In balance analysis, the margin of stability is often calculated using data from which trajectories?

• A. Center of pressure alone.
• B. Knee joint angle trajectories.
• C. Center of mass velocity alone.
• D. Center of pressure and center of mass trajectories (COP/COM) or simplified measures.

Answer: (D)

Margin of stability is a dynamic balance measure that tells us how far the body is from tipping relative to the base of support. To compute it, you need information that reflects both where the body's mass is moving and how that movement is being controlled at the feet. The center of pressure shows where the ground reaction force is applied and therefore reflects foot- and ankle-level control, while the center of mass tracks the body’s mass location (and, with velocity, allows extrapolation). By analyzing both COP and COM trajectories, researchers can calculate the margin of stability—often via the extrapolated center of mass or a related distance to the edge of the base of support. Using only a single trajectory (COP alone, COM velocity alone, or knee angles) misses essential dynamics between body motion and foot-ground control, so combining COP and COM data yields the most informative and accurate measure.