C of P vs. C of G changes during quiet stance and voluntary sway in the frontal and sagittal planes

Centre of gravity (C of G) and centre of pressure (C of P) were simultaneously analyzed, in both sagittal and frontal planes, during quiet stance and exaggerated sway about the ankle. Under these conditions, the C of P is dependent directly on the moments generated at the ankle. Thus Cc of P measurements reflect the neuromuscular system's regulation of postural sway which was quantified by the displacement of body C of G. During static posture, the C of P's average projection should coincide with the average C of G. This assumption was valid over the period of this study (7.16s). Therefore, any bias difference between C of G and C of P constitutes a C of G modeling error. The error was reduced by iterative changes to the anthropometrics. Quiet stance sway amplitude reached ±0.4 cm at very low frequencies while the C of P fluctuated more rapidly at amplitudes of ±.25 cm greater than the C of G. During exaggerated voluntary ankle sway, C of G was displaced 12.0 cm before returning to its equilibrium point. Throughout this movement, C of P overshot sway by ±1.5 cm. Angular sway acceleration was closely related to the magnitude of the difference between C of G and C of P. Thus in stance the body may be conceptualized as an inverted pendulum. Similar trends were observed in both sagittal and frontal planes. Whether during quiet stance or voluntary ankle sway, the proportionality of the CG-CP relationship was conserved.