Tennessee Titans legend Chris Johnson has ALS after noticing subtle symptom aged 39

Former NFL legend Chris Johnson has opened up about his ALS diagnosis after admitting the illness has 'progressed much faster' than he 'ever imagined'.

The former Tennessee Titans running back was diagnosed with the condition last year at the age of 39, but just a year on, relies on a computer to communicate.

Amyotrophic lateral sclerosis (ALS) is a degenerative disease that affects nerve cells in the brain and spinal cord, and it progresses rapidly.

But the sports star is now opening up about the 'unexpected' symptoms he experienced prior to his shock diagnosis, in order to help others spot the signs.

Johnson noted that he first started noticing symptoms when he had a weaker grip.

Speaking with Good Morning America, his wife Brittany said: "I thought because of football and, you know, his career, that it had to be something with that.

"Maybe ... a pinched nerve or something along those lines, but never ALS."

He said that while the condition has changed him physically, it hasn't changed who he is. (Good Morning America)

Opening up about coming to terms with the illness, Johnson, a dad-of-four, said: "Honestly, I don't know if you ever fully process it.

"At first, you're in shock. Then you realize you have two choices. You can give up, or you can fight. I chose to fight."

He also spoke about how rapidly the disease progresses, leaving little time to come to terms with the reality.

"It's continued to progress much faster than I ever imagined. I want people to understand just how quickly ALS can attack your body.

"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."

The star admitted that his symptoms have progressed more quickly than he imagined. (Jason Merritt/Getty Images)

Despite his decline, the star urged fans to remember he's still the same person.

"ALS has changed what my body can do, but it hasn't changed who I am," he said.

"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."

Early symptoms of the condition, as per Mayo Clinic, include trouble walking or doing usual daily activities, weakness in the legs, slurred speech or trouble swallowing, muscle cramps and twitching, changes in thinking or behavior, and more.

The experts note that symptoms vary from person to person and worsen over time.