History-dependent muscle forces for sensing and moving in normal and impaired movement

Muscles exhibit history-dependence where forces in response to stretch are higher if muscle is isometric prior to stretch. This short-range stiffness is critical for generating the initial stabilizing torques reactive balance after perturbations, before sensorimotor muscle responses occur. Moreover, history-dependent muscles spindle firing can be directly predicted from history-dependence force in intrafusal muscle fibers, and predicted using cross-bridge model. We show that muscle spindles encode intrafusal muscle fiber force and rate change in force (dF/dt, or yank), in contrast to the classical concept of encoding muscle length and velocity. Many previously unexplained phenomena about muscle function, including paradoxical muscle spindle firing during isometric contraction can be explained by a cross-bridge based muscle spindle model. Finally, this new insight led us to a new hypothesis and simulations demonstrating the role of history-dependent muscle forces and reflexes in spasticity associated with cerebral palsy.