Bit of a broken record on my part...but since my sciatica is mostly gone...
You might find this book helpful...
Tom Danielson's Core Advantage: Core Strength for Cycling's Winning Edge
https://www.amazon.com/Tom-Danielson.../dp/193403097X
Tom and his co-author also discuss the impact of overly tight hamstrings, as vtje mentioned above, on your lower back muscles and how this can distort you lower spine.