FIGURE 1.

The two-segments model of the foot during the stance phase. Using the RGB convention, {FF_rear} represents the FF of the rearfoot segment, {FF_fore} the FF of the forefoot segment, and {GF} the room’s global frame. Points p and q are arbitrarily placed on rearfoot, and forefoot segments, c′ and c are, respectively, hypothetic and optimum rearfoot-forefoot joint’s center. ${\overrightarrow{a}}_{c,q}$ is the acceleration at c estimated from q, ${\overrightarrow{a}}_{c,p}$ the acceleration at c estimated from p, ${\overrightarrow{a}}_{treadmill}$ the acceleration of the treadmill, and $\overrightarrow{g}$ the Earth gravitational acceleration. Finally, δ is the orientation difference (i.e., quaternion) between ${\overrightarrow{a}}_{c,p}$ and ${\overrightarrow{a}}_{c,q}$ while ${\overrightarrow{r}}_{pc}$ and ${\overrightarrow{r}}_{qc}$ are the distance vectors from point p to c and from q to c, respectively.