*2.4. Data Collection: Muscle Activation Measurement*

After taking a break for 30 min after the *MVC* measurement, dental work simulations were performed on a dental mannequin (Simple Manikin III, NISSIN, Kyoto, Japan) installed in the dental UCS, and muscle activity was recorded in eight EMG channels. The participants performed simulations for intraoral scanning and tooth preparation tasks for two days at intervals of one week to prevent fatigue accumulation between tasks, and the work order was randomly selected by listing all orders (Figure 2).

**Figure 2.** Electromyography measurements during dental simulations. (**A**) Tooth preparation simulation; (**B**) Intraoral scanning simulation.

 (µV) The digital integrated dental UCS (MEGAGEN, Daegu, Korea) used an intraoral scanner (i500; MEDIT, Seoul, Korea) and monitored the dental UCS, and the conventional dental UCS (Maxpert; SHINHUNG, Seoul, Korea) showed the scanning process on a separate monitor, other than that of the dental UCS, connected to an intraoral scanner. The participants performed all work procedures after adjusting the dentist's chair and the patient's chair to fit their posture and body.

 (%) = × 100 The intraoral scanning task was performed by consecutively scanning the maxillary and mandibular models for dental education (D85DP-500B.1; Nissin Dental, Kyoto, Japan) three times using an intraoral scanner (i500; MEDIT, Seoul, Korea; Figure 2). The scanning strategy was to scan the complete arch in the order of occlusal, buccal, and lingual, and all participants performed a scan so that there were no empty spaces in any of the teeth (Figure 2). The weight of an intraoral scanner used in the present study was 280 g.

ℎ 60 − ℎ 60

ℎ 60 × 100

(%) =

The tooth preparation task was performed by preparing the maxillary right first molar (D85DP-500B.1; Nissin Dental, Kyoto, Japan) for a single ceramic crown and chamfer margin using a high-speed dental handpiece (TG-98; W&H, Bürmoos, Austria; Figure 2). Participants performed the tooth preparation task without a magnification system (Figure 2).

One investigator (J.M.L.) recorded the muscle activity in real time only when the participant performed any action for the tasks and did not record the muscle activity unless the participant performed the simulation. In addition, all working times were recorded.

#### *2.5. Data Collection: Muscle Activation Analysis*

Muscle activation and muscle fatigue were calculated from the data measured using *EMG* measurement software (TeleScan ver 3.29; LAXTHA, Daejeon, Korea). *EMG* data from dental work were normalized and expressed as percentages, and the activation of each muscle was calculated as follows [9,12] (1):

$$\text{RMS\\_EMG}(\%MV\text{C}) = \frac{\text{Muscle activation during tasks (\mu\text{V})}}{\text{MVC}} \times 100\tag{1}$$

*RMS EMG* (%*MVC*) indicates muscle activation that occurs during dental work compared to MVC. As the *RMS EMG* (%*MVC*) increased, the risk of MSDs increased, and the ergonomic risk level according to the activation level of each muscle was evaluated according to previous literature: *MVC* in the range of 0–10% means "low risk"; 11–20% means "moderate risk," and more than 21% means "high risk" [9,12,13].

Muscle fatigue can be identified by increasing and decreasing median edge frequency (*MEF*) values, and as *MEF* decreases, muscle fatigue increases [22–25]. The *MEF* value can be obtained in the frequency range of 1–400 Hz after applying the fast Fourier transform, which transforms the *EMG* signal that changes with time into a frequency. Among the total working time, *MEF* in the first 60 s and next 60 s were calculated, and muscle fatigue was calculated according to the following formula [23,24] (2):

$$\text{Muscle\\_fatigue(\%)} = \frac{\text{MEF in the second 60 s} - \text{MEF in the first 60 s}}{\text{MEF in the first 60 s}} \times 100 \tag{2}$$

When *MEF* in the first 60 s of dental work was compared with *MEF* in the next 60 s, a negative value was obtained when the value of *MEF* in the second 60 s was low, indicating the increase in muscle fatigue [23,24].
