This study investigates the cognitive workload and body posture of operators in the manufacturing of office chairs, with a focus on understanding the relationship between cognitive demands, physical strain, and work performance in an assembly line setting. A multi-faceted approach was adopted, integrating subjective assessments, objective ergonomic evaluations, and performance metrics. The study aims to measure anthropometric dimensions, assess cognitive workload and task performance, conduct a Rapid Upper Limb Assessment (RULA), and explore the relationship between perceived cognitive workload, RULA scores, and task performance. It is hypothesized that increased cognitive workload and task complexity negatively impact task performance in office chair assembly, while poor posture further reduces performance and increases physical discomfort. The experiment involves nine office chair assembly operators and utilizes validated self-reported scales such as NASA-TLX and the Nordic Musculoskeletal Questionnaire (NMQ), along with observational techniques including RULA analysis conducted manually and via CATIA V5 software. Findings revealed a paradox where high-frequency production days were associated with lower perceived cognitive demands (from 58 to 49) and physical demands (from 63 to 56) but higher frustration levels (from 38 to 45) under conditions of increased workload. The RULA analysis highlighted ergonomic risks, leading to recommendations for ergonomic interventions. In conclusion, this study offers valuable insights into the cognitive and physical challenges faced by assembly line workers, advocating for task rotation, ergonomic interventions, and cognitive support tools. Therefore, it aligns with the SDG 8 by addressing factors that influence productive employment and decent work conditions in manufacturing.