The plank exercise focuses on strengthening your core and lower back muscles. The plank is effective because it increases core strength, balance and muscle endurance. Doing this exercise while doing the "up and down" movement allows you to strengthen your arm and shoulder muscles in addition to focusing on your core. Adding in the dumbbell slide will force you to engage your core muscles more than usual to stabilize yourself in the plank position. If you find that you are compromising your form doing the up and down movement, just do the normal plank either on your forearms or on your hands.
1) Start in a modified plank position on your forearms with your dumbbell on your right side.
2) To perform the "up" position, place your right hand under your shoulder, press your body up, and walk your left hand in to end up in a full plank position with both arms straight. ( do not sway your body or let your hips rock back and forth. Engage your core so your body stays straight)
3) Using your left hand, reach under your right arm and grab the dumbbell. Slide the dumbbell to the left side of your body.
3) Lower your left elbow followed by your right elbow down to the floor and return to your starting position.
4) Repeat on the other side and try 3 sets of 20 for a challenge.