Patients with neuromuscular disorders experience problems with the activities of daily living (ADL) due to limited muscle strength. Upper limb weight-bearing orthoses have been developed to increase quality of life by enabling patients to move their arms freely. Most existing designs help patients lift their arm by compensating the gravitational force on the sagittal plane. However, the essential ADL for a patient's actual life, such as eating, drinking, scratching the face, and receiving phone calls, require motion in 3D space that is mainly performed by forearm movement. To assist forearm motion, weight should be compensated by elbow flexion/extension and shoulder medial/lateral rotation. In this study, a weight-bearing mechanism that compensates the weight in 3D space with only two zero-initial-length springs is presented. We built a 4 degree of freedom passive weight-bearing orthosis (WBO) with three rotational axes on the shoulder and one rotational axis on the elbow to mimic the natural movement of a human arm. To design the WBO, joint placement was considered, and a slider-crank mechanism was used to solve the joint misalignment problem caused by the overlapping of the joint between the WBO and the human arm. A prototype was built to verify the function of the proposed mechanism. It showed sufficient weight compensation performance to enable self-reliance in ADL by reducing the effort required to move the arm.
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