I'm having issues with keeping my back upright/flat when I squat. I don't think its my back strength. I have a strong dead and abs are pretty good too. I put my pointer fingers on the rings. My feet are slightly wider than shoulder width, toes pointed out slightly. My head is looking forward. On my last few reps of a given set I just feel myself drifting forward? My form is good until i get tired and start drifting forward. WTF? Any ideas?