The upper extremities are not forced to doing something they were not anatomically designed for. The pains in the neck, shoulders and arms of day long rides disappear. The lower back is not forced into a hunched over position but is allowed to assume a relaxed natural reclining position. My wife made an observation three months after I began riding my Ryan Vanguard recumbent, I no longer complained of my back aching. Even though I had been a life long bicycle commuter, and had for the last five years been riding a conventional touring bike that had been properly fitted to me, over the last year my back was growing increasingly sore from my commutes.
Safety is another big medical bonus.
With the continual head up riding position the rider is far more aware of the riding environs. Eye contact with motor vehicle drivers is quite easy. The rider is closer to the road, is riding in a feet forward position and less likely to take a header in the event of a crash. His composite rear wheel disintegrated from brake overheating. Instead of flipping end over end as would have happened had he been riding a conventional road bike like mine, he was able to skid the recumbent to a stop on the rear chain stays and dragging his feet Fred Flintstone style.