Balance Control Strategies during Standing in a Locked-Ankle Passive Exoskeleton

This paper investigates sensorimotor adaptation strategies of sagittal postural control in healthy subjects under kinematic constraints. A passive exoskeleton named CAPTUR, with locked ankle joints and legs motion restrained to the sagittal plane is used to restrict and measure participant's movements. The aim is to assess the role of the orientation of the shank and the trunk segments in maintaining the body center of mass above its support base, while the ankle strategy is inhibited. Five young healthy participants were asked to keep standing, while their balance was challenged by five experimental conditions. Participants mainly regulated quiet standing balance by flexing/extending the knees, in order to affect the shank and feet angles, and move the contact patch along the sagittal axis. In this case, the orientation of the trunk segment changes synchronously with the shank angle to keep an upright posture. Responses to more dramatic excursions of the center of pressure are ensured by changing the trunk tilt angle in opposition of phase with the shank angle. These observations could be used to implement a bioinspired balance controller for such constrained lower-limb exoskeletons.