Former NFL star Chris Johnson says he's been diagnosed with ALS

Chris Johnson was a dominant running back at one point in his NFL career.

He ran for 2,000 yards in his second season with the Tennessee Titans in 2009 on his way to three consecutive Pro Bowl appearances. He was named the 2009 Offensive Player of the Year and made the Pro Bowl in 2009 thanks to his statistical prowess that season.

ZERO BS. JUST DAKICH. TAKE THE DON'T @ ME PODCAST ON THE ROAD. DOWNLOAD NOW!

Johnson, 40, on Monday announced a new and harrowing battle. He said he was diagnosed with ALS.

"T here's no history of ALS in my family," he said on ABC’s "Good Morning America." "My doctors believe my case is what's called sporadic ALS, which is actually how the vast majority of ALS cases happen.

FORMER FALCONS STAR TIM GREEN REFUSES TO LET ALS SILENCE HIM AS FATHER-SON PODCAST CREATES LASTING LEGACY

"That's one of the reasons this disease can be so shocking. It can happen to someone who never expected it."

Johnson said he started to notice a weakness in his right hand and that his grip hadn’t been as strong as it previously was.

"It's continued to progress much faster than I ever imagined. I want people to understand just how quickly ALS can attack your body," Johnson said. "Just over a year ago, I was picking up my 7-year-old daughter so she'd make a wish with her birthday cake. Today, I couldn't do that."

CLICK HERE FOR MORE SPORTS COVERAGE ON FOXNEWS.COM

There is no known cure for ALS – known as amyotrophic lateral sclerosis. It’s a progressive disease in which the brain loses connection with the muscles, according to the ALS Association. The afflicted slowly lose their ability to walk, talk, eat, dress, write, swallow and, eventually, breathe.

Johnson credited his family, including his wife Brittany, with giving him a "reason to keep going."

The former running back played in the NFL from 2008 to 2017 with the Titans, New York Jets and Arizona Cardinals.

Fox News’ Angelica Stabile contributed to this report.