Fig. 1
An inverted pendulum model shown at the point of heel strike
An inverted pendulum model shown at the point of heel strike can be used to consider the anterior-posterior (AP) Margin of Stability (MoS). The inverted pendulum consists of a mass (black circle) representing the Centre of Mass (CoM) atop a very light rod representing the leg length (solid black) attached to a hinge at the ground representing the ankle joint. The Extrapolated CoM (XcoM) (white circle) is the position of the CoM accounting for velocity, as described in more detail in text. The anterior limit of the Base of Support (grey circle) is positioned at the toe. In this example the XcoM is behind the BoS and calculating the MoSAP (BoS - XcoM) would give a positive value. Under these conditions and given the CoM is the origin, the Inverted Pendulum model would fail to reach vertical and would eventually return to it’s current position, which is considered “stable” for this systematic review. Conversely, if the XcoM was in front of the BoS the inverted pendulum would swing over vertical and proceed to fall to the ground (rightward on the planar image) – it would not return to its current position and is therefore considered unstable for this systematic review. This figure was created by FW