Not Cathe, but budding in anyway :
I think it's easier to let the back round on these (vs. regular dead lifts). Also, even if you are doing the movement correctly, I think the lower back is doing more work holding the isometric contraction. If your "sore" is muscle fatigue, there should be no problem. But if it's "sore" as in "pain" or sharpness, then watch your form carefully. Maybe don't go down as far. Or switch to a two-legged version when you start to feel your form slip.
12# may or may not be too heavy, depending on what you usually use for deadlifts. You would go lighter than what you usually use, because you have the added challenge of balancing.}(
While doing the single leg deadlifts in legs and glutes, my lower back was really getting sore. Am I using too heavy weights (I used 12 lbs.) or could I be using improper form? Also, could you tell me if you have new Cathe gear in your new catalog? Thank you!