Revisiting consumed endurance: A NICE way to quantify shoulder fatigue in Virtual Reality

Virtual Reality (VR) is increasingly being adopted in fitness, gaming, and workplace productivity applications for its natural interaction with body movement. A widely accepted method for quantifying the physical fatigue caused by VR interactions is through metrics such as Consumed Endurance (CE). Proposed in 2014, CE calculates the shoulder torque to infer endurance time (ET) - i.e. the maximum amount of time a pose can be maintained - during mid-air interactions. This model remains widely cited but has not been closely examined beyond its initial evaluation, leaving untested assumptions about exertion from low-intensity interactions and its basis on torque. In this paper, we present two VR studies where we (1) collect a baseline dataset that replicates the foundation of CE and (2) extend the initial evaluation in a pointing task from a two-dimensional (2D) screen to a three-dimensional (3D) immersive environment. Our baseline dataset collected from a high-precision tracking system found that the CE model overestimates ET for low-exertion interactions. Further, our studies reveal that a biomechanical model based on only torque cannot account for additional exertion measured when the shoulder angle exceeds 90° elevation. Based on these findings, we propose a revised formulation of CE to highlight the need for a hybrid approach in future fatigue modelling.