ADA doesn't apply to airlines. A separate federal law covers accessibility issues, the Air Carrier Access Act:

http://www.disabilitytravel.com/airl...arrier_act.htm

I doubt it requires that FAs lift heavy bags belonging to disabled passengers.

FAs have told me that their employer (AA) has told them that they lift passenger bags at their own risk and if injured doing so, they risk a workers' comp denial.

And I agree that if you can't lift your own bag, you need to check it or find another passenger who will help. FAs are not baggage handlers. Those guys work under the wing.