characteristic of the Fenn effect. The maximum energy utilisation was observed at maximum force isometric contractions, as has been experimentally observed in cardiac muscle. Changing the stiffness of the series elastic component did not change the Fenn-effect behaviour. Fenn-effect plots using crossbridge energy predictions showed behaviour similar to the FLE plots, but the FLE: crossbridge energy ratio declined with decreasing force even though the efficiency has been experimentally found to be constant.