## 2 Methods
### Experimental Procedures  
The experimental procedures were approved by the Institutional Review Board of Sungkyunkwan University and were performed following the Declaration of Helsinki (IRB #SKKU-20210107). Before the test, the participants were provided with detailed information about the purpose and procedure of the experiment and then gave their written informed consent. After disinfecting the participant's skin with alcohol cotton, electromyography bipolar electrodes (Ag/AgCl, with a 2.5 cm inter-electrode distance) were attached to the muscles to collect electromyography (EMG) signals. The EMG data were collected using the wireless system of the Noraxon Telemyo 2400 DTS (Noraxon, Scottdale, AZ, USA). The electrode locations were identified according to SENIAM guidelines [20]. The maximum voluntary contraction (MVC) and resting EMG were measured two times with a 60-s rest between trials for each muscle, and the mean of the two measurements was used as the MVC in this study. The EMG was sampled at a rate of 2000 Hz. Only data in the 20-400 Hz range were collected using bandpass filtering for the EMG signals, and all EMG signals were normalized based on the MVC. The methods for measuring the MVC of each muscle were as follows: (1) upper trapezius--lifting a shoulder in a sitting posture; (2) middle deltoid-raising arm laterally in a sitting posture; (3) biceps brachii--bending elbow while sitting with the elbow at 120\({}^{\circ}\) of extension; (4) triceps brachii--stretching elbow while sitting with the elbow at 120\({}^{\circ}\) of extension; (5) erect spinae--raising back and legs in a prone position; (6) rectus femoris--stretching knee while in sitting position with feet in the air; (7) biceps femoris--bending a knee in a prone position with the knee at 90\({}^{\circ}\) of flexion; (8) tibialis anterior--bending ankle while in sitting position at 90\({}^{\circ}\) of flexion with feet on the floor.  
Each participant performed a warm-up exercise for about 10 min to get used to the drilling task and exoskeleton devices. After the warm-up exercise, participants rested for about 10 min. Then, the participant performed a 10-min drilling task every 60 s (drilling task for 40 s and resting for 20 s, respectively) on the experimental frame, which was custom-made for this study (Figure 2). Three working heights (60, 85, and 110 cm) of the experimental frame were set up based on the 50 th percentile of knee, hip, and back heights for adult males aged 20 to 59 in South Korea [21]. The drill used in the test was an electric drill (JAYA, Seoul, Republic of Korea) that weighed about 1.5 kg and is typically found in manufacturing sites. This test did not limit the speed of the drilling task, and  
Figure 1: CEX (**left**) and Chairless Chair (**right**).  
participants were instructed to perform the task in their preferred style and speed for all working heights.  
Thus, all participants performed nine drilling tasks, including all combinations of three levels of intervention (WO, \(W_{\text{CEX}}\), and \(W_{\text{CC}}\)) and three levels of working height. The tests were carried out over three days to prevent fatigue, and the tasks were randomly conducted based on the three levels of intervention.