As far as comeback seasons go, it doesn’t get much better than what Adrian Peterson was able to accomplish in 2015. But something about that season has been eating at Peterson and he’s set out this summer to fix it.

After missing all but one game of the 2014 season due to a suspension and subsequent legal issues stemming from a domestic violence case involving his son, Peterson returned to the gridiron in 2015 at the age of 30 and didn’t skip a beat. He led the league in rushing yards with 1,485, carries with 327 and in yards per game with 92.8. The old AD returned with a vengeance and put the league on notice that he wasn’t done just yet.

With his help, the Vikings offense became somewhat potent again and that was enough, with a defense that ranked among the league’s best, to create a four-game improvement on Minnesota’s record. The Vikes captured the NFC North crown and made the postseason for the first time in three years.

But Minnesota wasn’t long for the playoffs and that continues to haunt Peterson, whose nine-year NFL career has featured just four playoff visits and only one trip beyond the first round. What has eaten at Peterson the most this offseason is how it happened this time, the role he played and a key fumble that was a sign of a much larger flaw in his game.

In their first playoff game in three years, the Vikings faced off with another of the league’s most elite defenses, the Seattle Seahawks, and the game was the defensive battle everyone anticipated. Still, Minnesota controlled the entire game and led 9-7 early in the fourth quarter. But that’s when a Peterson fumble, just two plays after Seattle had gotten on the board with a touchdown, swung momentum heavily toward the Seahawks and gave them great field position.

Seattle moved the ball just 12 yards over the next six plays but that was enough to get in position for what would end up being the game-winning field goal in a 10-9 Vikings loss.

On its own, the miscue by Peterson might be dismissed as a wrong place at the wrong time event, but Peterson sees it differently. It was his eighth fumble of the 2015-16 season, one shy of his career-high mark in that department, which he hit in 2008 and 2009. Since then he had been able to get his ball security in check and fumbled just 11 times in 58 games from 2010 through 2014.

However, after the long layoff in 2014, his fumbling woes resurfaced, so Peterson has spent the 2016 offseason working to erase them. As ESPN’s Ben Goessling writes, the four-time All-Pro has been hard at work in the gym doing anything he can to make sure the fumbling problem that plagued him in 2015 doesn’t come back to bite him again this coming season.

That process has involved long resistance workouts in which trainers literally try to beat the ball out of Peterson’s hands. As the workouts have gone on this summer, that’s become nearly impossible to do. The training techniques are similar to ones the running back used earlier in his career, after a two-fumble performance in a playoff game against New Orleans helped end Minnesota’s 2009 season early. He returned to them this offseason after an epiphany, inspired by an old teammate’s critique of his carrying technique, that there was a reason teams were trying so hard to dislodge the ball from his grasp.

“When you’ve got people who are more focused on,’OK, I kind of don’t want to tackle him anyways; let’s find another way to slow him down,’ they are focused more on punching that ball out,” he told Goessling. “That’s something I just have to realize and make sure I eliminate that.”

Peterson has altered the way he carries the ball slightly, eliminating some bad habits of carrying his arms high that he says come from his time as a track athlete, and strengthened his grip on the ball. He still thinks back to that fumble against Seattle often and credits the players on the Seahawks with a great play that changed the game. But that won’t stop him from doing everything in his power to make sure it never happens again.

“Don’t get me wrong: It was a heck of a play by those guys,” Peterson told Goessling. “One had my arm; the other ripped the ball. It’s not like it was punched out, or it was an easy give-up. They worked for it. But being able to have that arm pulled and still have that thing tight, or having the awareness to let your body weight go down and try to get your knee down, that could have prevented that. That’s what goes through my mind.”

It’s what goes through the mind of many veteran players in the offseason, finding ways to improve and adapt by any means necessary. Peterson will begin putting what he’s learned this offseason to the test soon and if it works, it will make one of the game’s most elite players even more difficult to stop.

FITZ OUT TO PROVE HE HAS MORE TO GIVE

Arizona Cardinals star wide receiver Larry Fitzgerald also went into this offseason looking for ways to improve and adapt, just as he always does. Last year, that tried and true technique worked in a way few anticipated.

After a few down years that had many questioning what he had left to give the game, Fitzgerald bounced back in his 12th NFL season with a monster 2015 campaign. He caught a career-high 109 passes, piled up 1,215 yards and scored nine touchdowns as part of an Arizona offense that ranked as the league’s most potent in the regular season. He kept humming in the postseason, posting one of his career best games, with eight catches for 176 yards and the game-winning touchdown in overtime in the Cardinals divisional round victory over the Green Bay Packers.

But the following week, Arizona’s Super Bowl quest was derailed in a NFC Championship Game thumping at the hands of the Carolina Panthers. It left Fitzgerald short of his ultimate goal for a 12th straight season, though he was so close he could taste it for the first time in quite a few years. It’s that quest for a championship that keeps Fitzgerald going entering his 13th season and has him spending his offseason improving and adapting to keep up with the young wideouts many believed had passed him by.

“I have a lot of good football left in me. How long I will play, I don’t know,” Fitzgerald told NFL Total access on Wednesday. “I feel great. I can still play at a high level. I just have to take it one year at a time and I got a really fantastic team I’m a part of.”

Being part of that team is one thing that has helped Fitzgerald adapt. After a couple inconsistent seasons, he was looked at by opposing teams as more of an afterthought in an offense that featured big play threats in Michael Floyd and John Brown. That allowed Fitz to thrive in a way he hadn’t since 2011. But after the bounceback 2015 campaign, attention will shift back to the soon to be 33-year-old as a prime piece of the Arizona attack. How he and the offense adapt to that change will go a long way toward seeing if he can duplicate his 2015 success.

However, there’s no doubt that Fitzgerald has put in the work this offseason to stay at that level. His work ethic combined with his talent is what has kept him in the league so long and what will keep him chasing a championship as long as his body can handle it. That’s endless pursuit and tireless push to improve is thematic of many, but not all, of the league’s veteran stars, and it’s that which separates the very good from the elite and Hall-of-Fame caliber talents like Fitzgerald and Peterson.