When we consider the boost in fitness a runner gains from interval training, we tend to focus on the running aspect of these workouts. After all, that's where the real work is done, right?
It turns out, it's not that simple. While the energy output during the running reps is undeniably important, it's the amount and type of rest taken in between them that produces a wide range of training benefits.
Running 10 x 400 meters with one minute of rest in between each rep is significantly more challenging than if you were to take two minutes of rest. Similarly, active rest and full rest during those moments serve different functions.
"Nothing is set in stone with interval training," says Cliff Latham, a coach and exercise physiologist in College Station, Texas. Having worked with runners training for a wide range of distances, from sprint events to marathons, he works to tailor all aspects of interval workouts depending on the intended effect.
In general, the amount and type of rest during interval workouts, along with the pace and distance of the reps, help target three separate energy systems that are important for endurance training. These systems include the phosphagen system, the glycolysis or anaerobic system, and the aerobic system.
It's through these three systems of metabolism that adenosine triphosphate or ATP is produced and used as energy to make your muscles function. By tailoring the rest a runner takes between hard reps, along with distance and pace, these various energy systems are trained.