Choose your workout method. Warm up for five to 10 minutes. Go hard for 10 seconds and easy for 10 to 20. Then go hard for 20 and easy for 20 to 40. Next, go hard for 40 and easy for 40 to 80. Finally, go hard for 60 seconds and rest for one to two minutes. Then repeat the sequence in reverse. Cool down for five to 10 minutes. If you swim, measure your intervals in lengths of the pool. Start by sprinting one length and coming back slowly. Next, go hard for two lengths and slow for two lengths. Continue building up to four lengths fast and four lengths slow before repeating the sequence in reverse.

The Fine Print

High-intensity training could be considered “microwave” fitness, offering many of the benefits of traditional, moderate exercise in just a matter of minutes. But, like a microwave, it comes with its own safety concerns and issues to consider.

Ease into it. Start with shorter or less intense intervals and build up to harder efforts. Developing strength (through resistance training) and a good aerobic base (think longer, slower cardio training) will prep your body to handle intense efforts, making your workouts more effective. It can also help you recover more quickly.

Play defense. Studies indicate that intense exercise can temporarily weaken your immune system. Combat this danger with good nutrition and plenty of sleep, as well as a day or two of minimal activity after hard workouts. Rest will also make your efforts more effective because your body rebuilds to become even stronger during post-workout recovery periods.