Evolution of human heterogeneity in assembly line balancing: traditional and innovative paths for workforce inclusion

This paper presents a systematic literature review of the Assembly Line Worker Assignment and Balancing Problem (ALWABP), an extension of the classical Simple Assembly Line Balancing Problem that explicitly incorporates worker heterogeneity into the balancing process. Originally developed to model Workers with Disabilities in sheltered workshop environments, the ALWABP has progressively evolved into a broader class of models capable of representing diverse workforce characteristics, including differences in skills, experience, performance, and physical capabilities. In line with the human-centric paradigm of Industry 5.0, this review analyses the evolution of the ALWABP literature, classifies existing contributions, and identifies key research trends and future directions. A systematic review methodology is adopted, with a final sample of 26 articles. The analysis highlights a growing research interest in recent years. Four main research streams are identified. First, workforce diversity has expanded from disability-focused models to general heterogeneity in worker characteristics. Second, human factors such as ergonomics, fatigue, fairness, and workload distribution are increasingly integrated. Third, Human-Robot Collaboration emerges as a key enabler for inclusion and flexibility through collaborative robotic systems. Fourth, uncertainty is addressed via stochastic, robust, and fuzzy approaches to capture variability in task times and worker availability. Conclusions highlight some research gaps and possible future research directions.