stationary bike/back problems question

Biking is an aerobic activity, if you're getting your heart rate up (and keeping it up for a period of time) you should be realizing gains in terms of cardiovascular fitness.

I'm not sure that the arm movements really provide any added benefit but you're still working your legs.

Sometimes problems require creative solutions and it sounds like you've done that, I don't expect it reduces the exercise value of your workouts. (When it's time to replace your bike have a look at recumbent models, they might help alleviate your back problems to)

You're not cheating because leaning your back against the wall doesn't put extra power into the pedals. Which is the point on a bike. You do whatever you can to be comfortable while turning the pedals over.

I ride a lot and have a power meter on all my bikes...power meters basically give you an objective measure of much force you are putting in the pedals. It's displayed in watts...so 200 watts up hill could be 6 mph while 200 on a flat road might be 22 mph. It's a cool device. Ok, now to your question. When I am doing squats twice a week in winter my back will start to hurt on my long trainer rides. When my hand are in the hoods my power might be 200 when I sit up it drops by 20-30% and feels the same level of difficulty. Plus my HR will increase 5 or so beats, which makes sense as my heart needs to push blood higher. Basically I am not using my muscles in the same way so I lose power. Less power = less calories burned, HR is crude estimator of calories, measuring actual force is usually within few percent. Is a super cool device. So if you sit up, try to pedal harder or just extend your workout a little if you have a calorie goal.

I do wonder if I tried riding that way all the time if my power would slowly start to match being on my brake hoods for same HR and effort. I would think HR would never line up based on body position.

John