Yes it happens and it's not three drunk hillbillies with wheelchairs stolen from the local nursing home. In 1997 when I first broke my spine in a single car accident due to black ice, I was laying in my bed at the hospital thinking I would never be able to downhill race again. The though was devastating. I tried to find alternatives to look into to stay active but wheelchair tennis wasn't cutting it and Quad Rugby is well . . . Only for Quadriplegics. That sounds strange but it's true it their sport and that is really cool.
So I picked up a magazine called Sports and Spokes. During my read through I came across a down hill racing mountain bike for paraplegics. I about fell out of bed. I saved for two years and picked one up. It was used and awesome! The company that made it was called Grove Innovations. The bike was called the DH1. It was aptly named and even cooler looking. (This pic is not me. Not sure who it is but that's the same bike)
Here is a newer version I'm not sure the maker.
The major down side to these bikes is that they are strictly gravity based. There is no crank, no electric power, just no way to go up hill at all. I'd love to find someone who is very knowledgeable about or involved in the production of electronic bike parts. This bike does absolutely nothing for your health to ride but it's fun as all get out and I'd love to figure out a way to power it. Hub motors come to mind but my knowledge of them is limited.
Can you just imagine a four wheel hub drive 8000w bike with 6" of independent travel and 72v at 24wh. Crazyfun!