For as long as helmets have collided, the fiercest opponent for NFL players has been time. They walk in the door on a spring day with strength, vitality and focus. They walk out the door years later with limps, scars and memory lapses. Time never loses.

As bodies have become bigger, collisions more impactful and pressure more intense, NFL players have aged more quickly. The league chews and spits faster than ever. So instead of looking at their NFL careers as marathons, more players may be looking at them as sprints, and moving up the finish line.

The latest example is Rashard Mendenhall. Rather than return for a seventh NFL season, the Cardinals running back plans to follow other dreams. It sounds like Mendenhall grew tired of a lot of things, including the entertainment aspect of the NFL. This is how the 26-year old explained it in a column he wrote for the Huffington Post: "… I've greatly enjoyed my time, but I no longer wish to put my body at risk for the sake of entertainment. I think about the rest of my life and I want to live it with much quality. And physically, I am grateful that I can walk away feeling as good as I did when I stepped into it."

He already has dealt with a fractured shoulder, courtesy of Ray Lewis, and a torn ACL. But you get the feeling this is about more than ice packs and hot tubs. When Mendenhall was growing up, he lived a couple towns over from my town. I went to see him play when he was in high school. He always has been a different kid. Mendenhall does not have the type of personality that lines up neatly with others on a cookie sheet. Certainly, he is a free thinker. He has written about a vision he had of the Blessed Mother, about living with few possessions and about finding his comfort outside of football.

One of the aspects of the NFL that bothered Mendenhall is the scrutiny that comes with being a star player. He wrote of feeling uncomfortable with "waves and currents of praise and criticism," and spoke of experiencing racism. He made himself a target with some tweets after Osama bin Laden was killed in 2011. Among his tweets: "What kind of person celebrates death? It's amazing how people can HATE a man they have never even heard speak. We've only heard one side." And: "We'll never know what really happened. I just have a hard time believing a plane could take a skyscraper down demolition style."

So Mendenhall was not the typical NFL player. But he is not the only NFL player who thinks the grass is greener beyond the playing field. Panthers left tackle Jordan Gross just retired at 33. The Panthers wanted him back, and if he had become a free agent, there would have been significant interest in his services. He probably could have made $10 million this season, in the estimation of people who know. But Gross said he felt he no longer could make the commitment it took to be an NFL player.

Last November, Broncos guard John Moffitt walked away from a hefty six-figure salary and a chance to be part of a Super Bowl team because he became disenchanted with the concept of trading his health for a paycheck. The 27-year old played in only 19 NFL games, but already there had been a blown-out knee, elbow surgery, shoulder injuries, sleep apnea and floaters affecting his vision. So after he left the Broncos to visit his girlfriend in Seattle during the bye week, he did not come back.

Linebacker Channing Crowder was a starter with the Dolphins in 2010. In 2011, he was released in training camp. At 27, Crowder still was a commodity. He worked out for the Patriots and said the Bills, Eagles and Bears were interested. But he was not. He walked away to spend more time with his family.

Brian Urlacher's contract was up a year ago. The Bears made him an offer that was not to his liking. They agreed to part ways. Urlacher still could have played at a fairly high level in 2013, but rather than wait for a new team to want him, he decided to play golf, go fishing and dabble in broadcasting.

Last June, the Bears signed free agent Sedrick Ellis to play a significant role after Ellis had visited several teams. He looked good in offseason workouts and created a buzz at Halas Hall. The defensive tackle was the seventh pick of the 2008 draft and still had potential. But after five NFL seasons, at the age of 28, he had enough. He was a no-show for training camp and gave back a $285,000 signing bonus.

Two years ago, at the age of 34, quarterback Marc Bulger could have had a starting job. But what was more appealing to him was the sound of spending more time with his charitable foundation. He quit, but NFL teams didn't stop calling for two years.

There have always been players like Bulger and Mendenhall who have left the NFL before they have to, even great ones. Jim Brown and Barry Sanders did it. But as the league becomes increasingly more difficult for older players, early exits are becoming more appealing. Instead of getting devalued, shuffled around and permanently impaired, players are jumping before they are pushed.

Boston Globe writer Ben Volin pointed out there were 312 players in the NFL last season who were 30 years or older. Just three years earlier, there were 380 such players. This is the new and improved NFL meat grinder.

I went to Steelers training camp in Latrobe, Pa., the year Mendenhall was a rookie. I remember his veteran teammates telling me he reminded them of Jerome Bettis, who had retired a few years earlier. In hindsight, the biggest difference between Mendenhall and Bettis is Bettis kept coming back over and over and over again. He hung around for 13 years and had more carries than every player in NFL history except Emmitt Smith, Walter Payton and Curtis Martin.

With the way the NFL is evolving, there will be fewer Jerome Bettises. There will be more Rashard Mendenhalls.