Sore back after straddle ups

Hi guys, Every time I do straddle ups I always get a sore back later on that night. However the tightness & ache is gone by the next morning. It kind of feels more of a quad or hip flexor thing rather than lower back I have no problems after doing the 5x15 tuck ups from the last progression. Any advice would be appreciated thank you. jd

Hey John probably you have strong hips flexor and weak abs. can you post a side view of the exercise?

Thanks Alessandro I will post in the next couple of days..