Forces on Fecobionics acting on the normal and along the anorectal segment. The forces are the contractile forces from the segment (P_w), the pressure in the bag (P_b), the friction force between the device surface and the wall (F_f), the pressure from the fluid within the segment at the inlet (rear pressure, P_r), the pressure from the segment and the atmosphere at the outlet (front pressure, P_f), and also the gravity (G) of Fecobionics. X, Y, and Z are global coordinates. S and θ are the local coordinates with arc of length s along the curve of symmetry and θ the polar angle, respectively.