plank variation- instead of holding yourself in a full up position from a pushup, brace yourself on your elbows and your toes and hold- when you've got that down, go to holding with one leg lifted.

Assume a plank position on a training ball with your elbows and forearms on the ball. Roll the ball in a circle to the right 10 times, to the left 10 times, and then forward and backward 10 times while holding your body straight and balanced on your toes. When you can do 10 increase to 20 or drop to a smaller ball.

side planks with and without hip lifts and holds either with your feet stacked or with your uppr leg lifted in a scissor position.

Guaranteed to hit your core without grinding anything except possibly your elbows.