## 5 Experiments
It was difficult for the foot force switch-based method to recognize walking forward and backward based only on the outputs of the force sensors of the feet. This was one of the drawbacks of the method. The gravity compensation method showed good result, but the walk tended to be slow because it did not assist the horizontal motion of the leg, even though it assisted vertical leg motion, such that the wearer had to firmly push his leg back. Table 1 supports the analysis in terms of the walking speed for the motion \"walking backward\". Figure 9: Image sequences of walking backward based on each method: the sampling time of the image capture was approximately 5.4 s.  
Figure 10 shows the average load on the wearer's shoulder while walking forward and backward. The proposed method and the foot force switch method maintained a load of approximately 100 N whereas the gravity compensation method maintained one of 350 N. If there was no assist control, the wearer had approximately 350 N on his shoulders. The proposed method successfully reduced the load. It depends on the motion database \(D\). When the datasets for the database were sampled, the powered exoskeleton was hung up on the gantry so that the wearer had no load due to the exoskeleton. Therefore, the proposed method lifted the exoskeleton. To maintain the back-drivability of the powered exoskeleton, we kept the control gain as small as possible. An approximately 100 N load on the shoulder was imposed because of the small control gain for back-drivability. The gravity compensation method did not reduce the load on the wearer's shoulder. If the control gains \(k_{k}\) and \(k_{h}\) in Equations (3) and (4) became large, the load on the wearer's shoulder in the upright position became small, but it became challenging for the wearer to swing the leg because the powered exoskeleton tried to keep the leg as vertical as possible. It eventually lost back-drivability. To retain back-drivability, the control gains \(k_{k}\) and \(k_{h}\) needed to be small, in which case the system failed to reduce the load on the wearer's shoulder. The foot force switch-based method was as good as the proposed method according to Figure 10.  
Figure 11 shows the horizontal front-back reaction force measured by foot force sensors while the wearer walked based on each control method. The reaction forces on the left leg under the proposed method and the force switch-based method were smaller than that for the gravity compensation method. This was because the gravity compensation method did not consider the motion of the feet in the horizontal direction. The proposed method used motion sensors to predict the posture of the  
Figure 10: Average load on the shoulders while walking. powered exoskeleton and successfully reduced the reaction forces on the feet in the horizontal direction. The force switch-based method also reduced the reaction forces because the pre-defined motion for the method fed the swinging leg forward and the standing leg backward. The reaction force on the right leg was comparatively small when the gravity compensation was applied because of the wearer's gait. Figure 12 shows the horizontal front-back force measured by the foot force sensors while the wearer walked backward based on each control method. The proposed method showed the smallest magnitudes of forces during this. This was because it appropriately recognized the wearer's motion and controlled the legs of the powered exoskeleton based on the estimated motion. The gravity compensation method yielded the highest resistance force to the wearer's legs because it did not consider the motion of the feet in the horizontal direction, as mentioned above. The force switch-based method failed to support backward walking because it could not distinguish between walking forward and backward, and the pre-defined motion for the method was designed for forward walking. Eventually, the wearer had to push the swinging leg more strongly. The reaction force on the right foot was small because of the manner of the wearer's walk.