Maximum allowable force on a safety harness cable to discriminate a successful from a failed balance recovery

A safety harness system is essential to ensure participant safety in experiments at the threshold of balance recovery where avoiding a fall is not always possible. The purpose of this study was to propose a method to determine the maximum allowable force on a safety harness cable to discriminate a successful from a failed balance recovery. Data from 12 younger adults, who participated in experiments to determine the maximum forward lean angles that participants could be suddenly released from and still recover balance using three different limits on the number of steps, were used. For each participant, the coefficients of an asymptotic exponential regression, between the maximum vertical force on the safety harness cable and the initial lean angle at each trial, were evaluated by a least squares method. A proposed threshold for the maximum allowable vertical force of five force constants ensured that the initial lean angle reached 99% of its steady state value with respect to its initial value. It should thus discriminate well a successful (below the threshold) from a failed (above the threshold) balance recovery. Furthermore, although the amplitude of the horizontal forces should not be neglected in safety harness system designs, the contributions of the medial-lateral and anterior-posterior forces can be neglected in experiments at the threshold of balance recovery. Finally, although our five force constants method could be used, the actual value obtained for the maximum allowable vertical force may vary with other safety harness systems and postural perturbations. (C) 2009 Elsevier Ltd. All rights reserved.