## 4 Discussion
### Physical Stress  
HRV, as previously outlined, was used to assess the impacts of task complexity and instruction method on physical stress. The physical stress analyzed in this study represents the measurable portion of physical resources expended during the performance of the two maintenance tasks using different instruction methods. HRV analysis, derived from electrocardiogram (ECG) data, is a highly responsive diagnostic technique used to assess the activity of the autonomic nervous system (ANS) under stressful situations [85]. The ANS's branches modulate the heart's electrical activity via the sympathetic (SNS) and parasympathetic (PNS) divisions, influencing heart rate and rhythm. PNS activity promotes relaxation and regeneration and SNS activity mobilizes the body for activities such as the \"fight or flight\" response [45; 46]. The time domain of HRV showed significant effects of task complexity on m-RR, m-HR, SDHH, RMSSD, HRV triangular index, and TINN. As task difficulty escalated from simple to complex, m-HR, SDHR, HRV triangular index, and TINN indicators increased, while m-RR and RMSSD indicators decreased, irrespective of the instructional approaches employed. This indicates that increased task complexity results in higher physical activity levels, leading to enhanced sympathetic activity through central and autonomic nervous system integration, a finding consistent with several other studies [86; 87; 88]. Paper-based instruction was associated with lower values of RMSSD compared to AR-based instruction, regardless of task complexity. The statistical analysis showed that for the paper-based instruction group, the complex task significantly reduced m-RR and RMSSD values and increased m-HR, SDHR, and TINN values compared to the simple task. Comparing the two instruction methods during a complex task, AR instruction significantly reduced physical workload level by an average of 13.66% (10.95% for m-RR, 10.09% for m-HR, 19.96% for SDHR, and 53.13% for RMSSD). However, no changes in physical workload were observed when comparing the two instructional methods for a simple task. In addition, using AR for both complex and simple tasks resulted in the same physical workload. A higher workload has been reported to be associated with lower values of m-RR and higher values of m-HR [89], and a decrease in RMSSD of HRV has been used as an indicator of fatigue and stress [90; 91]. The variations in the time domains of HRV in this study acted as indicators of a high physical workload that suggest increased sympathetic activity. The observed reductions in RMSSD indicated a decrease in vagal modulation of the HR [90; 91; 92]. Based on the time domain of HRV parameters, the results indicated that the utilization of AR training for the complex task led to a notable decrease in workload when compared to the traditional paper-based instruction approach. These results align with other research demonstrating a correlation between alterations in task difficulty and corresponding variations in physical load [93; 94; 95; 96; 97; 98]. Furthermore, previous studies have reported that HR increases when task difficulty [53; 99; 100; 101] or stressful conditions [102; 103] increase. Theoretically, based on the measures used in this study, the complex task is associated with high stress levels, which explains the changes observed in the HRV time domain variables. HRV analysis through nonlinear domains focuses on the complex time behavior of heart rhythm. The results demonstrated a statistically significant effect of task complexity on SD1, ApEn, and SampEn. These measures exhibited a drop in values during complex tasks in comparison to simple tasks, irrespective of the instructional techniques employed. Our findings are in line with those reported in previous studies [98; 104]. [PERSON] et al. [104] found that nonlinear metrics decreased significantly during stressful conditions. The results also showed that for paper-based instruction, the SD1 value was significantly reduced when performing the complex maintenance task.