Center of Pressure (CoP)

The single point where the ground's total supporting force on a foot effectively acts, used to judge balance.

When a foot presses on the ground, the ground pushes back at every point of contact; the center of pressure is the single point where all of that distributed force can be combined into one equivalent ground reaction force. Biomechanics researchers measure it with a force plate — while standing, it drifts slightly as the body sways, and while walking, it travels from heel to toe. By definition, it can only fall inside the support polygon (the convex region enclosed by all the ground-contact points); once it reaches the edge, the foot is about to start tipping over that edge. It comes from a different source than the zero moment point (ZMP): CoP is computed from contact forces, while ZMP is derived from the whole robot's motion and acceleration — but the two coincide whenever there's a single contact surface or the robot is walking on flat ground, which is why classical humanoid walking control often uses “keep the ZMP/CoP inside the support polygon” as its stability criterion, measuring CoP in real time with a six-axis force sensor in the foot.

ExampleAs a standing person leans forward, their center of pressure shifts from the middle of the foot toward the toes; lean too far and, once it reaches the edge of the toes, it can't move forward any further — the only options left are to take a step or fall forward.

Also called

CoP, Center of Foot Pressure

Related

Zero Moment Point · Support Polygon · Ground Reaction Force (GRF) · Foot Force Sensor · Center of Mass (CoM) · Balance Control

Sources

Center of pressure (terrestrial locomotion) - Wikipedia
Zero-tilting moment point - Stéphane Caron