Clark made the announcement in a statement posted on Sunday to the website of former 49ers owner Ed DeBartolo Jr.
“In September of 2015, I started feeling weakness in my left hand. I was mildly paying attention to it because since my playing days, I’ve constantly had pain in my neck. I was thinking it was related to some kind of nerve damage because it would just come and go,” Clark wrote.
“After months of tests and treatment, I got some bad news,” he continued. “I was diagnosed with amyotrophic lateral sclerosis. I have ALS, also known as Lou Gehrig’s disease. Those words are still very hard for me to say.”
In recent years, ALS, commonly referred to as Lou Gehrig’s disease, has been linked to the degenerative brain disease chronic traumatic encephalopathy, ESPN noted. CTE is found in athletes and other individuals with a history of concussions and other repetitive brain trauma.
Clark addressed questions surrounding the connection between his football career and the diagnosis.
“I’ve been asked if playing football caused this. I don’t know for sure. But I certainly suspect it did,” Clark wrote. “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.”
Clark is perhaps best known for being the receiver in the play football fans simply call “The Catch.” The six-yard touchdown pass thrown by Joe Montana lifted the 49ers to victory over the Dallas Cowboys in the 1982 NFC Championship game.
The San Francisco 49ers CEO Jed York released the following statement regarding Clark’s announcement: