A curve-fitting procedure to explain changes in muscle force–velocity relationship induced by hyperactivity

Experiments have shown that a period of hyperactivity induces changes in the muscle force–velocity relationship. The goal of this study was to explain such changes by taking into account that the myosin heavy chain (MHC) composition of a muscle is a primary determinant of its shortening velocity. For this purpose a mathematical model was developed where the force–velocity relationship of the whole muscle was built by summing the force contributions of individual components at each of a series of shortening velocities. An individual force–velocity relationship was assigned to each component, i.e. each type of MHC. Experimental data were obtained on control and hyperactivated epitrochlearis muscles from rats. In the controls rats, fitting of the model with experimental data was satisfactory. In hyperactivated muscles, parameters of the fastest MHC component had to be modified. This improved the fit between model and experimental data and accounted for possible changes in myosin light chain composition.