(CNN) Dwight Clark and Gale Sayers are both NFL legends, and their struggles with brain disease were revealed this week.

In an open letter on his website , San Francisco 49ers legend Clark announced that he has amyotrophic lateral sclerosis.

The neurodegenerative disease -- also known as Lou Gehrig's disease -- disrupts the connection between the brain and the muscles and can eventually leave sufferers paralyzed but with brain function, or "locked in."

Some experts have hypothesized that the condition may be due to exposure to repeated head trauma

In his letter, Clark said that he had constant pain in his neck since his football days and in September 2015 developed weakness in his left hand. Months later, he was diagnosed with ALS.

"While I'm still trying to wrap my head around the challenge I will face with this disease over the coming years, the only thing I know is that I'm going to fight like hell and live every day to the fullest," Clark wrote.

San Francisco 49ers Dwight Clark's iconic reception in 1982.

"In addition to losing strength in my left hand -- which makes opening a pack of sugar or buttoning my shirt impossible -- I have now experienced weakness in my right hand, abs, lower back and right leg. I can't run, play golf or walk any distances. Picking up anything over 30 pounds is a chore. The one piece of good news is that the disease seems to be progressing more slowly than in some patients," he said.

Clark played as a wide receiver for the 49ers from 1979 to 1987. He is best remembered for "The Catch" in the 1982 NFC playoffs, which took the 49ers to victory against the Dallas Cowboys in the final minute of play.

Sayers' struggle

Research suggests that professional football players are four times more likely to have ALS and three times more likely to die from neurodegenerative diseases like ALS or Alzheimer's than the general population.

In a recent interview with the Kansas City Star , Ardie Sayers, wife of former Chicago Bears great Gale Sayers, opened up about her husband's dementia. Sayers, the "Kansas Comet," was the youngest person to be inducted into the Pro Football Hall of Fame, at the age of 34 in 1977.

Although he was diagnosed just four years ago, Ardie believes the disease began surfacing as far back as 2009.

Sayers' story was immortalized in the 1971 movie "Brian's Song," starring Billy Dee WIlliams as Sayers and James Caan as Brian Piccolo, which told about the friendship they developed as they became the first interracial NFL roommates while competing for a spot with the Bears.

Today, his wife says, 73-year-old Sayers is healthy "as a horse" and works out with a trainer several days week, but mentally, he is not as strong. He has a hard time holding a conversation, and she said he had recently washed his hands with carpet cleaner.

It is not known whether Sayers' condition is directly a result of his football career, but his wife says doctors have no questions about whether it is a factor.

'A part of this has to be on football'

"Like the doctor at the Mayo Clinic said, 'Yes, a part of this has to be on football,' " Ardie Sayers told the Kansas City Star. "It wasn't so much getting hit in the head. ... It's just the shaking of the brain when they took him down with the force they play the game in."

Much of the concern about repetitive hits has been over their link to the neurodegenerative disease called chronic traumatic encephalopathy. Some experts have posited that it can prompt the development of other diseases, such as ALS, as was the case with former Philadelphia Eagles player Kevin Turner.

In November, doctors said that Turner had CTE when he died and that it had brought on his ALS. Turner joined a roster of former football players diagnosed with CTE, including Frank Gifford, Junior Seau and Dave Duerson.

Despite the fact that Hall of Famer Mike Webster had the first case of CTE diagnosed in a professional football player in 2002, it was not until March 2016 that the NFL publicly acknowledged a connection between the sport and brain disease.

In his new letter, Clark says, "I've been asked if playing football caused this. I don't know for sure. But I certainly suspect it did. And I encourage the NFLPA and the NFL to continue working together in their efforts to make the game of football safer, especially as it relates to head trauma."

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In 2012, 80 concussion-related lawsuits on behalf of more than 2,000 NFL players were combined and filed as a single class-action lawsuit in federal court. The players accused the NFL not only of negligence but of failing to notify them of the link between concussions and brain injuries. Turner was a lead plaintiff on the complaint.

Three years later, a federal judge gave final approval to a settlement. By this time, over 5,000 former NFL players had filed suit and were eligible for the settlement. This agreement provided up to $5 million per retired player for serious medical conditions associated with repeated head trauma.

The deal also called for baseline medical exams for retired NFL players and monetary awards for those who are diagnosed with Alzheimer's, Parkinson's disease, dementia, ALS and certain cases of CTE.

Hope for a cure

Clark's letter focuses on fighting the disease and the support provided to him by his family, friends and former 49ers teammates.

"Every single one of my 49ers teammates that has contacted me has said whatever I need, anytime I need it, they will help. That's just the kind of guys they are. They were so giving as players and now they are the same as friends," he said.

"I'm not having a press conference or doing any interviews. That time will come. Right now, I've got work to do. I've got to devote all my energy preparing for this battle and I would hope you can respect my family's privacy as I begin this challenge. My ultimate hope is that eventually I can assist in finding a cure for ALS, which disrupts the lives of so many and their loved ones."

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Like Clark, Ardie Sayers is focused on the future -- not just of her husband but of others like him.

"It's hard, yes, I'm not saying it isn't. And it's challenging at times," she said. "But then when I stop and think about the people around me and people that are willing to help and family that are willing to come ... we're blessed that way."