>>( just did a little research, and it seems a big difference is that with no forward passes you don't have people moving at the speeds of NFL wide receivers and kickoff returns in NFL are particularly brutal as two sides move toward each other at a dead run. also, if I understand correctly you have a rule against obstruction, not allowing contact with players not carrying the ball. In football that is blocking, it is an integral part of the game and most players will block or be blocked - often hard enough to knock them down or knock them out - on every play. )
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>An opinion; I think that if one wears protection or not also determines how one plays, attacks, defends, etc. If you and the opponent have no protection to speak off (rugby) then you will both tackle and defend in a particular way which is probably much less hard than if you both have extensive protection. The protection allows both parties to hit harder but also possibly suffer less.
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>There is an upper limit to how hard a human can attack another in rugby or America football. Does the upper limit on the protective effects of the body armour counter the maximum attacking force of a human athlete? Hypothetical question .. I don't know.

Good points. But I think the real issue is that the rules of the sports have evolved with the equipment or lack thereof. Helmets were introduced in football in the early 20th century (albeit leather things similar to scrum caps ) to keep the game from being outlawed outright.

Facemasks have been introduced within my memory.

But a big factor is that the popularity of the sport has lead to a Darwinian selection of players to where we have athletes running harder and faster and playing for bigger stakes than anything imagined before. And there is a lot of flying through the air at full speed and colliding. The padding only protects to some degree, but neck, back, knee, joint injuries are only exacerbated by getting hit by something a lot harder than a human body.

And again, the rules of rugby don't allow obstruction. You can only go after the guy with the ball, and all defense is zone, since you have to protect against the pitch back. Football *requires* very hard, violent "obstruction" by virtually every player about 70 times a game. Sometimes one guy is blocked or tackled by three or four other people and they don't have to link arms - they each come in full speed and more often than not don't try to wrap him up but rather to knock him down hard enough to dislodge the ball - and his head.

No doubt the rules of football would be dramatically different without this level of equipment and changes to the rules in rugby might very well mean making more equipment necessary.

I think it would be cool if there were some off season time when a good rugby side could do an intense 3 month training camp in American football and a comparable US Football team would do the same with rugby and then they could play a series of games alternating sports. I'm sure it would be a big international TV draw.