UC Berkeley

Upper-limb prosthesis users continue to reject devices despite continued research efforts. Today, the passive topology of body-powered prehensors, which physically transmits grasp force and position data between user and device, results in improved performance over myoelectric alternatives. However, the loads and postures on the user's body also result in discomfort, fatigue, and worsened grasp force control. Despite the long history and everyday adoption of body-powered prehensors in society, the measurement of how specific body loads and postures affect grasp performance and user experience has yet to be systematically studied. In this work, we present a body-powered prosthesis emulator to independently change required input forces and motions to study the positive and negative effects provided by the inherent haptic feedback. Using a simulated grasping task, we collect functional and qualitative data from 15 participants using a shoulder harness interface. Outcomes show that reducing input loads and excursions to below 24 N and 45 mm, respectively, significantly reduces negative outcomes, but further reductions beyond those thresholds produce limited benefit. Coupling load and excursion, as is the case in real passive body-powered transmissions, demonstrates the importance of the force-motion tradeoff within the tested range of transmission ratios from 0.5:1 to 2:1. The purpose of this study is to inform future prehensor designs that leverage the transparency of body-power to deliver high functionality while mitigating user discomfort.