Wearable motion assist device usually has single axis joints, unlike some normal joints. This gap of the joint motions might cause pain or a decrease in assisting efficiency. We calculated how the efficiency affected by the gap, in case of the device to assist the knee extending torque, as standing up from a squatting posture. We considered two types of the gap on each segment, sliding along the limb axis and rolling around the intersection of the limb and device. These four gaps were in two degrees of freedom. As a result, rolling gap was under 10 degrees and did not affected much to the efficiency, on the condition without sliding gap. On the other hand, the sliding gap had the bigger impact. We proposed the noble mechanism to use the effect of the sliding gap to control the effective torque on a knee.