Influence of a passive lower-limb exoskeleton during simulated industrial work tasks on physical load, upper body posture, postural control and discomfort

This study investigated the effect of wearing a passive lower-limb exoskeleton on physical load, kinematics, postural control, and discomfort. 45 healthy males participated and were exposed to three 21-min simulations, including screwing, cable-mounting, and clip-fitting. Each exposure comprised one of three exoskeleton statuses (standing, high and low sitting on exoskeleton) and three working distances (optimal, far, very far). The order of exoskeleton status and working distance were randomized across subjects. A force platform was used to calculate the mean center of pressure (COP) and absolute (SSABS) and relative static postural stability (SSREL) as measures of postural control as well as the weight transferred to the exoskeleton supports as indicator of physical load. Neck and back angles were recorded together with electrical activity of four bilateral muscles (trapezius, erector, vastus, gastrocnemius). Discomfort was recorded before and after each exposure on an 11-point numeric rating scale. Physical load decreased due to the exoskeleton carrying up to 64% of the subject's body mass. The COP remained within the base of support with the lowest values of static postural stability for high sitting (27%). During sitting, vastus activity increased (similar to 95-135%) while gastrocnemius activity decreased (similar to 25%) compared to standing. Trapezius and erector activity levels showed only minor differences between exposures. Larger working distances resulted in a more anterior COP and increased erector activity. Standing without exoskeleton was related to less discomfort (0.5) than sitting on the exoskeleton (similar to 1.3). Working postures and distances changed SSREL and activity levels of the vastus, gastrocnemius, and erector, but not SSABS. However, postural stability did not approach a critical state in our simulations without external perturbations. Therefore, investigating exoskeletons in the field will provide useful information about their effectiveness and usability in dynamic working situations where external forces could occur.