Chris Johnson built his NFL career around what his body could do. The former Tennessee Titans star was once one of the fastest and most explosive running backs in football, rushing for 2,006 yards in 2009 and earning the nickname “CJ2K.” Now, as he lives with ALS, Johnson says the game that defined so much of his life has become impossible to separate from questions about why he developed the disease.
Johnson was diagnosed with amyotrophic lateral sclerosis in May 2025 at age 39 after first noticing weakness in his right hand. In a new interview with PEOPLE, he said doctors have never told him football caused his ALS and have not identified a single explanation for why he developed it.
Still, Johnson said he “can’t help but wonder” whether years of hits and physical punishment played some role. He raised the same question in a Sept. 10 letter to his four children, while repeatedly cautioning that he is not a medical expert and does not know what caused his disease.
There is research behind the question, but not an answer about Johnson himself. A major study found ALS rates were significantly higher among NFL players than among U.S. men overall, while the ALS Association says the exact cause of the disease remains largely unknown and likely involves a combination of genetic and environmental factors.
Johnson Says He ‘Can’t Help But Wonder’ About Football
Johnson spent 10 seasons in the NFL, including six with Tennessee after the Titans selected him in the first round of the 2008 draft. Football had been part of his life long before that, so his playing history naturally became part of discussions with doctors after his diagnosis.
“Football has come up throughout this process when we’ve talked with doctors because I played the game basically my whole life,” Johnson told PEOPLE. “That’s a big part of my history.”
He stressed that his doctors have not told him the sport caused his ALS. But after years of collisions and the physical demands of professional football, he acknowledged that the possibility stays on his mind.
Johnson made the same distinction in his Players’ Tribune letter. He wrote that he has seen numerous neurologists and visited major ALS clinics around the country, and said doctors often seemed to take notice when they learned about his football career.
The NFL Study Found an Association, Not a Cause
The study Johnson referenced was published in JAMA Network Open and examined 19,423 players who appeared in at least one NFL game after debuting between 1960 and 2019.
Researchers identified 38 players who had been diagnosed with ALS. After adjusting for age and race, the study calculated an ALS incidence rate about 3.6 times higher and an ALS mortality rate about 3.9 times higher than expected among U.S. men.
The NFL players who developed ALS also had longer professional careers on average. They played approximately seven seasons, compared with about 4.5 seasons among matched players who did not develop the disease.
Those results were statistically significant, but the authors did not conclude that football itself caused ALS. They wrote that their study could not identify which exposure or combination of exposures might account for the elevated rate.
The researchers also lacked direct information about players’ histories of repetitive head impacts or traumatic brain injuries, as well as genetics, environmental exposures, smoking and other potential ALS risk factors.
The ALS Association similarly lists brain and spinal injuries among factors that research suggests may increase ALS risk, particularly alongside professional sports participation, while emphasizing that increased risk is not the same thing as identifying the cause of one person’s illness.
His First Warning Sign Was Weakness in His Right Hand
Johnson’s road to diagnosis began with something much less dramatic than the symptoms he deals with now. In his Players’ Tribune account, he recalled that his right hand began feeling strained in early 2025. During a family road trip that March, he picked up a water bottle he regularly carried and suddenly found it unusually heavy. His wife, Brittany, lifted the same bottle and did not notice anything unusual.
After struggling to schedule an appointment, Johnson contacted a former Titans team doctor, who helped arrange a nerve-conduction study in Nashville.
The initial test did not give Johnson an ALS diagnosis. Instead, paperwork referenced possible anterior horn cell disease and he was told the problem could be serious. Johnson and Brittany began searching the terminology themselves while he waited at the airport and discovered ALS was among the possibilities.
Sports Illustrated reported that Johnson underwent further testing before receiving the official diagnosis in May 2025 from neurologist James Wymer in Florida.
When Johnson publicly revealed the diagnosis during a June GMA interview, he said there was no known history of ALS in his family and that doctors considered his case sporadic.
Brittany said football initially entered her mind for a much less serious reason. When Johnson’s hand weakened, she thought years in the NFL might have left him with something such as a pinched nerve.
ALS Has Progressed to the Point That Johnson Uses His Eyes to Speak
The physical changes since those first symptoms have been severe. By the time Johnson appeared with Michael Strahan on Good Morning America in June, he was communicating through a speech-generating device controlled by his eye movements. His voice had been recorded earlier so the technology could reproduce a version that sounded more like him.
A September Sports Illustrated profile described Johnson as unable to speak or walk without significant assistance. PEOPLE later reported that he uses an electric wheelchair and needs help with feeding, changing and other daily activities, with Brittany serving as his primary caregiver.
Johnson has emphasized that, in his own case, those physical limitations should not be mistaken for a loss of awareness or identity. He told ABC News that he still thinks, dreams and loves his family the same way even though his body no longer cooperates.
He Still Supports His Twin Sons Playing Football
His 14-year-old twin sons, CJ and Kaden, play competitively. Johnson told PEOPLE that he does not look at his diagnosis and conclude that they should stop, although he now thinks more carefully about the risks and wants them to pay attention to their health.
Brittany expressed a similar conflict. She said the possible risks concern her, but football has become an important part of their sons’ lives and neither parent wants fear alone to determine what they do next.
Johnson went further in his letter to the children. He told CJ and Kaden that their football journey belongs to them and encouraged them to follow it if that remains what they want. He also wrote that he believes the game is safer now than when he played and hopes safety continues improving.
His Family Has Become the Center of His Fight
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He and Brittany are raising daughter Honey and twin sons CJ and Kaden, while older son Lee is in college. Johnson has said one of the hardest parts of ALS is watching responsibilities shift onto the people around him, including his children.
After revealing his diagnosis publicly in June, Johnson posted a message on Instagram thanking Brittany for becoming his caregiver, advocate and voice while continuing to hold their family together. Us Weekly independently reported the message.
His Ring of Honor Ceremony Was Postponed
The Titans had planned to formally induct Johnson into the franchise’s Ring of Honor during their Sept. 13 season opener against the New York Jets.
That ceremony was postponed after Johnson and his family were no longer able to travel to Nashville. In its statement, Tennessee cited “unforeseen circumstances” and said the formal induction would take place later when Johnson and his family could attend. The Titans still dedicated their halftime presentation to Johnson and his career.
Few seasons explain why the honor matters better than 2009. Johnson rushed for 2,006 yards and added 503 receiving yards, setting an NFL single-season record with 2,509 yards from scrimmage. He was named the league’s Offensive Player of the Year.
NFL.com noted at the time that Johnson became the first player in league history to finish a season with at least 2,000 rushing yards and 500 receiving yards.
The Titans have said Johnson’s formal Ring of Honor induction will be rescheduled for a date when he and his family can be there in person.