For the Plank, lift off the ground while supporting your weight under your toes and elbows. Make sure both your elbows and toes are supported directly below their respected joint. Squeeze your butt and slightly raise your head to keep your spine in its natural neutral position. This positioning will take the negative stress off of your lower back and neck. Try to maintain this straight line positioning throughout the exercise.
This exercise focuses on the contraction of the core in order to maintain a rigid body. It also requires activation of the muscles in your shoulders, chest, hips, and butt to compliment the stability of the core.