We numerically compute the spatially resolved stresses in an ovine stifle joint during aktive motion. For this, a sheep gait was measured using bone markers and the bone geometry was recorded by a computer tomograph. The motion was then applied as time-dependent boundary data to a finite element model of the femur and tibia of the sheep. A novel numerical method allowed to compute the contact stresses with high precision and speed. It was therefore possible to investigate the influence of the motion error on the stresses, in particular on the medio-lateral distribution of the total contact force.