the tandem bicycle "benefit" was discovered by serendipity, i put quotes around benefit because personally i don't think the studies are large enough/long enough to definitively prove it yet.
they've shown forced exercise in mice and primates is beneficial so there is a lot of evidence that the rate has to be intense.
i am signing up for a tandem forced exercise study, it requires one hour sessions 3 days a week for 10 weeks and with the terrible traffic in seattle i'm looking at maybe 4 hrs/session so i don't want to go "easy" on the exercise with that time/effort commitment, i'm happy to be their guinea pig. i'd run a marathon if it would help.

any benefit ultimately is biochemical in nature, i'm sure the ultimate goal is to discover what it is and synthesize the biochemicals. i don't think it's realistic to expect that level of exercise to treat a disease in an aging population nor as a country do i think everyone should do forced exercise to try to prevent brain diseases, well now that i think of the obesity rate .... the drain on the economy would be terrible.

i certainly don't expect to nor want to do forced exercise for years to come, it would wear out my joints. i'm not saying ymca's, etc. are jumping the gun by setting up classes, i see it as more as research/"fun".
but as someone with pd i see this as ultimately developing a reasonable exercise/drug regime if the benefit is proven maintainable. so i would still view this as research so yes, they are still asking for vigorous rates of exercise with the limited funds/volunteers they have. obviously if they want to look for changes in biochemicals/dna expression, brain scans, etc., go for the biggest changes you can achieve.

a motorized stationary bicycle mfg is certainly using forced exercise in their advertisements.