Though I naturally agree with the Americans on this one, and have fought this battle several times, Oxy you are still a shameless troll.
US football is a game of strength and quickness. A sprint.
Soccer (UK football, world football, whatevz) is a game of endurance and bursts of speed. A marathon.
Few American football players could play professional (world) football except for perhaps some of the Wide Receivers or Defensive Backs.
Few world players could play any positions in the NFL except as kickers, or maybe... and this is a very, very big maybe due to their never having used their hands before, as Wide Receivers or Defensive Backs.
Rugby players truly are badasses, and anyone saying otherwise is bullshitting, but if anyone thinks US football players aren't the strongest Athletes in any modern team sport they are out of their minds. Rugby is still a marathon game, and while those men are clearly tough as nails they do not suffer impacts from men 100 pounds heavier than they are at speeds nearing 15 mph or greater.
You don't need to be some uber-guru to understand this either. A simple measure of the size of your average athlete in any of the given sports tells the story here. (For US Football you kind of have to vary it by position to be accurate, separating linemen from linebackers and running backs/tight ends, and defensive backs and wide receivers from both of those groups. Specialists like QBS, kickers, and punters, don't translate well to the other sports that simply lack special positions).
American football players do not have the endurance to play Rugby though NFL running backs, Linebackers, and possible Receivers and Defensive Backs could likely be trained to play it. I'm not sure about training Rugby players for offensive positions in the US game, again because of the hands aspect, but they could certainly be trained to play any defensive position save as Linemen, as well as kickers or punters.
Anyway, I hate this fight. It's two totally different sports (or three, actually) and each specializes in its own thing that none of the others can really pull off.
Damn shit starting J00z!
"When do you ask yourself,
'Maybe everyone else isn't wrong for using the definitions of words; maybe I'm wrong for making up new definitions of words and then using them as crude slurs' -TiG