## 4 Discussion
It has also been reported that long-term repetitive work causes fatigue and pain in the entire upper limb [2]. However, fatigue and pain decreased with our power-assist device. [PERSON] pointed out that, in vocational training tasks when working at an average load level (P0.5) of 1 h or more, muscle load becomes excessive when the %MVE value exceeds 10% [25]. The %MVE at the average load level (P0.5) in this study was 9.4 %MVE (with assistance) and 12.7 %MVE (without assistance), and the muscle output was reduced to 10 %MVE or less by power assistance. [PERSON] et al. reported that the static load level (P0.1) was 2-6 %MVE when measuring the muscle activity of the extensor/flexor muscles of the forearm during haircutting movements of a female hairdresser [3]. The %MVE of the static load level in this study was similar (6%), but was attenuated by approximately 2% with power assistance. These results also indicate that the power-support device compensates for the muscle burden. Fatigue associated with muscle activity results in a temporary decrease in skeletal muscle strength and capacity [26], with impaired activation of the motor neurons that drive the muscle fibers [27]. Therefore, it is inferred that the attenuation of the load due to the assistance maintained the skeletal muscle resources of the ADP, and fatigue was less likely to occur. In a report on cosmetologist work and risk, [PERSON] et al. found that sustained muscle activity correlates with pain [28], and pain due to repetitive movements increases inflammatory markers due to muscle damage [29]. Controlling the muscles that participate in movement is effective in the prevention of muscle damage [30]. Therefore, it can be inferred that pain in the hands may be reduced as a result of decreasing the muscle burden with scissors equipped with a power-support device. This study had several limitations. First, the participants in this study were female university students, and not cosmetologists. The results of this study cannot be applied to hairdressers, as it has been reported that the average working hours of hairdressers is 8 to 12 h per day [3,31], and the experimental time in this study was 100 s. Compared to the subjects of this study, hairdressers routinely work with scissors for long periods, so there is a difference in endurance and skill levels. Therefore, in future studies, hairdressers should be used as subjects, and an analysis comparing number of years of employment and experimental time should be performed. Second, muscles and joints other than the ADP and ECR were not included in the analysis. The angle of the wrist and shoulder joints affects the load on the fingers when operating scissors [32], and tension from the neck to the back tends to increase as a result of working in the same position [2]. In this study,to determine the effect of power support, the measurement conditions were minimized, and wrist joint dorsiflexion and shoulder joint position were kept constant. To maximize the effect of power-support scissors and prevent occupational diseases of hairdressers, it is necessary to create an optimal model of the working limb position, including the posture-control muscles, and estimate the burden on the fingers. Third, the force of the power-assist scissors was fixed. A mechanism that can adjust the force according to the muscle strength of the fingers and the size of the hand should be developed. Fourth, in this study, we performed a simple opening and closing operation with barber scissors of a general shape, and did not verify whether power support can be obtained for each type of scissors, opening and closing angle, and procedure. Hairdressers use different tools and techniques such as using thinning scissors and cutting with a slight movement of the cutting edge. In order to extend the effect of the power support scissors we have developed, new verification with tools and techniques should be done. Fifth, the analysis of hair cutting pace was not included to optimize the effect of power support.