CATHE,
I HAVE HAD PROBLEMS WITH MY TOES SLIDING OUT FROM UNDERNEATH ME DOING THE PLANKS. I STARTED KEEPING MY HEEL AGAINST THE COUCH, WHICH HELPS FROM SLIDING, BUT IS THIS TAKING AWAY SOME EFFECTS FROM THE EXERCISE?
Not Cathe here (obviously!) but have you tried doing these on a yoga mat? The mat can give just enough traction to hold your toes in place. (And if you 'tuck your toes under' they may have less tendency to slide.)
What surface are you doing them on? Are you barefoot? I find doing them on carpeting with barefeet best. If I am at the gym, I use a mat and usually my shoes keep me from sliding.
