Stability analysis of human stance control from the system theoretic point of view

This paper analyzes basic aspects of the human stance control. It proceeds from a linear model of the human body as an inverted pendulum. The model is used to show how humans achieve control stability despite considerable neural time delays of the sensory feedback loops that pass through higher brain centers and allow for context dependent and modifiable responses. Control stability is attributed to the fact that these long-latency (LL) loops are combined with short-latency (SL) loops of `primitive' (stereotype) reflexes via spinal cord and brainstem and with passive loops with virtually no time delay given by the intrinsic musculoskeletal stiffness and damping. The tool used for our stability analysis is an extended Nyquist Criterion. Weighting the combination of the three sets of feedback loops is identified as a key to the adaptability properties of the human stance control and its energy efficiency.