I was diagnosed with (lumbar) spinal stenosis early this past summer. Not sciatica but similar problems. My spine doc (who is a keen cyclist and rides a Trek Madone) suggested getting a better bike. The saddle has nothing to do with my problem. Sitting upright is the last thing I should be doing. Luckily I've always ridden road bikes.
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The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.