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

Former NFL star Chris Johnson announced that he's been diagnosed with ALS, also known as Lou Gehrig's disease.

Johnson, now 40, said he received the diagnosis last year. He shared the news in a "Good Morning America" interview with Michael Strahan, a retired NFL player himself and co-anchor of the talk show. The interview aired Monday.

"If sharing my story helps even one person get diagnosed sooner, inspires more research, or gives another family hope, it's worth it," said Johnson.

He told Strahan that doctors believe he has "sporadic" ALS, because there's no history of ALS in his family. The vast majority of cases — about 90% — are considered sporadic, meaning there isn't an inherited or genetic link, according to the National Institutes of Health.

ALS, or Amyotrophic Lateral Sclerosis, is a progressive neurodegenerative disease that impacts nerve cells in the brain and spinal cord, severing the connection between the brain and muscles. People with the illness eventually lose their ability to walk, speak, eat and breathe, according to the ALS Association. Doctors are developing new treatments that help ease symptoms or slow down their progression, but they have not found a way to stop or cure it.

For Johnson, the disease progressed quickly. On "Good Morning America," he used a device to speak for him, which he maneuvered with just eye movements that the machine picked up through sensors.

"Just over a year ago, I was picking up my 7-year-old daughter so she could make a wish for her birthday cake," said Johnson, who said the disease "can be so shocking," for a number of different reasons.

He appeared on "Good Morning America" beside his wife, Brittany Johnson, with whom he shares four children.

"All I could think about was our kids and how young they are," she said, recalling her response when her husband was first diagnosed. "The life that we previously had is now a thing of the past, but we're still hopeful."

Johnson said he initially began to notice weakness in his right hand. Symptoms started with "little things," he told Strahan, "like my grip didn't feel right and I wasn't as strong as I've always been."

Over a 10-year NFL career, Johnson rushed for 9,651 yards, scoring 55 touchdowns on the ground for the Tennessee Titans, New York Jets and Arizona Cardinals. He is one of only nine running backs to rush for over 2,000 yards in a season, setting that mark for the Titans in 2009, when he was named Offensive Player of the Year. A first-round draft pick out of East Carolina, Johnson was a Pro Bowl selection in each of his first three years in the NFL.

He said he hopes his "Good Morning America" interview helps debunk some assumptions people may have about ALS and those who live with it.

"First, I want people to know that I'm still me. ALS has changed what my body can do, but it hasn't changed who I am," said Johnson, adding: "Your mind stays sharp. People sometimes look at the physical disability and assume you're not still the same person inside. I still think the same, I still dream, I still love my family. My body just doesn't cooperate."

Johnson is among a handful of celebrities who became advocates for ALS research following their diagnoses, including the late actor Eric Dane. Gehrig, the New York Yankees player for whom the disease was informally named, brought global awareness to ALS after his diagnosis in the 1930s.