A Novel Hill-type Two-mode Model of Skeletal Muscle to Simulate Mechanisms Underlying Position Control on the Descending Limb of the Force–Length Relation

In a new Hill-type two-mode model of skeletal muscle, a stretch-evoked force enhancement mode is introduced in addition to a sliding filament mode on the descending limb of the isometric force–length relationship. Dynamic behaviors of muscle on the descending limb are examined in computer simulation using the model. First, force responses of the model to ramp-stretch of muscle are similar to those of frog semitendinosus muscle at different levels of muscle activation. Second, length–velocity phase trajectories of the model in isotonic tetanus contraction are similar to those of rabbit skeletal muscle. Third, firing rates of some motor units of human finger extensor muscle are slightly lower in static position control than in isometric contraction. Computer simulation is performed to reveal mechanisms underlying these behaviors. A hypothesis of static position control on the descending limb based on the simulation results is proposed; i.e., some active motor units of the finger extensor muscle operate in a stretch-evoked force enhancement mode, such that the stable finger position can be maintained owing to the spring-like property of these motor units.