An apparatus interferes with voluntary locomotion of a target by conducting a current through the target. The apparatus includes a current delivery circuit, a detector, and a processor. The current delivery circuit delivers the current in accordance with a goal for causing pain or skeletal muscle contractions that interfere with voluntary locomotion by the target. The detector detects the current delivered through the target to provide a result. The processor adjusts the goal in accordance with a binary search in response to the result.