A plank can be completed on your hands, elbows, knees, or toes. You can also complete a side plank on your forearm or outstretched arm. Center and side planks work on balance, strength, and endurance, and help you trim inches off your waist. It's common for beginners to start in the half-plank position on their knees and build up to a full plank position on your toes. Keeping your core muscles tight, body in a straight line, and continuing slow and controlled breathing is important during planks, as it's very easy to hold your breath during isometric and difficult exercises.