Dissociation between muscle activation, spinal joint load, and subjective perception in passive back support exoskeleton use

One important purpose of back support exoskeletons (BSEs) is to reduce the stress to the lumbar spine and alleviate the low back pain. However, most previous studies introducing BSEs have argued for their effectiveness based only on easily measurable physiological outcomes such as muscle activation. Motivated by this gap, we examined whether the muscle activation of the back muscle during load handling with a passive BSE can faithfully reflect the estimated L5-S1 joint reaction forces and subjective perceptions. Experiments involving 12 healthy young adults revealed that the BSE significantly reduced average thoracic erector spinae muscle activation during both lifting and lowering the load, and lumbar erector spinae muscle activation during lowering. The BSE also reduced integrated thoracic erector spinae muscle activation during lowering. L5-S1 joint reaction forces were also significantly reduced by the BSE (p < 0.001). Participants reported a lower rate of perceived exertion (p < 0.05). However, the lack of associations among all metrics suggests that inferring one measure from another remains limited when evaluating the efficacy of a BSE.