Straddle ups

When doing straddle ups I feel like I'm using my lower back not core. I'm trying to keep that posterior pelvic tilt to reduce this but it doesn't seem to be effective. Has anyone got any tips or perhaps I need to go back to even more basic moves? I have very flexible hamstrings and lower back.

Hi Franny,

Thanks for reaching out to us.

Could you please submit a video of these? This way it'll be easier for us to give you more specific advices and further feedback