Influence of varied assistance levels provided by a dual-joint active back-support exoskeleton on spinal musculoskeletal loading and kinematics during lifting.

An active dual-joint back-support exoskeleton with motors at both lumbar and hip level was designed to reduce spinal musculoskeletal loading and preserve lumbar flexibility during lifting. A subject-specific controller estimated the moment actively generated by back muscles to counteract gravitational forces on the upper body, minimising a counter-productive abdominal muscle contraction. Eight subjects lifted a 15 kg load using free technique with four assistance levels, i.e. 0%, 30%, 50%, and 70% of the active moment. Time-averaged L5S1 compressive force and back muscle active moment estimated by an EMG-driven biomechanical model, decreased by 5.5-9.3% and 14.9-28.6%, respectively, with non-zero assistance. Higher assistance did not yield larger L5S1 compression reduction but did gain further reduction in the time-averaged back muscles active moment. No significant changes in abdominal muscle activity and minor changes in lumbar flexion were observed suggesting the controller and dual-joint design achieved their objectives.