## 4 Discussion  
We investigated the EMG activity of the ADP muscle during the operation of power-assisted scissors compared with conventional scissors in healthy females. Our findings indicated that the MF of the ADP was better maintained with the use of power-assisted scissors under 45\({}^{\circ}\) wrist dorsiflexion. Additionally, the power-assisted scissors resulted in  
Figure 4: Comparison of muscle activity levels with and without power assistance. The probability of maximum voluntary electrical activity (MVE) of the ADP is plotted for all subjects. (**A**) APDF analysis plot of the ADP in the middle position of the wrist joint, which was used to compare muscle activities. (**B**) APDF plots at 45\({}^{\circ}\) wrist dorsiflexion, which were used to compare muscle activities. The broken lines (left side of panels (**A**,**B**)) represent APDF values calculated from the surface electromyogram during scissors manipulation (y-axis: amplitude probability; x-axis: %MVE; z-axis: subject). The APDF analysis results indicate the means of the 10 th, 50 th, and 90 th percentiles of %MVE. The yellow line indicates power support, and the gray line indicates no power. * \(p\) < 0.05.  
lower static load levels (P0.1), mean load levels (P0.5), and peak load levels (P0.9) of %MVE in the ADP, accompanied by reduced subjective fatigue and pain. These results support the hypothesis that power-assisted scissors can mitigate muscle fatigue during operation. The implications of these findings are significant for reducing the risk of MSDs in hairdressers. The results of our study indicated that power-assisted scissors effectively maintained the MF band in the ADP under wrist dorsiflexion conditions, thereby preventing the onset of muscle fatigue. Muscle fatigue is defined as a decline in the muscle's ability to generate force [33] and is affected by both peripheral and central factors [34]. Peripheral fatigue can be caused by restricted blood flow in the muscles, limiting anaerobic metabolism and preventing the neutralization of lactic acid, which impairs muscle relaxation [35]. Previous studies have indicated that individuals with greater muscle mass experience less fatigue after exercise at the same intensity [36], and this is corroborated by the relationship between muscle mass and fatigue indicators based on sEMG measurements [37]. The relatively small mass of the ADP muscle, being intrinsic to the wrist, may make it more susceptible to fatigue under conditions of increased intracarpal pressure during wrist dorsiflexion [26]. Reports defining fatigue during repetitive finger flexion tasks using sEMG indicate that fatigue occurs in a linear regression pattern over time (50% MVC: 300 s; 60% MVC: 240 s; 70% MVC: 50 s) [38]. The phenomenon of slow-wave formation being prevented at 45\({}^{\circ}\) wrist dorsiflexion in this study suggests that power-assisted scissors effectively supplemented the muscle output. In our data, the %MVE (P0.5) at 45\({}^{\circ}\) wrist dorsiflexion was 16.7% MVE with assistance and 20.6% MVE without assistance, indicating a reduction in the EMG discharge volume under the power-assisted condition. The developed power-assist mechanism provides a consistent force in the adduction direction of the thumb while accurately fixing the index, middle, ring, and little fingers. The extensor digitorum, which fixes the upper blade, functions extrinsically from the forearm, and the finger positioning is directly affected by the wrist angle. During wrist extension, the fingers naturally curl, forming a tenodesis grip [39]. This phenomenon indicates that extrinsic finger flexors stretch depending on the wrist angle, impacting flexor tension [40, 41]. Furthermore, finger movements generate torque at the wrist [42], necessitating a counteracting torque to maintain the wrist angle. sEMG studies on the relationship between the finger extensor muscle activity and the wrist dorsiflexion angle have indicated changes up to 70% [43].