Might it be that you are allowing your low back to do some of the work to do the rear leg lifts? The low back waist area works more if you allow the pelvis to move as well as the leg. I don't think that's what's intended, I think instead you activate your abs to hold the pelvis in neutral, and lift the leg while the pelvis stays still. You can try this lying on your stomach, contract the abs to hold the pelvis still, keep that, then bend one leg 90 degrees and lift foot to the ceiling. You feel that in your low butt/thutt area. Then try not holding the pelvis still and lifting - see if that is what hurts your back. I think after a long while, once the abs strengthen, you can gradually stabilize your pelvis and keep the tug of the abs pulling the pelvis in the direction of a tuck while the low butt/thutt muscles are working.
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