Frequency-dependent contributions of sagittal-plane foot force to upright human standing.

Quiet standing is a mechanically unstable postural objective that humans typically perform with ease. Control of upright posture requires stabilization of both translational and rotational degrees-of-freedom that is accomplished by neuro-muscular coordination. This coordination produces a force at the ground-foot interface (F) that is quantified by magnitude, direction (θF), and point of application (center-of-pressure, CP). Previous research has shown that the nervous system controls muscle activation such that CP motion occurs at both slow and fast time scales. However, it is unknown how θF varies with respect to CP and how that relationship varies across time scales. We present a novel method for assessing the frequency-dependent relative variation in θF and CP. The center-of-pressure (CP) and direction of the ground-on-foot force (F) in the sagittal-plane during quiet standing were decomposed into 0.2 Hz-width frequency bands within 0.4-8.0 Hz. The relation between the direction and CP was approximately linear with a slope positively related to frequency. These frequency-dependent features of F have critical implications for understanding balance strategy because the translational and rotational acceleration effects of F were coupled, but with opposite phasing at high versus low frequencies. Such results suggest a system tuned for one stability mode at low frequencies and another mode at higher frequencies. This frequency-wise approach to examining the translational and rotational effects of humans' preferred F may be useful for establishing balance rehabilitation metrics, directing study of the underlying neural mechanisms responsible for the observed coordination, and for setting a biometric standard to inform biomimetic prosthetics and robotics.