Plank Run

Here is another plank variation: Get in the plank position, with your back and hips aligned. Bend your knees as if running. For variation that works your oblique muscles more, bring your knee to the opposite hip bone when you bend each leg.

Do this for 10 to 20 seconds at a time with 5 to 10 seconds of rest in between, or whatever variation works for you for a length of time that is effective. You should get a burn but not be in pain.