Knee to Elbow
Add some oblique engagement to the basic plank by bringing your knee to your elbow. Start in the basic plank position. Hold the top of the plank with the entire body flexed. Lift your right foot and try to touch your knee to the outside of your right elbow. Pause. Return your foot to the starting position. Perform the same move with your left foot. Alternate between the right and left foot for 10 repetitions on each side or for 60 seconds.
Reach and Extend
Start in a plank position. Lift your right hand and left foot. Then, as the name suggests, reach your right arm forward extending it as far as you can, while doing the same with your left foot. After you've reached and extended to your limits, hold it for five seconds before switching to the left arm and right foot.