CHALLENGING YOURSELF DURING a workout shouldn't be a matter of overcomplicating your routine. You don't need a ton of time or equipment to do it, either—just a plan and a limitless well of effort.

Looking for a cardio finisher? This 'leg matrix' gets the job done in just 90 seconds of squatting, lunging, and jumping. Cardio tends to get boxed up as a time-consuming form of exercise that takes a ...