A Human-Stiffness-Based Simulator for Ergonomic Evaluation of Pistol-Grip Power Tool Reaction

Powered pistol-grip fastening tools can produce substantial handle rotation during torque build-up, which may contribute to operator overuse injuries. This study developed and evaluated a human-stiffness-based physical simulator for a controlled relative comparison of fastening variables affecting peak handle rotation. The simulator used a DC-powered pistol-grip tool, which was constrained to rotate about its spindle axis, with a double-acting pneumatic cylinder providing resistance set according to a specified human-derived stiffness. The evaluation used stiffness data from 10 operators at 5 and 7.5 N·m, with simulator peak displacement compared to in vivo measurements using paired analyses. The simulator was then used as a screening platform to examine the effect of target torque and, at 7.5 N·m, the effects of fastener material, drive style, head type, and spindle speed. The simulator overpredicted human peak displacement, with mean biases of 14.72° at 5 N·m and 10.29° at 7.5 N·m. The peak rotation increased by 15.0% when torque was increased from 5 to 7.5 N·m (p = 0.022). Torx Plus style fasteners produced 11.5% less rotation than Hex drive fasteners; fastener material significantly affected rotation; head type did not; and rotation was substantially lower at 160 rpm. Although the simulator did not reproduce absolute human displacement without bias, the results support its use as a screening platform for relative comparison under fixed mechanical boundary conditions rather than as an absolute predictor of human displacement or injury risk.