[DadofTwins]

Question about Philadelphia's last challenge flag Sunday night:

Is a ball carrier down if his elbow/knee/whatever touches a defender lying on the ground, or can he keep going forward until his body hits grass?

Reid's challenge was based on where McNabb's elbow touched the turf. But the last few feet of McNabb's movement came while lying on top of a defender.

I can't find anything in the NCAA or NFL rule books to address the question.