Effects of a soft back exoskeleton on lumbar loading during a manual handling task: A pilot simulation study
Résumé
INTRODUCTION: Back exoskeletons are wearable structures that provide an assistive torque to one or multiple joints and can prevent low back disorders during manual handling tasks (1). A previous study already reported a reduced low back muscle activity using the CORFOR® exoskeleton during stoop liftings (2), but the influence of this exoskeleton on lumbar loading hasn’t been investigated yet. The aim of this study was thus to assess the effects of wearing such a soft back exoskeleton on lumbar compression force and flexion moment.METHODS: Eight male participants (178.6 ± 2.6 cm, 72.9 ± 7.1 kg, 21.4 ± 2 years) performed 10 repetitions of a lifting task in the sagittal plane using the stoop technique (8 kg; 15 cycles / min). Participants were familiarized to the exoskeleton and to the task before starting the experiment. They carried out the task with (EXO) and without (FREE) wearing a CORFOR® soft back exoskeleton. This exoskeleton is made of a pair of elastic bands attached to shoulders and knees, stretched by hip flexion. The 3D trajectory of a full body marker set was recorded using an optoelectronic camera system (Arqus A12, Qualisys, Sweden) sampled at 200 Hz synchronized to 3D ground reaction forces and moments measured using force plates (BMS600900, AMTI, USA) sampled at 1000 Hz. These data were then imported in OpenSim 4.2 to estimate compression force and flexion moment at L5/S1 intervertebral joint. The OpenSim Lifting Full-Body model (3) was scaled to each participant anthropometric characteristics and completed by adding the exoskeleton. Exoskeleton elastic bands were modeled as springs. Related slack length and stiffness were measured at the end of each experiment and participant-specific spring path was obtained during EXO task using 16 markers taped on the exoskeleton. Lumbar compression force and flexion moment were estimated through an inverse dynamics-based static optimization procedure. Student’s t-tests were finally performed to assess the effects of the exoskeleton by comparing EXO and FREE.RESULTS: Statistical analyses showed that L5/S1 compression forces were significantly lower (p = 0.02, d = 1) in EXO (38.8 ± 3.3 N/Kg) than in FREE (41 ± 2.4 N/Kg). Flexion moments were also significantly lower (p ≤ 0.001, d = 2.8) in EXO (1.8 ± 0.12 Nm/Kg) than in FREE (2.1 ± 0.08 Nm/Kg).CONCLUSION: This study suggests that wearing the CORFOR® exoskeleton may reduce lumbar loading and thus contribute to prevent low back disorders. These results are consistent with the reduction of back neuromuscular activity observed while using this exoskeleton (2). Future experiments should focus on the effect of such an exoskeleton during other handling tasks (e.g. squat, asymmetric lifting). Simulating the effect of different stiffnesses and paths of the spring on lumbar loading may also help to optimize the exoskeleton design.REFERENCES:1) De Looze et al., Ergonomics, 20162) Schwartz, Theurel and Desbrosses, IJERPH, 20213) Beaucage-Gauvreau et al., Comput Methods Biomech Biomed Engin, 2019
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