Fig. 2

FPA definition. A dynamic foot frame, that is initialized in every stance phase i (for left (L) and right (R) separately) and is maintained until the consecutive stance phase of the same foot, is used for calculating the FPA as the angle between the foot direction and the walking direction. The x-direction of this dynamic foot frame (\(\psi _x^{F_{L/R,i}}\)) aligns with the foot direction. All signals are integrated in this dynamic foot frame to obtain a foot trajectory that ends at the next stance phase. This walking direction is shown in red and defined as the position vector between the calcaneus of two consecutive stance phases (with \(p^{F_{L/R,i}}_{t_{end}}\), where \(t_{end}\) is the step duration). This allows for direct computation of the FPA