Former NRL star Jarryd Hayne has revealed how two hits in his rookie NFL season left him "rattled" for days.
Back in Sydney during the off-season, Hayne said he needed to strengthen his neck in order to prepare for the bigger impact of the American game.
"Obviously the endurance side of things is different but I got hit twice head on and my head was rattled for a couple of days," he said.
"Definitely your neck has to get a lot stronger. It's pretty tough. It's like having a helmet and running through a brick wall. If you do that, you can make it in the NFL."
The 27-year-old played eight games with the San Francisco 49ers in his first season as a professional NFL player, but isn't guaranteed a return.
Franchise aren't permitted to talk with players for another two months, but the 49ers do hold his rights and can offer him a spot when team activities are organised for the end of April.
Hayne, who was used primarily as a punt returner under since-sacked coach Jim Tomsula, has planned a return to California at the end of March and aims to become a more versatile player.
"I like being out wide like a wide receiver. Running back's also very enjoyable as well. I think for me, it's just about being versatile and playing anywhere they put me," he said.
He was also willing to be used as a slot receiver, a position he experienced while training in opposed sessions at the 49ers.
"You've got to run every other play for the other team coming up that week and I actually did a lot of slot receiver, which I thoroughly enjoyed," he said.
"But for the coach, I've just got to stay fit and play wherever they want me."
Either way, a burning desire to complete his first career touch down has proven to be the ultimate motivation for coming back this year.
Hayne came close when he was being used as a running back against the New York Giants.
"I got close twice this year. I was devastated," he said.
"It was actually a route that I ran that if I had run it a bit different, I would've scored against Detroit. I come close, and then obviously that one against the NY Giants as well.
"Definitely next year, it has to be done. It has to be done."