NFL Video Rulebook Down by Contact
Rule 7 Section 2 Article 1
Rule 7 Section 2 Article 1
An official shall declare the ball dead and the down ended:
Note: If, after contact by an opponent, any part of a runner’s leg above the ankle or any part of his arm above the wrist touches the ground, the runner is down.
Note: If a quarterback does not immediately drop to a knee, and contact from a rushing defender is unavoidable, it is not a foul unless the defender commits some other act that would constitute unnecessary roughness.
Notes: