Raw tracing of the location of the underfoot centre of pressure (COP) in the anterior–posterior (AP) and medial–lateral (ML) directions during one trial. The black dashed line indicates the average location of the COP pre-perturbation, while the grey dashed line indicates maximal anterior displacement of COP following perturbation. The minimum margin of safety (MMOS) is derived as the minimum distance between the COP and anterior boundary of the base of support (BOS) defined by the toes.