Identification of the time varying mechanics of isolated skeletal muscle

A method for identifying the mechanics of systems with rapidly changing characteristics is presented for the case in which the characteristics of the system are changing in the same order of time as its dynamics. Results of muscle twitch experiments on frogs are presented. The length perturbation input and muscle force output data ensembles were analysed to yield instantaneous measures of muscle stiffness throughout a muscle twitch. The results clearly show muscle stiffness rising and then decaying during twitch. Stiffness as a function of force exhibits no hysteresis, and except at low forces, the relationship is linear. The variance accounted for at low forces is 0.6 compared with values greater than 0.9 at moderate and high forces. This departure from linearity at low forces indicates that nonlinear analysis of muscle mechanics must be performed