Figure 1.

The two-step trajectory algorithms used to track the ankle position are illustrated. A shows the algorithm for the fixed-trajectory paradigm. At all times throughout the desired trajectory, the robotic arm is guiding the ankle of the rat to the desired position. Only when it reaches the desired position does it move to the next trajectory point (filled circle). The open circle depicts the current desired position, and the arrows are the forces that are applied to the ankle to reach the desired position. B shows the algorithm for the AAN paradigm. If the ankle of the rat is within the open circle, the robotic arms are passive and the leg is able to move freely in any direction within that area (open arrows). When the ankle leaves the circle, the robotic arms guide it back into the open circle. The robotic arms also provide a small force guiding the ankle to the next point in the programmed trajectory. The large circle represents the area where no forces are applied. The filled arrows show forces that adjust the ankle position to this area which are proportional to the distance from the desired position.