THE BIG IDEA: High-intensity intervals force your body to adjust quickly.
WHY: Your body adapts when it's pushed to the limit. Redlining for short periods lets you reap the benefits of high output while allowing you to recover between efforts. Researchers at Ontario's McMaster University have shown that two weeks of high-intensity training produces the same performance benefits as two weeks of endurance training, but in 75 percent less time. "The key is progressive overload, which means adding work in amounts incrementally more than your body is used to," says Derick Williamson, a sports scientist at Durata Training. "Intervals use high intensity to get the most overload in the least amount of time."
DO IT: Once a week, perform short intervals (three to five minutes) at 100 percent effort to boost your VO2 max, the peak quantity of oxygen your body can take in and utilize. After at least one recovery day, do a workout composed of longer intervals (eight minutes or more) at 70 percent effort to increase your lactate threshold—the level of output your body can sustain over time.