Active control vs. passive stiffness in posture and movement coordination

The neuro-physiological mechanisms involved in postural stabilization are not well understood. Human body mechanically resembles an inverted pendulum that is inherently unstable. Active and passive mechanisms at muscle and spinal level as well as visual and vestibular processes are attributed to postural stabilization. At the same time, intrinsic delays in the reflex pathways and the low-pass characteristics of the muscle response tend to limit the effectiveness of active mechanisms of stabilization. The motivation for this research was to study the relative contribution from active control (feedforward mechanisms) and passive stiffness (feedback mechanisms) in maintenance of posture and coordination of voluntary movement. We develop a multi-segment sagittal model with three degrees of freedom that included the rotation at ankle, knee, and hip joints. We propose an optimal LQR controller as the central nervous system analog in posture and movement coordination. We present analytical and simulation results to support an active-passive model of postural stabilization. Besides expanding our understanding of the stabilization processes in the body, the insight gained from this study is expected to promote awareness of the existence of optimal trajectories in the coordination of skilled voluntary movements.