How does a runner's motion compare to the ground?

A runner's motion involves a combination of forward propulsion and vertical movement, as the legs push off the ground and then lift off during each stride. While the runner moves forward, their feet repeatedly contact the ground, creating a cycle of acceleration and deceleration. The ground remains stationary, providing the surface for the runner to push against, while the runner's body experiences changes in speed and direction relative to that fixed ground. Overall, the runner's motion is dynamic, characterized by alternating phases of support and recovery with respect to the ground.