Cable core
 

Cable core rotation

Hold a cable with both hands to your right side, so your left arm is straight and stretched across your body. Only moving your arms, pull the cable to the opposite side until your right arm is straight. Return the cable back to starting position and repeat.

Top tip: Lock out your arms and shoulders, resisting the urge to drag the weight around with these muscles. The more you are able to isolate your core the quicker your six-pack will start to emerge.