# Verification of Industrial Worker Walking Efficiency with Wearable Hip Exoskeleton
## 4 Discussion
### Ascending Stairways  
The walking efficiency of a wearable hip exoskeleton was verified through a stairway ascension experiment. The results showed that the average muscle activity of the RF, ST, TA, and GCM decreased when wearing the WIM, and that a statistically significant difference was observed in all the muscles except the TA. Additionally, when wearing the WIM, fatigue in the ST, TA, and GCM, but excluding the RF, tended to decrease; however, only the TA showed statistical significance. Treadmill walking significantly reduced GCM fatigue, whereas ascending stairs significantly reduced TA muscle fatigue. During stair ascension, the plantar flexors (gastrocnemius) and vastus medialis lateralis primarily exert vertical propulsion during the stance phase, whereas the gluteus maximus and hamstrings act as the driving force for forward propulsion. Additionally, during the swing phase, the antagonist muscles across the hip, knee, and ankle joints control the leg by distributing the force from the leg to the rest of the body [37]. This study did not measure the muscle activity of all prime movers used in ascending stairs. However, the fatigue of the GCM, which is the driving force of vertical propulsion, decreased by 76.1%, and the muscle fatigue of the ST, which is involved in forward propulsion, was also reduced by 206.2%, indicating that the assistive power of the WIM was effective. Meanwhile, RF muscle fatigue significantly increased during stair walking, whereas the average muscle activity decreased by 16.8%. As mentioned, it has been confirmed that the patterns of muscle activity and fatigue are different [31]. In addition, muscle fatigue increased despite a decrease in the RF muscle activity when climbing stairs. This suggests that a factor causing muscle fatigue was acting on the muscles, even with the WIM device assisting. In fact, the participants in this study reported that although the WIM made it easier to walk up stairs, they complained of discomfort owing to the mismatch between leg movements and the thigh support frame when climbing stairs. This is believed to not provide optimal control because unlike walking on level ground, the range of the hip flexion angle increases when climbing stairs. It is presumed that these factors may have caused muscle fatigue by interfering with the normal muscle activity of the RF involved in hip joint movement. Improvements in this regard should be addressed going forward. Previous studies related to verifying the efficiency of walking-assistive exoskeletons have mainly analyzed muscle activity. [PERSON] et al. (2017) interpreted the decrease in muscle activity of the RF as being because of a decrease in muscle activity and an increase in walking efficiency from wearing the exoskeleton [13]. In addition, [PERSON] et al. (2020) reported that the muscle activity of the RF decreased by up to 47% when walking up stairs using the soft exosuit system [38]. As mentioned in previous studies, the results of muscle activity and fatigue do not necessarily match, because fatigue can increase even if muscle activity is low. Therefore, discussions on walking efficiency based on decreased muscle activity should be viewed with caution, as fatigue can be caused by prolonged use despite reduced muscle activity. Studies of wearable exoskeleton walking efficiency should verify muscle activity and muscle fatigue. In analyzing the energy expense of stairway ascension, it was found that there was no significant difference in the performance time with and without a WIM. However, while this was not true for the total EE, the oxygen uptake, heart rate, EEM, and net EEM showed statistically significant decreases. Wearing a WIM significantly reduced the oxygen uptake by approximately 14%, the heart rate was significantly reduced by 7.9%, the EEM by 12.9%, and the net EEM by 15.7%. Unlike treadmill walking, the significant decrease in heart rate during the more difficult stair walking was interpreted as a significant benefit of the assistive power of the WIM during stair walking.