## 3 Results  
### Main Findings  
The systematic literature review identified 25 studies investigating mental workload in HRC across various industrial applications. These studies range from early implementations of machine learning for adaptive collaboration to recent advancements in neuro-ergonomic assessments, multimodal workload measurement, and robot adaptation strategies. Table 1 presents a chronological overview of the reviewed studies, a summary of their objectives from the perspective of the authors of this review, and information regard  
Figure 1: PRISMA 2020 flow diagram for the systematic literature review (EC: exclusion criteria). Adapted from [30].  
ing the publication type of each entry. When the publication year is considered, 6 papers were published between 2020 and 2022, 7 papers were published in 2023, 10 papers were published in 2024, and 2 papers were published in 2025. As for their publication types, 18 were published in scientific journals, while 7 were published in conference proceedings.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Publication** & **Sample Size** & **Task Category** & **Workload Measure** & **Collected Data** \\ \hline  & & & & & Performance & Quality checks, quality issues, productivity, operating costs, variability of job, risk of accidents \\ \cline{3-6}
[26] & CP & 4 & Assembly & & Subjective & NASA-TLX; Borg CR-10; Job engagement \\ \cline{3-6}  & & & & & Physiological & HR; HRV; EDA \\ \hline  & & & & & Subjective & NASA-TLX; RS9 \\ \hline  & & & & & Physiological & EEG \\ \hline  & & & & & Physiological & EEG \\ \hline  & & & & & Subjective & NASA-TLX \\ \hline  & & & & & Physiological & Cardiac Measures; EDA; Hand temperature \\ \hline  & & & & & Physiological & EEG \\ \hline  & & & & & Physiological & EEG; EMG \\ \hline  & & & & & Physiological & EEG \\ \hline  & & & & & Performance & Number of errors/mistakes \\ \cline{3-6}
[38] & JP & 9 & Assembly & & Subjective & NASA-TLX \\ \cline{3-6}  & & & & & Physiological & EEG \\ \hline  & & & & & Performance & Number of errors/mistakes \\ \cline{3-6}
[4] & JP & 36 & Assembly & & Subjective & NASA-TLX; SAM; BDM \\ \cline{3-6}  & & & & & Physiological & HRV  
Moreover, it is observed that, regarding trends over the years, the data do not indicate consolidated patterns, showing variations in the combination of different physiological measures, whether applied in isolation or in conjunction with subjective and performance metrics, without a consistent adoption of approaches that reflect the researchers' perspectives.  
However, the widespread adoption of subjective measures stands out, as they were used in 19 out of the 25 analyzed studies, with the integration of physiological and subjective measures establishing itself as the dominant and transversal approach in this review. Another relevant aspect is that performance measures, when applied, are always associated with subjective measures. In 11 studies, the joint application of all three cognitive workload assessment approaches was observed.  
A notable aspect is the almost exclusive use of central nervous system-related metrics between 2020 and 2023, reflecting researchers' strong interest in analyzing brain activity and its behavior concerning workload in HRC settings. Additionally, central nervous measures were employed in 13 out of the 25 studies, followed by cardiac measurements and EDA, used in 10 and 7 studies, respectively. Ocular measures, mentioned in only four studies, were only observed starting in 2023. Finally, among the physiological metrics analyzed, temperature was employed by only two authors.