HandMECH: Biomechanically Inspired 3D-Thumb-Trajectory for a Body-Powered Hand Prosthesis

According to the type of actuation, the hand prosthesis can be classified as passive (opening or closing using a terminal device), body-powered (opening or closing using the opposite shoulder and harness)(BP), or active prostheses. Passive prostheses have many limitations, and most of them can perform only a limited number of tasks. Although robotic prostheses provide precise motions and adequate forces to perform the tasks, their operation time and motor control challenges are still limitations of these actuators. This study presents the realization of a novel body-powered (BP) hand prototype (HandMECH) that emphasizes mechanically grounded thumb function. Inspired by the human hand, a three-dimensional (3D) thumb trajectory is implemented to replicate thumb movement paths required in multiple grasp types while reducing the required degrees of freedom (DoF). Elastic elements enable the fingers to return to a neutral posture after the user releases the shoulder-driven tendon input. The system utilizes a harness to translate shoulder movements into finger-grasping movements, and eliminating the need for manual cable manipulation. The prototype provides 8 DoF in the fingers and is additively manufactured in ABS. Prototype-level functional evaluations with one able-bodied participant, speed and repetitive functional tests, Box and Blocks Test, and different activities of daily living (ADL) demonstrate feasibility for pinching, grasping, and power gripping. A preliminary feasibility assessment involving one able-bodied participant, consisting of speed and repetitive function tests, Box and Blocks, and ADL. are conducted to evaluate the basic functional potential of the mechanism. The results demonstrate the mechanical feasibility of the design for pinching, grasping, and power grasping. The HandMECH prototype is able to perform a functional evaluation test in 29 s and pick and place 16 blocks in one minute during the BBT. The proposed hand prototype is capable of holding objects with a thickness of 1-120mm. HandMECH is 22% lighter than the lightest body-powered prosthesis reported in the literature. Force measurements further indicate that approximately 78% of the applied user force is transmitted to the fingertips in the tested configuration, providing a quantitative characterization of transmission performance in a friction- and compliance-dominated BP mechanism. The recorded engagement time of 0.2 s demonstrates a dynamic responsiveness that is highly competitive with reported benchmarks for state-of-the-art robotic prostheses, highlighting the low latency advantage of direct cable-driven transmission in body-powered architectures.