This article discusses one of the key biomechanical principles used to objectively measure movement.
Ground reaction forces (GRF) are the forces that we can measure based on our interaction with the ground. A person standing motionless on the ground exerts a contact force on it (equal to the person’s weight) and at the same time, an equal and opposite ground reaction force is exerted by the ground on the person. Similarly, in order to move, we put force into the ground and the ground applies forces back (GRF) onto us which then propels us forward. Every human movement is the result of a force being applied!