On a bike you have a little bit of flexibility due to the way the seat post works. Both in how the seat attaches and adding curves to the post, particularly for triathletes who like to favor their hamstrings over their quads, and sit considerably farther forward than any 'normal' cyclist would.

I know I've seen wheelchairs where the back was a tube that went into a tube. If you put the curve in the replaceable part you get more adjustment but less support. Generally the tolerances on bikes are very tight and medical equipment seems to be all over the place.