The Plank is one of my favorite poses/exercise in Pilates and yoga. I believe it is hard to achieve correctly. Start with your knees on the ground, hands underneath shoulders, and feet up off the group. Spread your fingers wide and grip the earth. On an exhale press your hands into the ground and extend arms long. Draw your shoulder blades towards your hips and find neutral shoulder blades. Do not let the sternum sink. Get your pelvis into neutral. Pull abdominals in. Engage your pelvic floor. Do not lose the core. If you are feeling strong and maintaining neutral spine/hips, put balls of feet on the ground and extend legs. Hold for 30 seconds. Work up to 5 minutes. Enjoy!!
Below is a SIDE Plank. Make sure you line up your wrist with your shoulder. Do not put all of your weight into the bottom arm. Exhale and use your TVA, Obliques, and Lats to push up into a side plank. Lift your top side torso toward the sky. Do not attempt if you do not have stability in your shoulder joint. Enjoy! Breathe!