Define center of mass (COM) and center of pressure (COP) and explain why their relationship matters for stance control.

• A. COM is the weighted location of the body's mass; COP is the point of application of ground reaction forces on the support surface. Stability relies on controlling the relationship between COM and COP so that the projection of COM remains within the base of support.
• B. COM is the center of the base of support; COP is the body's mass; balance does not depend on their relation.
• C. COM is the application point of the ground forces; COP is the mass distribution; their relation is irrelevant.
• D. COM is the center line of the spine; COP is the visual axis; their relation ensures alignment.

Answer: (C)

Understanding how COM and COP relate is essential for stance control. The center of mass is the weighted average location of all the body's mass, essentially where the mass is concentrated in space. The center of pressure is the point on the support surface where the total ground reaction force acts. Stability in stance comes from how the projection of the COM falls within the base of support and how the COP moves in response to sway.

When you sway, the body uses muscle activity to shift the COP under the foot to generate a counteracting torque, nudging the COM projection back toward the center of the base of support. The closer the COM projection stays to the center, the easier it is to maintain balance with small adjustments. If the COM projection moves toward the edge of the base, the body must shift the COP further in the opposite direction or take a corrective step to prevent tipping. In short, the dynamic interplay between where the mass is located (COM) and where the ground pushes (COP) governs how effectively the system can maintain stance without falling.