3D Path Control: Can we use lower limb inter-joint coordination to assist gait and balance?

Maintaining balance during walking is a critical yet under-addressed challenge in the control of lower-limb exoskeletons, especially for users with progressive neurological conditions such as multiple sclerosis and muscular dystrophy. While assist-as-needed strategies have enabled flexible support in the sagittal plane, most exoskeletons lack active control in the frontal plane, limiting their ability to support mediolateral (ML) balance. In this study, we introduce a 3D Path Control strategy that enables coordinated assistance across hip abduction/adduction, hip flexion/extension, and knee flexion/extension. The controller is designed to provide partial gait assistance while preserving user autonomy and the ability to modulate step width, an essential mechanism for maintaining ML balance. Two experiments with healthy participants were conducted to evaluate the approach. The first experiment showed that increasing ML assistance improved alignment with a nominal coordination pattern and allowed modulation of hip abduction/adduction range of motion, and consequently, lateral foot placement. The second experiment demonstrated that even with constraining controller settings, users could still deviate from the desired path and adopt different step widths. These results suggest that 3D Path Control can simultaneously assist gait and support balance by combining structured inter-joint coordination while still providing flexibility in foot placement to the subjects.