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A Low-Back Exoskeleton can Reduce the Erector Spinae Muscles Activity During Freestyle Symmetrical Load Lifting Tasks

Low-back wearable robots are emerging tools to provide support to operators during handling of goods and repetitive operations. In this paper, we present and validate a novel control strategy for an active pelvis orthosis, which operates intuitively and effectively to assist workers during lifting operations. The proposed control strategy has a hierarchical architecture: the first layer (the intention-detection module) aims to detect online the onset of the lifting movement; the second layer (the assistive strategy) computes the reference torque profile to assist the movement, after the movement onset is detected; the third layer (the low-level control layer) aims at setting the current to drive the actuators. The control strategy relies on the angle measurements acquired by the encoders integrated in the robotic device and does not need additional sensors to detect the event. The system was tested on a pool of five healthy subjects, who were requested to perform repetitive lifting movements: first, the subject was requested to bend the trunk, grasp the box, lift it up and place it on a table; second, the subject was requested to grasp the object from the table, lower it down, place it on the floor and get up without the load. The tasks were executed with the exoskeleton controlled in transparent and assistive modes. Results show that the assistive action allows to perform the lifting movement faster. Significant reductions of the activity of the Lumbar Erector Spinae muscles were observed in the assistive mode compared to the transparent mode: a 16% reduction was observed when extending the trunk while holding the weight and a 33% reduction when extending the trunk without holding the load.