The answer is NFL (American Football). A basic consideration of physics tells me that the body contact sports I follow in Australia (Rugby League and Rugby Union) do not generate hits as hard as NFL. The hardness of a hit can be measured in terms of Force. Good old Isaac Newton derived the law of Force = Mass times Acceleration. Of course, for "acceleration" you can read "deceleration" for football hits. The result comes to the same thing in terms of Force. NFL players have more mass. NFL players weigh from about 200 pounds to 300 pounds or more. Rugby players weigh about 180 pounds to 250 pounds. This is not even counting equipment and NFL players have heavier equipment. NFL players are also faster on average than rugby players. This is not true of the very heavy scrimmage line players over 300 pounds in NFL but the rest of them are certainly faster on average than rugby players. If there is more mass AND more speed to be decelerated in a collision, then it is an open and shut case. The NFL hits are harder by a considerable margin. Why are NFL players bigger and faster than rugby players? NFL as a game selects for bigger and faster players with the emphasis on explosive speed and bulk. NFL plays are short, full speed bursts with breaks between snaps. Rugby has less breaks and more continuous plays. Rugby League suits up 17 players for 13 on-field players for two halves of 40 minutes of almost continuous play. NFL suits up 46 players for 11 on-field players for 4 x 15 minute quarters of snaps with rests in between (except for no-huddle offense). NFL players come onto the field in rotations, rested and at near full power. Rugby players come on and stay on, for the most part, even when fatigued. Rugby players require more stamina but less explosive speed than NFL players. The way humans are built, more stamina is achieved by having less explosive speed and less bulk. These are unavoidable trade-offs. It's a new day (04/04/2016) so I will add a little. The above discussion leads on to the old chestnut: "Which sport is the toughest?" The answer goes like this. It all depends on what you mean by toughness. However, I would define "toughness" in this context like this. Which sport is harder to play at the highest level? Jarryd Hayne (rugby league player crossing over to NFL) gives us the answer after his rookie season in NFL. In rugby league he was our MVP (most valuable player). In NFL, he looked good in pre-season trials and then he struggled in real season games. Actually, he was a "curate's egg". He was good in parts and bad in parts. Of course, he was new to NFL and he did not have the knowledge, experience, game instincts and muscle memory for the game. My assessment is that if he gets opportunities in the next 2 years he can become an average journeyman running back and/or punt returner in the NFL. That is pretty good but he will never be a Beastmode type player in my opinion. This answers the question. NFL is tougher to play. Our rugby league MVP looks like becoming a journeyman at best not an MVP in NFL. That would still be an enormous accomplishment. It is a very elite athlete group. Why does the USA produce a tougher game? Simple. USA population = 320 million. Australia population = 22 million and only about half of them are rugby states. All other things being equal, the USA will produce 15 elite athletes for every 1 elite athlete Australia produces. And all other things are not equal. The USA is wealthier and more advanced in sports science, training facilities and so on. If this leverages to a factor of 2 (quite likely) then the USA will produce 30 elite athletes for every 1 elite athlete Australia produces. Not only does this give more super-elite athletes at the right end of the bell curve, it shifts the whole bell curve to the right to higher performance.
I think american football should be the toughest of them all, hockey too but with the second most of the energy is acquired, not absorbed. I did play a little american football when I lived there and it looked dangerous ( we played with no armour and the american guys usually called me too brutal, maybe I played too dangerously haha ). Wimbledon of Vinnie Jones could also join.
Now, Jarryd Hayne has quit the NFL after trying it out for one year. He started out okay in the 2015 preseason but fumbled and stumbled at the start of the regular season. He came back and made a few good runs with limited snaps. Then he was relegated to the practice squad mid-season and brought back for two games at the end. The SF49ers were a hot mess in 2015 so that certainly didn't help Hayne. It looked possible that Hayne could come back in his second year and maybe do better under Chip Kelly's system. But then Hayne suddenly retires after a forgettable first year and some off-season training where he claimed he was still focused on the NFL! He now says his dream is to play rugby sevens for Fiji. All I can say is WTF! Hayne has shown himself to be a peripatetic dilettante. It's obvious that his dream to play NFL was not backed by desire, passion or commitment. I would understand if he had medical concerns but this has not been mentioned. I would understand if Chip Kelly had indicated he had no place for Hayne. But Chip actually said he was intrigued by Hayne's size, speed, evasiveness and skills. So there is no good reason advanced for why Hayne has quit. I guess Hayne as a free person is entitled to do what he likes (as we all are) but a professional sports coach or manager of any franchise, in any code, would now wonder, "How committed is this guy to anything? His track record on commitment is poor. He would be a poor hire." SBS Comedy writes: "Jarryd Hayne retires to puruse lifelong dream he has had for half an hour." "Australian media have already claimed Hayne’s retirement from NFL is the most impressive of all time, with reports that the coaching staff of the 49ers were really impressed with the ‘vigour’ and ‘determination’ with which he quit the sport."
