Walking and standing with an exoskeleton for the lower limbs: effects of mass and inertia on gait and postural control

Passive lower limb exoskeletons have been recently used in the industry to prevent musculoskeletal disorders. Their assistive behavior aims to reduce the effort at specific joints, specially the knees. Most of them are designed to alleviate the mechanical load on the knee in tasks that require partial crouching or squatting positions. However, the exoskeleton user needs to stand and walk safely around his working environment. Therefore, our purpose was to determine if wearing a lower leg exoskeleton affected locomotion or posture. To do so, five subjects participated in gait and posture experiments with and without a passive exoskeleton for the lower limbs (LegX from SuitX, USA), while their cardiac frequency and kinematic gait data were collected. First, they walked on a treadmill for 10 minutes and, afterwards, they performed three postural control tests on a force plate: standing and crouching with eyes opened, and standing with eyes closed. Our findings show that walking at the self-selected speed with LegX increases heart rate, induces gait temporal asymmetry with longer steps in the non-dominant side, and reduces the medio-lateral displacement of the center of pressure. These results indicate that this passive lower limb exoskeleton affects medio-lateral (M-L) stability during quiet stance, widening the base of support and increasing the M-L stiffness. Moreover, walking with an exoskeleton increases metabolic cost and gait temporal asymmetry, probably related to safety perception to maintain the speed of the treadmill. Nevertheless, it is likely that with longer periods of use, the wearer may learn to use the exoskeleton to exploit the M-L configuration to increase stability and to reduce the gait temporal asymmetry.