Dr. Michael Farrell:

I do not work on the sidelines but would point out that in American football where the players are genuinely considerably bigger than are the players in rugby, there are two issues. One is the wearing of a helmet, which can be used as an offensive weapon. In other words, one leads into a head to knock out or injure the other player. Second, there are off-the-ball tackles in rugby which are not allowed in football. An unsuspecting player can be gang-tackled by two or three people and one does not need to have one's head hit to be concussed; the mere rapid acceleration or deceleration associated with a tackle, for which one is not prepared, is what separates American football, in many ways, from rugby, football and other sports. The first issue is that the helmet is used as an offensive weapon and the second issue is third-man-tackle or the off-the-ball tackle by one or two other players on an unsuspected player.