Fig. 5.

Surface plot of the logistic regression that can best predict the likelihood of falls following a sudden, unannounced slip during gait as function of its two best determinants: the stability (s) and limb support quotient (S_hip) at the instant immediately prior to the fall recovery step touchdown. The logistic regression model is $p\left(\text{fall}\right)=\frac{1}{1+e^{31.0+25.0s+146.8S_{hip}}}$ (p < 0.05 for all model coefficients). Also shown is the boundary (the outline of the limits of recovery) formed at the middle (50%) bisecting the surface. The projection of the boundary on the bottom plane (i.e. the s-S_hip domain) is also demonstrated, which is the boundary demonstrated in Fig. 3. One recovery trial (the open diamond) was wrongfully classified as a fall by the limits as a type II or β error.