For the Side Plank, rotate to your side and support the weight of your body on your feet and the elbow closest to the ground. Foot positioning is based on preference. Some prefer one foot on top of the other, and others like to place one foot in front of the other. Pick your hips off the ground and try to maintain a rigid straight line from your head down to your feet. Again, squeeze your butt and slightly raise your head to keep your spine in a neutral position. The arm not being used should be lifted straight in the air to assist with balance.
This exercise focuses on the side of the core that is maintaining hip elevation. Muscles of the shoulder, torso, and hip (on the adjacent side) must also be activated to hold the straight line throughout the exercise.