I also do push ups on my knees. I can't do the push up/glide thing either. I can get the legs working but I can't do a full push up. I tend to do half push ups for as many reps as I can but I am usually just holding plank and gliding my legs for the last few reps. As for the moving risers in plank exercise in Ab Stacker, I did as many reps as I could on my toes and then went to my knees to finish the set. I also couldn't keep my feet elevated when moving the risers side to side.