Nearly two years after Adrian Peterson stunned the NFL and surprised the medical field by returning from a devastating injury to threaten the NFL's single-season rushing record, the Vikings running back's left knee remains the standard by which all other surgically-repaired knees are measured.

Peterson tore the anterior cruciate ligament and the medial collateral ligament in his knee late in the 2011 season, an injury that turns most NFL running backs into mere mortals. But Peterson not only returned to the field eight months later, he rushed for 2,097 yards and a dozen touchdowns.

Looking back Wednesday on his recovery and his record-setting season, Peterson acknowledged that he made things difficult for his peers by creating unreasonable expectations for ACL recoveries.

"I knew that when I came back and had the kind of season that I had, I knew that it was going to be hard for anyone to duplicate that type of success after an ACL," Peterson said on a conference call to promote Hyperice, an ice compression wrap he says aided him in his recovery two years ago and also after his groin surgery this winter. "Why do I say that? I say that because just coming back is one thing. That work that I put in, I can't really express to you how hard I worked, how hard I grind."

But Peterson tried. Over the next 60 seconds or so, he described a rehab program that made most, if not all, of the muscles I have ache. Every day, he would ride an exercise bike and do other things to rehab his knee. Then he did upper-body workouts in the gym and performed other exercises to strengthen his quads, hamstring and groin. And that was all before lunch. He then would go meet up with his personal trainer, who had him do plyometrics and other activities to help him strengthen his lower body and regain his flexibility and range of motion in his left knee.

"And I did that for months," said Peterson, who turned 29 three weeks ago.

After rushing for 230 yards in the first three weeks of the 2012 season, he eclipsed 100 rushing yards in 10 of his final 13 games and topped 200 yards in two of them. Peterson came up eight yards short of Eric Dickerson's single-season rushing record, but he carried the Vikings into the playoffs.

"I was better and I knew just the work alone would be hard for anybody to duplicate," Peterson said. "So I set the bar high and I knew it was going to raise some trouble for some other people."

There were 63 ACL injuries during the 2012 season, according to NFL, and some were suffered by standout players such as Washington Redskins quarterback Robert Griffin III, Seattle Seahawks defensive end Chris Clemons, Baltimore Ravens cornerback Lardarius Webb, Houston Texans inside linebacker Bryan Cushing and New York Jets cornerback Darrelle Revis. While the injuries to and the circumstances for each player are different, each of those players experienced a drop-off in their play and production with some (Griffin and Clemons) steeper than others (Revis) in 2013.

No ACL last season was scrutinized more than the one in Griffin's right knee. Griffin, who reached out to Peterson's trainer for advice after suffering his knee injury, struggled at times last season before he was benched by (now former) Redskins coach Mike Shanahan for the final three games.

So what made Peterson the exception to all the rules about ACL recoveries two offseasons ago? While he acknowledged that genetics were probably a factor, he mostly chalked it up to hard work.

"That's not to say that other guys didn't work hard to come back," Peterson said. "But I know the kind of work that I put in and I tried to share that with people, but people have their own ways. That's perfectly fine as well. But I knew it was going to be extremely hard for someone to come out and put in the work that I put in."