'lando, I haven't done the research, but I'd guess that there were fewer serious injuries in Rugby than American football. This is based on the assumption that since Rugby players have to play a full 80 minutes of game time with only a 10-15 minute half time break their training is focussed less on power and more on fitness/stamina, therefore the 'hits' are not as intense in Rugby. Also, an American football tackler is going to feel more confident of administering a big hit on an attacker because they are encased in body armour.