Dr Lieberman gave the impression of a happy retirement for Ali - who was known as Cassius Clay before joining the Nation of Islam and changing his name - and stressed that, despite some reports, there is no reason to fear for his health.
He said: 'He's had Parkinson's since about 1984, that's almost 30 years, that's a long time in Parkinson's.
'He's in good spirits, he has some trouble walking but overall for having had Parkinson's for 30 years, he's doing okay.'
He added: 'Muhammad is now 72 so you can have a heart attack or you can have a stroke.
'I don't know that he's more or less at risk than anyone else but anything can happen to you.
'How do people with Parkinson's Disease die? They don't die of Parkinson's Disease, they develop trouble swallowing and they develop pneumonia and he doesn't have trouble swallowing.
'They fall, they bang their head - his family takes extraordinary care of him.
'I could have a heart attack or a stroke and die but I don't see anything immediately that leads me to think he's gong to die in six months or die in a year. I can't say that.'
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