New year, new you, right? Well here's our first 30-day fitness challenge of 2014 - PLANK! The plank is one of the best exercises for a flat, toned stomach because it works all the muscles in your core. It’s a static exercise where you use your arms to raise yourself off the floor and hold the whole body straight and rigid, like a plank of wood. You can do it anywhere, you don’t need any equipment and it only takes a minute (literally). What’s more, it is more effective than sit-ups and crunches because these work only the superficial abdominal muscles. So who's up for the challenge?