Adjective = noun
Recumbent adj: flat, horizontal, level, prostrate, supine, reclining, sprawling.
Only in the world of cycling can they get away with arrogant abuse of English grammar, and assume that everyone will understand you when you refer to a bicycle (in this case a tricycle) as a recumbent (noun). It might easily have been called a supine or a prostrate, or even a horizontal. But somehow, I don’t think any of those would have caught on so easily.
So here I present you with a supremely comfortable supine rider, sitting (almost) horizontally prostrate on his recliner, waiting to go at a flat-out speed on the level, until he finds himself sprawling with fatigue………….the recumbent rider.