Locomotion

Ground Reaction Force (GRF)

Ground reaction force is the force exerted by the ground on a body in contact with it, equal and opposite to the force the body applies to the ground. In legged robotics, GRFs at each foot are the only external forces available to accelerate the robot's center of mass, making their planning and control the essence of locomotion; they are constrained to unilateral contact with friction cones and are measured by foot sensors or estimated from joint torques.

Why it matters for physical AI

Every locomotion controller, from ZMP walking to RL policies, is ultimately shaping ground reaction forces; accurate GRF estimation and control determine stability on slippery, soft, or uneven real-world terrain.