Abstract
BackgroundUpright standing requires control of an inherently unstable multi-joint human body within a small base of support, despite biological motor and / or sensory noise which challenge balance. Without applying perturbations, system identification methods have been regarded as inadequate, because the relevant internal biological noise processes are not accessible to direct measurement. As a result, unperturbed balance studies have been limited to investigation of behavioral patterns rather than possible underlying control strategies.MethodsIn this paper, we present a mathemathically rigorous system identification method that is applicable to study the dynamics and control of unperturbed balance. The method is derived from autocorrelation matrices with non-zero time lags and identifies the system matrix of a discrete-time dynamic system in the presence of unknown noise processes, without requiring any information about the strength of the noise.ResultsUnlike reasonable ‘least-squares’ approaches, the performance of the new method is consistent across a range of different combinations of internal and measurement noise strengths, even when measurement noise is substantial. We present a numerical example of a model that simulates human upright balancing and show that its dynamics can be identified accurately. With a biomechanically reasonable choice of state and input variables, a state feedback controller can also be identified.ConclusionsThis study provides a new method to correctly identify the dynamics of human standing without the need for known external perturbations. The method was numerically validated using simulation that included realistic features of human balance. This method avoids potential issues of adaptation or possible reflex responses evoked by external perturbations, and does not require expensive in-lab, high-precision measurement equipment. It may eventually enable diagnosis and treatment of individuals with impaired balance, and the development of safe and effective assistive and / or rehabilitative technologies.
Highlights
Upright standing requires control of an inherently unstable multi-joint human body within a small base of support, despite biological motor and / or sensory noise which challenge balance
External perturbations are applied to challenge participants’ balance, e.g. by applying pushing/ pulling forces or translating/rotating a platform on which they stand. Those perturbations have traditionally been regarded as necessary to identify the dynamics of human postural control, because the input and output are directly measured, allowing application of well-established closed-loop system identification techniques to obtain a robust and reliable input-output dynamic relation [2,3,4, 6]
Equipped with the new method, natural human postural dynamics and control can be studied in depth without concern for adaptation or possible reflex responses evoked by external perturbations, or any need for expensive high-precision measurement equipment
Summary
Upright standing requires control of an inherently unstable multi-joint human body within a small base of support, despite biological motor and / or sensory noise which challenge balance. External perturbations are applied to challenge participants’ balance, e.g. by applying pushing/ pulling forces or translating/rotating a platform on which they stand Those perturbations have traditionally been regarded as necessary to identify the dynamics of human postural control, because the input (external perturbation) and output (motion in response to the perturbation) are directly measured, allowing application of well-established closed-loop system identification techniques to obtain a robust and reliable input-output dynamic relation [2,3,4, 6]. The closed-loop dynamics and control estimated in this way may not well represent those of daily activity
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