Wheelchair–Exoskeleton Hybrid Robot With Retractable Parallelogram Lift for Sit-to-Stand Support: Motion Analysis and Muscle Activity Assessment

This paper introduces a novel wheelchair–exoskeleton hybrid robot featuring a retractable parallelogram lift, developed to assist individuals with mobility impairments in transitioning between ambulation over uneven terrain and seated navigation on smooth surfaces. The system enables stable sit-to-stand and stand-to-sit transitions by maintaining ground contact through a wheelchair equipped with a front differential drive and rear omni wheels. In the upright position, the foldable lift retracts and repositions the driving platform vertically behind the user, removing obstructions during walking. The exoskeleton structure supports the lift’s weight, allowing full ground contact through the user’s feet for improved mobility. The parallelogram lift produces a curvilinear seat trajectory, with design parameters derived from kinematic analysis to closely emulate natural sit-to-stand motion. A dynamic model was formulated to estimate the seat reaction force and actuation requirements across various postures, incorporating hip and knee moments provided by the exoskeleton to enhance physical human–robot interaction. Preliminary experiments were conducted with four healthy participants performing sit-to-stand and stand-to-sit motions under three distinct support conditions: no support, lift support, and combined hip support. Consistent with the theoretical simulation, the seat reaction forces recorded from multiple participants confirmed that the lift mechanism supports up to approximately fifty percent of their body weight. Surface electromyography recorded from the quadriceps and hamstring muscles provides insight into the underlying muscle activation patterns and inter-muscular coordination during assisted movement. These results contribute to the understanding and optimization of human–robot interaction in hybrid assistive systems.