Destabilizing effects of time-delay in lambda-model for human postural control

The time-delay existing in the human postural control system will cause destabilizing effects and trajectory errors. The objective of this article is to investigate whether the physiological λ-model with co-contraction can compensate the influence of time-delay, and identify the input variables of the model. In the λ-model, two neurophysiological commands from the central nervous system and a characteristic factor maintain the posture, which are the reciprocal command (R), the coactivation command (C) and the damping factor (μ). It will be shown that if the time-delay in the feedback loop is within a limit, the intrinsic feedback control can provide a stabilized posture control. The acceptable time-delay limit under which the system remains stable, is determined through simulation. We also show that the reciprocal command R only predetermines the final equilibrium point, and there is no influence on the time-delay limit; however the coactivation commands C and the damping factor μ have the influence on the limit. Therefore, they must be selected properly to ensure the system stability.