Final answer:
Newton's Third Law of Motion explains the ground reaction force measured in biomechanics labs, as it states that for every action, there is an equal and opposite reaction.
Step-by-step explanation:
The principle of the ground reaction force measured in biomechanics labs when a person stands on or walks over a force platform can be explained by Newton's Third Law of Motion. This law states that whenever one body (e.g., a person) exerts a force on a second body (the ground), the second body exerts a force that is equal in magnitude and opposite in direction back on the first body (e.g., the reaction force from the ground). Therefore, when a person pushes against the ground while standing or walking, the ground pushes back with an equal and opposite force that can be measured by the force platform.