A chronic drop foot patient walks on a treadmill at constant, self selected speed of about $1.8{\scriptscriptstyle \frac{km}{h}}$. At each stride of the paretic foot, FES is applied with constant intensities on both stimulation channels from heel-rise to initial contact. We determine the roll and pitch indicators, as defined in (1) and (2), for various combinations of *u*_{L} and *u*_{F} within ranges that provide sufficient support to enable the patients to walk properly: At first we decrease *u*_{L} in small steps from 1 to 0 while keeping *u*_{F} constant. This is done for various values of *u*_{F}. Since hysteresis effects are common in FES, we repeat the process vice versa, i.e. keeping *u*_{L} constant while decreasing*u*_{F} from 1 to 0. Subplot (c) of Figure 2 shows pitch and roll indicators plotted against the Cartesian-like stimulation parameters *u*_{L} and *u*_{F}, while subplot (d) presents the result of the entire experimental procedure carried out for the polar-like stimulation parameters *u*_{Σ} and *ρ*.

For each constant *u*_{Σ}, the roll indicator increases almost (i.e. we simplify the discussion by ignoring very short non-monotonous sections) monotonically with *ρ*. The foot pitch, however, is almost constant if *ρ* ≥ 0. For the Cartesian-like coordinates, foot roll depends on *u*_{L} only if *u*_{F} ≤ 0.75 and foot pitch is influenced by *u*_{F} only if *u*_{L} ≥ 0.5. Recall that our goal is to generate and maintain physiological foot pitch and roll by manipulation of *u*_{L},*u*_{F} or *u*_{Σ}, *ρ*. Based on the observations from Section 1, the most intuitive approach would be to simply raise/lower *u*_{F} (or *u*_{Σ}) whenever the pitch is too low/high, respectively, and to raise/lower *u*_{L} (or *ρ*) whenever the roll is too low/high. The results in Figure 2 indicate that such a decentralized approach would be successful. For example, consider any parameter combination *u*_{Σ}, *ρ* in subplot (d) that yields too little roll. Then an increase of *ρ* eventually leads to roll values within the physiological ranges found in healthy foot motion. Likewise, a decrease of *ρ* brings too high roll indicators back to the physiological range. Analogous properties are found for the *u*_{Σ}-pitch-relation. However, at least in the considered patient, it is not possible to avoid slightly exaggerated foot roll if *u*_{F} ≥ 0.9, and the *u*_{F}-pitch-relation shows that it is not possible to avoid slightly exaggerated foot pitch if *u*_{L} ≥ 0.9.

Beyond the sole reachability of physiological foot motion, Figure 2 also allows discussion of the cross couplings, i.e. the influence of *u*_{L} and*ρ* on pitch and the influence of *u*_{F} and *u*_{Σ} on roll. If they were zero, the lines in each plot would lie on top of each other. This is approximately the case for several ranges of both coordinates. However, modifying *ρ* has a significant effect on pitch when *u*_{Σ} ≥ 0.8, and *u*_{F} as well as *u*_{Σ} influence roll for some values of *u*_{L} and *Σ*, respectively. Therefore, we find that neither of the two proposed parameterizations eliminates these cross couplings completely. Similar results are obtained in another chronic drop foot patients.

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