Former NFL player Chris Johnson revealed the first symptom he experienced of ALS after announcing his shock diagnosis on Monday. 

The ex-Tennessee Titans and New York Jets running back had teased a big interview with fellow football icon Michael Strahan on Good Morning America, but fans were left stunned when the reason for the broadcast was revealed.

Amyotrophic lateral sclerosis (ALS) - also known as Lou Gehrig's disease - is a progressive neurological disease that slowly destroys motor neurons in the brain and spinal cord over time.

Johnson - who was one of the fastest running backs in NFL history and 2009 NFL Offensive Player of the Year - was only diagnosed last year at the age of 39, but has already lost the ability to speak, using his eyes to trigger a voice machine as he sat alongside his wife for the ABC interview.

He said he noticed something was off about his health when he felt his right hand become weak. He said the first subtle signs were 'little things, like my grip didn't feel right, and I wasn't as strong as I've always been.'

Johnson, now 40, and his wife said they expected the issue to be related to his long football career, with his wife Brittany adding: '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.'

The ex-NFL star was in what he describes as the 'prime of his life' when he received the diagnosis and had been working out every day and spending time with his wife and four children. 

'We hoped it was something else, but after thorough testing, they finally came down with a diagnosis of ALS,' Johnson explained. 

'They told us about a medication that might extend life by a few months, then they told us to get our affairs in order.'

Brittany went on to reveal that she was initially 'in denial' about the diagnosis, and that her thoughts immediately went to their four children. 

'You want it to be anything else, you want the doctors to be wrong,' she revealed. 

'The life that we previously had is now a thing of the past, but we're still hopeful that a breakthrough will happen, and that God, a miracle will happen.'

Johnson added that Brittany 'hasn't left my side' since the diagnosis, and that his children give him reason to carry on. 

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

'Just over a year ago I was picking up my seven-year-old daughter so she could make a wish with her birthday cake. Today, I couldn't do that.'

When asked why he had chosen now to share his story, Johnson revealed: 'Because if sharing my story helps even one person get diagnosed sooner, inspires more research or gives another family hope, then it's worth it.' 

'First, I want people to know that I'm still me,' he began. 'ALS has changed what my body can do but it hasn't changed who I am.'

His rapid decline left fans in tears as they watched him explain his diagnosis and the subsequent effects on his body. 

Johnson has no history of the disease in his family and doctors believe his condition to be a case of 'sporadic ALS', which is the most common form of the disease and occurs randomly.

'It can happen to someone who never expected it,' Johnson revealed. 'I don't know if you can 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.'

After his diagnosis, doctors recorded Johnson's voice, so that even when he is using a machine to speak, it still sounds exactly like him. 

Johnson ended his professional career in 2017 after three seasons with the Arizona Cardinals, totaling 11,000 yards in his ten years in the league.

He is best known for his six seasons in Tennessee, during which he was voted to the Pro Bowl three times and won Offensive Player of the Year, before moving to the New York Jets.