Planks are always more challenging than they look, and that is just one reason why I love them. They work wonders for your core as well as your shoulders and require no equipment at all. With planks, it's easy to track your progress; the stronger you become, the longer you'll be able to hold the positions. Start by holding your plank for at least 30 seconds and add on in five- to 10-second increments to keep challenging yourself. Here are four planks to mix up your routine!