2023 Volume 26 Pages 43-52
In the descending limb, where the muscle length is longer than the natural length, isometric force decreases with increasing muscle length. This implies negative stiffness, and the system is theoretically unstable. In vivo, however, stable positional control is achieved. Few studies have explored position control in the descending limb. We herein report a new two-mode Hill-type muscle model created based on the results obtained from physiological experiments of isolated frog semitendinosus muscle. Next, based on the experimental results of the human extensor digitorum communis muscle operating in the descending limb, we show that there exists a motor unit in which the firing frequency is lower during static position control than during isometric force control. By comparing the physiological results of this motor unit with the behavior of the model, we constructed a stable static position control mechanism in the descending limb experimentally and theoretically.
