Does the margin of stability measure predict medio-lateral stability of gait with a constrained-width base of support?

This study aimed to determine the validity of the centre of mass position (COM) position and extrapolated COM (XCOM), relative to the base of support, for predicting medio-lateral stability during a walking task where the base of support width is limited. Nine young healthy participants walked on a narrow beam. Three-dimensional motion capture was used to calculate the COM and XCOM relative to the base of support. Steps were classified as having either the COM or XCOM inside or outside the base of support, and were classified as successful (stable - foot placed on the beam) or failed (unstable - foot stepped off the beam). If the COM or XCOM are valid measures of stability, they should be within the base of support for successful steps and outside the base of support for failed steps. Classifying the COM and XCOM inside or outside the base of support correctly predicted successful or failed steps in 69% and 58% of cases, respectively. When the COM or XCOM were outside the base of support, walking faster seemed to help participants to maintain stability. The further the COM or XCOM were outside the base of support during a successful step, the more likely participants were to fail on a subsequent step. The results of this study suggest that both COM and XCOM are valid measures of stability during a beam walking task, but that classifying COM and XCOM as inside or outside the base of support may be over-simplistic.