Hi all,
I've been having my first rides on my recumbent and the posture which the seat puts my back in is quite different. On a diamond frame, I would straighten the back quite a bit in order to protect it while pedalling hard. Now when lying down, the back is worryingly "passive". Aren't we supposed to engage the back lower back muscles to protect it? Foremost I'm worried about my pelvis rounding up in order to fit the anatomical shape of the hardshell seat.

Thanks in beforehand!

Apples and oranges. Your back is fully supported on a recumbent.

I don't think I have ever heard of someone's pelvis or spine being permanently deformed by a seat (of any type) they sit on.

Core work, stretching, etc. is a virtue, regardless of what kind of bike your ride, and what shape the seats are you sit on. Riding a bike is riding a bike. It's not exercise for your back. Exercise your back using back exercises.

I'm not concerned that the act of resting my back on the seat would hurt it, I'm concerned that while pushing hard on the pedals it would put a lot of stress on the lower back in that position. You wouln't round your pelvis/lower back while doing reps on a leg press machine, so I don't see what would make the recumbent different from that.

In the past, I've given myself a really sore back by having the pedals too far out and 'slouching' in the hardshell to reach them better. But with proper reach? Not a problem.

Are you having stress strain on your lower back? I have been thinking about you issue when riding as I have had a flare up with my osteoarthritis in my back and that has me off of 2 wheels onto my recumbent trike now and often several times a year. I have no issues with injuries or structural abnormalities but arthritis can bring structural abnormalities to the back, It seems to me something is amiss with the set up as I am hyper sensitive to back issues and have zero issues in my recumbent. My general position is as any shop I believe would recommend, seat back a comfortable angle that provides good lumbar support, full contact with the backrest for solid support and hips level while pedaling w/o rocking. I know some folks like a what to me is a very aero position but I like being more upright enjoying the view w/o strain.