Richard Simmons is helping people struggling though these days of self-isolation -- an area of expertise for him -- by making a comeback of sorts ... strictly online.
The beloved, former fitness mogul has been out of the public eye since February 2014, but 3 weeks ago his YouTube channel began posting for the first time in 6 years ... right around the time Americans began getting ordered to stay inside to stop the spread of coronavirus.
The material is nothing new -- it's old 'Signature Moves' clips, workouts and motivational messages -- but sources close to Simmons tell TMZ .