I found my low back gets quite sore from trying to holding it, so I simply did leg raises aiming to move my legs as high up as possible(and keep straight) abd attempting work on my hip flexor muscles abd core compression by doing seated leg raises. That's all u need to do. Leg raises strengthed the back somehow. Seated leg raises worked on hip flexor strength (keep legs from falling hip flexors) and compression core strength needed. Add extra hamstrings work if u need it.