## 4 Discussion  
This study was performed to identify the effects of not wearing exoskeletons and wearing lower-limb exoskeletons (CEX and Chairless Chair) on muscle activity and subjective discomfort ratings in the drilling task. Participants recruited for this study also performed a 10-min drilling task at three working heights (60, 85, and 110 cm) to determine the effect of the exoskeleton device at different working heights. The effect of wearing a lower-limb exoskeleton on the lower-limb muscles was statistically significant in this study. The activity of the rectus femoris, biceps femoris, and tibialis anterior muscles was reduced by 68.8-69.7%, 78.5-71.5%, and 57.9-97.1%, respectively, compared to without an exoskeleton for all working heights. Similar findings were found in previous research. Many previous studies have reported the effect of reducing the activity of lower-limb muscles when performing tasks while wearing lower-limb exoskeletons. Figure 4: Interaction effect between working height and exoskeleton on lower-limb muscles. Different letters indicate significant statistical differences (\(p<0.05\), [PERSON]’s test). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Body Part**} & \multicolumn{5}{c}{**Interventions**} & \multicolumn{5}{c}{**Working Height**} \\ \cline{2-9}  & **WO** & **WCC** & **WCEX** & \(p\)**-Value** & **60 cm** & **85 cm** & **110 cm** & \(p\)**-Value** \\ \hline Neck & 2.1 & 1.6 & 1.5 & 0.677 & 1.7 & 1.8 & 1.7 & 0.053 \\ \hline Shoulder & 2.1 & 1.7 & 1.6 & 0.541 & 1.7 & 1.8 & 1.9 & 0.152 \\ \hline Chest & 1.5 & 1.5 & 1.2 & 0.265 & 1.4 & 1.4 & 1.4 & 0.057 \\ \hline Arm & 2.3 & 1.8 & 1.8 & 0.476 & 1.8 & 1.9 & 2.1 & 0.189 \\ \hline Back & 2.7 & 2.3 & 2.3 & 0.327 & 2.5 & 2.6 & 2.2 & 0.258 \\ \hline Leg & 2.3 & 2.8 & 2.5 & 0.474 & 3.0 & 2.4 & 2.3 & 0.823 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mean values of subjective discomfort rating according to intervention and working height. [PERSON] et al. (2020) [10] evaluated the activity of six muscles (lumbar erector spinae, thoracic erector spinae, tibialis anterior, rectus femoris, semitendinosus, and lateral gastrocnemius) when performing bolting tasks while wearing the Leg X exoskeleton, and they reported that the rectus femoris muscle's activity decreased significantly when wearing the exoskeleton. [PERSON] et al. (2021) [23] reported that the lower-limb muscle activity (vastus lateralis, biceps femoris, rectus femoris, and vastus medialis) decreased by 44.8 to 71.5%, and the plantar pressure also decreased by 58.5 to 64.2%, when performing an assembly task with lower-limb exoskeletons in a squatting posture. [PERSON] et al. (2019) [12] also reported an approximately 25% reduction in gastrocnemius muscle activity when wearing an exoskeleton compared to working in a standing posture without the exoskeleton. [PERSON] et al. (2018) [13] showed that the muscle activity of the erector spine and the biceps femoris was decreased by 12-15%% and 5% in tasks involving lifting a box of 7.5 kg and 15.0 kg, respectively, when wearing an industrial exoskeleton. [PERSON] et al. (2016) [24] also showed that the muscle activity of the lower back muscles and the biceps femoris muscle decreased by 35-44% and 20% during an assembly task, respectively, when wearing the exoskeleton. They also reported a 37-50% reduction in lower back muscle activity and a 24% reduction in biceps femoris muscle activity when wearing the exoskeleton in a static holding task. The effect of wearing a lower-limb exoskeleton showed different aspects depending on the upper-limb muscles and working height in this study. The muscle activity of the upper trapezius (UT) and the middle deltoid (MD), which were the upper limbs, when wearing the lower-limb exoskeletons decreased by about 32.5-40.0% and 58.1-77.7%, respectively, compared to without the exoskeletons at a working height of 60 cm.