Thank you for your response, I believe you are right. Although it’s an upright bike, and I sit straight, I think I must be trying to reduce the weight on my lumbar spine by leaning forward. My core just isn’t strong enough yet. I feel I made an error waiting so long to have my surgeries, the doctors all told me to put off a fusion until the pain is too much. 10 years before my first fusion, then two and a half years again for my successful second. I will concentrate on building up my core again in other ways.
A tale of caution to everyone who reads this, don’t let fear of surgery keep you from trying to get better. Yes sometimes it doesn’t help, but it often does!