Background Maintaining upright posture is an unstable task that requires sophisticated neuro-muscular control. Humans use foot–ground interaction forces, characterized by point of application, magnitude, and direction to manage body accelerations. When analyzing the directions of the ground reaction forces of standing humans in the frequency domain, previous work found a consistent pattern in different frequency bands. To test whether this frequency-dependent behavior provided a distinctive signature of neural control or was a necessary consequence of biomechanics, this study simulated quiet standing and compared the results with human subject data. Methods Aiming to develop the simplest competent and neuromechanically justifiable dynamic model that could account for the pattern observed across multiple subjects, we first explored the minimum number of degrees of freedom required for the model. Then, we applied a well-established optimal control method that was parameterized to maximize physiologically-relevant insight to stabilize the balancing model. Results If a standing human was modeled as a single inverted pendulum, no controller could reproduce the experimentally observed pattern. The simplest competent model that approximated a standing human was a double inverted pendulum with torque-actuated ankle and hip joints. A range of controller parameters could stabilize this model and reproduce the general trend observed in experimental data; this result seems to indicate a biomechanical constraint and not a consequence of control. However, details of the frequency-dependent pattern varied substantially across tested control parameter values. The set of parameters that best reproduced the human experimental results suggests that the control strategy employed by human subjects to maintain quiet standing was best described by minimal control effort with an emphasis on ankle torque. Conclusions The findings suggest that the frequency-dependent pattern of ground reaction forces observed in quiet standing conveys quantitative information about human control strategies. This study’s method might be extended to investigate human neural control strategies in different contexts of balance, such as with an assistive device or in neurologically impaired subjects.

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Background: Maintaining upright posture is an unstable task that requires control of translational and rotational motions. Humans use foot-ground interaction force, characterized by point of application, magnitude, and direction to manage body accelerations. Previous work identified a point of intersection of the foot-ground interaction force vectors that exhibited consistent frequency-dependent behavior. Methods: To test whether this frequency-dependent behavior provided a distinctive signature of neural control or was a necessary consequence of biomechanics, this study simulated quiet standing and compared the results with human subject data. If a standing human was modeled as a single inverted pendulum, no controller could reproduce the experimentally observed frequency-dependence of the intersection point height. The simplest competent model that approximated a standing human was a double inverted pendulum with torque-actuated ankle and hip joints. It was stabilized by a linear feedback controller based on position and velocity errors of each joint. Results: When the relative cost between state deviation and control effort was varied, the frequency at which the intersection point crossed the center of mass position shifted. A similar effect was obtained by varying the relative cost between the ankle and hip control effort. The relative strength of ankle and hip actuation noise added to the simulated system affected the intersection point height at high frequencies. Conclusions: As a range of controller parameter sets could stabilize this model and produce the observed change in the vertical position of the intersection point with increasing frequency, the decrease in intersection point height appears to reflect a biomechanical constraint and not a consequence of control. Among the several controller parameter sets considered, that which best reproduced the human experimental results used minimal control effort and more ankle torque than hip torque. This suggests that the neural strategy employed by human subjects to maintain quiet standing balance engages at least two degrees of freedom and is best described by minimal control eort and emphasizing ankle torque.