Low Back pain when doing Tuck ups

I've progressed consistently through the other two ab exercises (hollow body and twisting exercises) but I've been stuck with regards to the Tuck Up. Two main problems I have, pain in my lower back when I'm executing the movement, and towards the last few reps my legs/knees seperate and flare wide in order to finish the set. 

Hi Peter,

Thanks for reaching out! When it comes to low back pain, I always recommend eliminating movements that cause it until you can see a medical professional. An in-person diagnosis is always best when it comes to these situations. Once you have a diagnosis, let your body heal, and can train the movement without any pain or discomfort, you can begin to re-build towards mastery. 

Knees coming apart near the end of the set is a compensation method. It means that your core is having trouble meeting the demands of your current rep scheme. You always want to perform each rep with perfect form and tempo before advancing to the next step. For now, however, work on addressing the back pain. 

Please keep us updated and don't hesitate to post follow-up questions here.