Center of gravity motions and ankle joint stiffness control in upright undisturbed stance modeled through a fractional Brownian motion framework

The authors modeled the center of gravity vertical projection (CG(v)) and t he difference, CP - CG(v), which, combined, constitute the center of pressu re (CP) trajectory, as fractional Brownian motion in order to investigate t heir relative contributions and their spatiotemporal articulation. The resu lts demonstrated that CG(v) and CP - CG(v) motions are both endowed in comp lementary fashion with strong stochastic and part-deterministic behaviors. In addition, if the temporal coordinates remain similar for all 3 trajector ies by definition, the switch between the successive control mechanisms app ears for shorter displacements for CP - CG(v) and CG(v) than for CP traject ories. Results deduced from both input (CG(v)) and muscular stiffness (CP - CG(v)) thus provide insight into the way the central nervous system regula tes stance control and in particular how CG and CP - CG are controlled.