To challenge your arm and core muscles simultaneously, try this variation of a plank position. Move slowly so you can keep your arms and shoulders stationary.
-- Karen Voight
1 Kneel on the floor and place your wrists directly below your shoulders, fingers facing forward. Straighten your legs behind you and come to the top of a push-up position. Shift your weight over your left foot and raise your right leg to hip height. Press your heels back and slide your shoulder blades down and away from your ears. Keep your hips and shoulders squared to the floor.