## 1 Introduction
[44] assessed 3D visual fatigue using heart rhythm and humans' autonomic balance. Their study showed that viewing 3D video destabilizes heart rhythm, increases heart rate, and causes a shift in autonomic balance due to increased activation of the sympathetic nerves. [PERSON] and [PERSON] [49] evaluated the physical stresses associated with viewing 3D and 2D displays using HRV. Their results indicated that viewing the 3D displays significantly increased physical stresses compared to 2D displays. This was evidenced by significant increases in HR and decreases in the standard deviation of heart rate (STDHR), mean time between heartbeats (m-RR intervals), very low frequency (VLF) power, and LF/HF power when viewing 3D displays compared to 2D displays. [PERSON] et al. [52] studied visually induced motion sickness (VIMS) and the visual impacts of head-mounted displays (HMDs). They showed that the experience of HMDs caused higher visual discomfort (i.e., simulator sickness and visual strain) compared to the experience of TV and desktop displays. Their evaluation, however, was limited to research using HMDs for virtual reality. [PERSON] et al. [51] assessed visual fatigue when viewing virtual magic cube tasks through optical see-through HMDs (OST-HMDs) compared to viewing real objects in the real world. They found that participants were more likely to experience discomfort when viewing virtual objects presented by OST-HMDs. Also, their study provided suggestions that HRV measures could be used to explore how visual fatigue is induced by AR content. Increasing task difficulties has been reported to cause an increase in HR [53]. Also, HR has been found to increase during multitasking conditions [54] or when an additional memory load is introduced [55]. RR intervals have been observed to decrease during complex tasks compared to simple tasks [56]. An increased mean HR was found during a Stroop test, and a reduction in high-frequency (HF) power was noted, with a large reduction in low-frequency (LF) power seen during tasks of high complexity [57]. The same findings regarding LF power were also reported by [PERSON] and [PERSON] [39] and [PERSON] et al. [58] with regard to increased task difficulties. The LF/HF ratio of HRV spectra is a proxy for the degree of sympathovagal balance present in healthy individuals [59]. It has been suggested that the standard deviation of RR intervals (SDRR) is the best indicator of total HRV (overall autonomic activity). While LF has been primarily utilized as a measurement of sympathetic modulation activity, HF, RMSSD, and PNN50 have been used as indicators of vagal modulation activity [44; 49]. In the comparison of AR against traditional instruction in previous studies, there is a substantial paucity of experiential data from user studies assessing the potentially negative consequences of AR-based training techniques. Furthermore, the comparisons and evaluations were mostly accomplished with non-industrial products using tasks with low levels of difficulty and few variations, which may not be representative of real industrial settings [60; 61; 62]. Consequently, the conclusions drawn from such studies are limited and specific, and may not apply to the wide variety of tasks in industrial settings, particularly where there is a high degree of product variability. To gain a deeper understanding of the features of AR-based instruction methods compared to paper-based approaches, it is beneficial and important to address more thoroughly the potential negative impact of AR-based instruction methods using objective techniques. To better reflect actual industrial situations, the design of the experiment should involve a variety of complex tasks such as fitting, screwing, fastening, etc. Hence, this research is essential as it will contribute to the body of knowledge by advancing our understanding of how to evaluate the effects of AR systems on a user's physical stress while performing maintenance tasks.