What is the margin of stability (MoS) in clinical balance assessment?

• A. The margin of stability is the distance between the boundary of the base of support and the extrapolated center of mass; computed from COP/COM trajectories or simplified measures to reflect how close the system is to losing balance.
• B. The margin of stability is the difference between predicted and actual step timing.
• C. The margin of stability is the distance from the ground to the center of mass.
• D. The margin of stability is the difference between center of pressure and the foot position.

Answer: (A)

The margin of stability measures how close the body’s current dynamic state is to tipping over by comparing the extrapolated center of mass to the edge of the base of support. The extrapolated center of mass (XCoM) combines where the body’s center of mass is and how fast it’s moving, providing a predictor of where balance would be given the present momentum. The margin of stability is the distance from the boundary of the base of support to this XCoM. A positive value means you’re still within the stability boundary, a small (near-zero) value means you’re close to losing balance, and a negative value indicates instability.

In practice, MoS can be computed from COP/COM trajectories during standing or walking, or estimated with simpler measures, to reflect how near the system is to losing balance. Clinically, a smaller MoS signals reduced stability or higher fall risk and can help track balance impairments or improvements after intervention.

The other options don’t fit because they describe timing differences, vertical height of the center of mass, or a direct COP-foot position difference, none of which capture the dynamic relationship between the base of support boundary and the moving body’s momentum that MoS represents.