Borland became the most prominent NFL player to retire from football because of concerns over brain injuries. He is the third NFL player to retire despite being age 27 or younger this week, joining Jake Locker of the Tennessee Titans and Jason Worilds of the Pittsburgh Steelers.
St. Louis Rams defensive end Chris Long tweeted in support of Borland.
Borland told "Outside The Lines" that he had been thinking about leaving football as the 2014 season went along, and wrote a letter to his parents late in the year. After the season, he consulted with prominent concussion researchers and former players to affirm his decision.
"I've thought about what I could accomplish in football, but for me, personally, when you read about Mike Webster and Dave Duerson and Ray Easterling, you read all these stories, and to be the type of player I want to be in football, I think I'd have to take on some risks that, as a person, I don't want to take on."
Borland was referring to former NFL greats who were diagnosed with the devastating brain disease chronic traumatic encephalopathy, or CTE, after their deaths. Duerson and Easterling committed suicide.
Borland said he began to have misgivings during training camp. He said he suffered what he believed to be a concussion stuffing a running play but played through it, in part because he was trying to make the team.
"I just thought to myself, 'What am I doing? Is this how I'm going to live my adult life, banging my head, especially with what I've learned and know about the dangers?'"
Former Ohio State running back Maurice Clarett, now a motivational speaker, wrote a series of tweets, starting with:
Former NFL player Damien Woody, now an ESPN analyst, said on Twitter:
Borland played in college at Wisconsin for two seasons with John Moffitt, who retired in November 2013 in the middle of the season with the Denver Broncos. At the time, he said he'd lost the love for the game and was tired of risking his health for a paycheck.
"I've saved enough. It's not like I'm sitting here and I'm a millionaire," Moffitt said at the time. "That's what I kind of realized. I'm sitting here and I got to this point and I was like, what is the number that you need? How much do you really need? What do you want in life? And I decided that I don't really need to be a millionaire.
"I just want to be happy. And I find that people that have the least in life are sometimes the happiest. And I don't have the least in life. I have enough in life. And I won't sacrifice my health for that."
Green Bay Packers director of player personnel Eliot Wolf, in a tweet, said that Borland's decision hasn't changed the number of players he sees wanting to play in the NFL.
Wisconsin athletic director Barry Alvarez released a statement Tuesday to offer support for Borland's decision.
"Chris Borland is one of the most impressive young men I've come across in my many years involved in college athletics," Alvarez said. "His exploits on the field are well known as he is one of the best linebackers in school history and one of the best I've seen up close. Off the field, he earned his degree from Wisconsin and devoted countless hours to community service. He represents all that we want our Wisconsin student-athletes to be.
"Knowing Chris, his decision to retire at this time was well thought out and made after much consultation with people he trusts. He obviously feels that this is the best decision for him, personally. He will always be a part of the Badger family and we wish him the best in whatever his future may hold."
Wesley Walker, 59, who played 13 years with the Jets, said he's had surgeries on his neck, back, shoulders, knees and Achilles that he attributed to his football career. He said that he suffers from spinal stenosis and nerve damage, that he feels pain in his arms and fingers, and that he's had 14 screws and a plate inserted into his neck and 10 screws and two rods inserted into his back.
"I admire [Borland] for what he did. I admire him for being man enough and smart enough to know what's more important in life," Walker told ESPN.com's Ian O'Connor. "If I had to do it over again, and I knew I'd end up in the amount of pain I'm always in, there's no way in hell I'd play football again. With all of my injuries, including my neck, I took a chance of breaking my neck and ending up in a wheelchair. I look back and ask, 'What was I thinking?' "
"Every individual has to make his own decision, and there's so much money to be made these days. But is money more important, or is your life more important? I could never see myself hurting myself, but there have been times when I've thought, 'God, I wish you'd just end this right now.' I don't sleep, I'm in constant pain, I haven't felt my feet in 20 years. I feel like there are times when my whole body shuts down. Sometimes I feel like I'm 90 years old.
"[Commissioner]Roger Goodell is a good friend of mine. But I want the NFL to tell truth about what's happening with players, and I think they sugarcoat everything."
Gore Respects Borland's Decision To Retire
Frank Gore talks about his former 49ers teammate Chris Borland's decision to step away from football and retire at the age of 24 because of concerns about the long-term effects of repetitive head trauma.
Chris Borland On His Decision To Retire
49ers linebacker Chris Borland speaks to Mark Fainaru-Wada about his decision to retire from the NFL over concerns about the effects of head trauma.
49ers' Borland Retires, Says He Fears Brain Injury
Mark Fainaru-Wada discusses Chris Borland's retirement from the NFL due to concerns about the long-term effects of repetitive head trauma.