Ryan Williams has admitted that he was "scared to run the ball" during the 2012 NFL season.

The Arizona Cardinals running back was worried that his surgically repaired knee would not be strong enough to survive the impact of tackles.

The 23-year-old told Arizona Republic: "It's a scary sight being a running back and being scared to run the ball. That's something that no running back should feel. I was pretty much scared all last year to get the ball in my hands.

"As soon as I saw [New England Patriots linebacker Brandon] Spikes coming to hit me, I curled up and forgot about the ball. It was something where I was trying to protect my leg instead of trying to protect the football.

"That was my least favourite play, because running to the outside, my whole right side was exposed."

Williams managed just five games for the Cardinals last season, having sustained a shoulder injury in October.