Review of balance recovery in response to external perturbations during daily activities.

Balance is an essential capability to ensure upright standing and locomotion. Various external perturbations challenge our balance in daily life and increase the risk for falling and associated injury. Researchers try to identify the human mechanisms to maintain balance by intentional perturbations. The objectives of this work were to point out which areas of perturbation based research are well covered and not well covered and to extract which coping mechanisms humans use to respond to external perturbations. A literature review was performed to analyze mechanisms in response to external perturbations such as pushes to the body or ground level changes during standing, walking, running and hopping. To get a well-structured overview on the two dimensions, the perturbation type and the task, the Perturbation Matrix (PMA) was designed. We found that multiple studies exist for the tasks walking and standing, while hopping and running are covered less. However, all tasks still offer opportunities for both in-depth and fundamental research. Regarding the recovery mechanisms we found that humans can recover from various types of perturbations with versatile mechanisms using combinations of trunk, as well as upper and lower limb movements. The recovery movements will adapt depending on the perturbation intensity, direction and timing. Changes in joint kinetics, joint kinematics and muscle activity were identified on the joint level and leg stiffness and leg length on the global leg level. We believe that the insights from the extracted mechanisms may be applied to the hardware and control of robotic limbs or lower limb exoskeletons to improve the balance and robustness during standing or locomotion.