In the course of my (somewhat limited) flying experience, I have rarely seen exit row seating assigned to 'able bodied*' passengers. I'm guessing that these are reserved for those who pay extra for the leg room.
I've been involved in a few evacuation tests at Boeing and I know how critical proper and rapid handling of these exit doors can be to evacuation timing**.
*I've seen old people, fat people (that look like they'd plug an overwing exit) and tall, skinny people (who need the leg room but don't look like they could lift a plug door). I've even seen parents with kids (I guess they can play on the floor). They do ask that you inform the crew if you feel unable to handle the overwing exit door. But that appears to be voluntary. And after someone paid extra or was awarded the deluxe seat based on frequent flyer/business class status, I'd guess that most people just shut up.
**During evacuation tests, they instruct the 'passenger' adjacent to the door to remove it and place it on the seat. I've always thought SOP was to turn it sideways and pitch it out of the opening. The politically correct response was that they couldn't expect every passenger to accomplish this (even though I can fling one of those like a Frisbee). An engineer told me that the real reason was that they conduct the tests with brand new airplanes and they didn't want me putting a big divot in the top of a wing.