Optimal Manipulation Plane for Individual Thumb and Its Inspiration for Future Ergonomic Developments of in-Palm Intelligent Devices

Smartphones are widely operated one-handedly with the thumb, creating notable ergonomic conflicts with conventional flat screens. This work builds a thumb biomechanical model to quantify manipulation comfort across its 3D reachable workspace. Using the kinematic Jacobian matrix to compute the manipulability ellipsoid and construct an optimization framework, we locate the spatial plane maximizing overall thumb performance. This optimal manipulation plane has a defined tilt angle and palm offset. Statistical tests against users’ natural holding postures show it lies beyond the 99% control ellipsoid of spontaneous preference, meaning users cannot naturally attain it without assistance. Accordingly, we propose a new phone case (shell) design to align the device interface with this optimal plane. It intends to boost comfort, mitigate muscle fatigue and raise input efficiency for one-handed use. This study validates biomechanics integration in product design and informs ergonomic innovation for palm-held smart devices.