Rockit--lower back & shoulder

Any of you ever get a sore lower back after working out with the
Rockit? For some reason, my lower back gets sore--regardless of using
heavy or light resistance. Also, when I use heavy resistance, my
shoulders get sore. I've tried varying my feet position, but it
doesn't seem to make much difference. I never really liked squats with
the muscle machine, even using a manta ray attachment, but I'm finding
that the Rockit is not much of an improvement. Any help appreciated.

Tom
Ashland, KY