The 21-day HIIT challenge includes some of our most popular HIIT workouts to give you three weeks of quick, but effective training. The workout plan consists of interval training, walking/jogging, and rest days. It’s easy-to-follow and the quick workouts are ideal for anyone who doesn’t have hours to spend at the gym. It’s the perfect way to tone your muscles, improve your endurance, and burn some serious calories! Download the calendar and complete the workout assigned each day.