Assisting Global Balance Recovery Responses During Perturbed Walking with Ankle Exoskeletons

While ankle exoskeleton research has mainly focused on reducing metabolic energy consumption, recent studies also aimed to support walking balance which is also important for individuals with mobility impairments that typically have an increased fall incidence. Balance supporting controllers have been designed by mimicking the balance recovery action of healthy subjects. In a previous study we designed and evaluated an exoskeleton controller that mimics the complete ankle function during balance recovery, including the contribution of muscle mechanical impedance, local feedback (e.g. stretch reflex) and global balance recovery responses based on sensory integration. While this controller successfully assisted walking balance, it is unclear if assisting muscle mechanical impedance and local feedback with an exoskeleton is beneficial, as exoskeleton assistance influences muscle activity and hence the mechanical impedance of muscle fibers. Here we developed a biomimetic exoskeleton controller that aims to assists only the slow global balance recovery response without counteracting the contribution of intrinsic mechanical impedance of muscle fibers and local feedback. Across 10 subjects, the neuromuscular controller improved balance support after perturbation compared to a minimal impedance controller. Muscle activity is reduced in response to push and pull perturbation by respectively 7.5% $(p=0.0 6)$ and 19% $(p < 0.0 1)$ with respect to the minimal impedance controller.