Critical delay as a measure for the difficulty of frontal plane balancing on rolling balance board.

The relation between balancing performance on rolling balance board and reaction time is investigated. Ten young healthy adults performed balancing trials on a rolling balance board with different wheel radius R and stance widths d in the frontal plane. A 2- and a 3-degree-of-freedom models subject to delayed state feedback with a single lumped reaction delay were created in order to describe hip and hip-lumbar strategies. The critical delays of the underlying models, for which state feedback stabilization is still possible, were determined. This critical delay is a good measure of the difficulty of the balancing task: the smaller the critical delay, the more difficult the balancing task. Subjects' response time to visual stimuli correlates with the critical delay of the models, therefore, it can be used as an indirect indicator for balancing skill. Hip-lumbar strategy was found to be superior to pure hip strategy in the sense that it allows larger reaction delay. Overall, rolling balance board is a simple and convenient tool to assess human's balancing skill. The difficulty of the task can be tuned by employing different wheel radius with different stance widths while it can be measured by a single number: the critical delay of the underlying model. Improvement in balancing skill therefore can easily be monitored during balance therapy and physical rehabilitation.