Quantifying the Synergistic Fatigue Effect of Height and Duration on Construction Workers: An sEMG-Based Threshold for Work-at-Height Safety Management

The construction industry is characterized by high physical demands, where workers are regularly engaged in manual handling and labor-intensive tasks, making fatigue a common occupational challenge. This study aims to quantify the combined effects of working height and task duration on physical fatigue among construction workers and to establish dynamic fatigue thresholds based on surface electromyography (sEMG) signals. A two-way repeated-measures field experiment was conducted on an actual construction site with eight healthy male workers. Each participant completed four working-height conditions (9, 20, 33, and 46 m), and at each height, physiological and subjective measurements were obtained at four repeated time points (0.5, 1.0, 1.5, and 2.0 h). This yielded 32 participant-by-height trials and 128 repeated observations in total. The results demonstrated a significant interaction between working height and task duration, with the increase in heart rate from 0.5 h to 2 h at 46 m being approximately 2.2-fold higher than that at 9 m during a two-hour working period. Frequency-domain analysis of sEMG signals revealed significant declines in median frequency (MDF) and mean power frequency (MPF) across all monitored muscles over time (p < 0.001), with cubic regression models showing excellent goodness-of-fit (R2 up to 0.947). Based on MDF and MPF trends, fatigue progression was classified into three phases: development (0–30 min), stability (30–75 min), and instability (after 75 min), identifying approximately 75 min as an exploratory transition point from compensation to decompensation. These preliminary results provide exploratory empirical evidence that may inform the future optimization of work–rest schedules and real-time fatigue monitoring strategies in high-rise construction operations, pending validation in larger, more diverse populations.