A CROSS-BRIDGE MODEL THAT IS ABLE TO EXPLAIN MECHANICAL AND ENERGETICPROPERTIES OF SHORTENING MUSCLE

The responses of muscle to steady and stepwise shortening are simulate d with a model in which actin-myosin cross-bridges cycle through two p athways distinct for the attachment-detachment kinetics and for the pr oportion of energy converted into work. Small step releases and steady shortening at low velocity (high load) favor the cycle implying simil ar to 5 nm sliding per cross-bridge interaction and similar to 100/s d etachment-reattachment process; large step releases and steady shorten ing at high velocity (low load) favor the cycle implying similar to 10 nm sliding per cross-bridge interaction and similar to 20/s detachmen t-reattachment process. The model satisfactorily predicts specific mec hanical properties of frog skeletal muscle, such as the rate of regene ration of the working stroke as measured by double-step release experi ments and the transition to steady state during multiple step releases (staircase shortening), The rate of energy liberation under different mechanical conditions is correctly reproduced by the model. During st eady shortening, the relation of energy liberation rate versus shorten ing speed attains a maximum (similar to 6 times the isometric rate) fo r shortening velocities lower than half the maximum velocity of shorte ning and declines for higher velocities. In addition, the model provid es a clue for explaining how, in different muscle types, the higher th e isometric maintenance heat, the higher the power output during stead y shortening.