If you’re like a lot of runners, you might think that the most crucial element of interval training is the faster running—repetitions usually run at current or goal race pace. You figure that you run a bunch of reps at race pace, gradually cut down the recovery interval between reps over the course of a few weeks, and then finally cut out the recovery intervals altogether as you toe the start line for your target race.
Only one problem: Your body doesn’t work that way.
Instead, your goal for interval (or repetition) training should be to target a specific physiological component of your running fitness—not a pace—and then accumulate the maximum amount of work possible for that component. These components can include your nervous system, cardiovascular system, muscles, mitochondria (the aerobic power plants in your muscles), energy systems, and more. The key to applying a correct workload begins with understanding that there are two crucial elements—not one—in your workout: the faster repetition and the recovery interval. It’s the correct balancing of both that results in a successful workout.
A Brief History Lesson of Interval Training
To understand the relationship between repetitions and recovery interval, it’s valuable to recall the birth of interval training. But first, let’s clarify how we’ll define the two elements of interval training in this article:
Repetition: The workout’s higher-intensity running periods (e.g., the “400s” in a 12 x 400m session).
Interval: The recovery period between repetitions.
In the late 1930s, German coach Woldemar Gerschler, influenced by cardiologist Hans Reindell, created interval training as a way to increase the amount of blood pumped by the heart with each beat, better known as “stroke volume.” Gerschler had his athletes run short, intense repetitions (100-400 meters) to raise their heart rates to 180 beats per minute, at which point they quickly transitioned to a recovery interval (e.g., walking or slow jogging). During this recovery interval, returning blood created a momentary increase in pressure within the heart’s chambers, stretching the chambers and triggering an adaptation that would eventually result in an average heart volume increase of 20 percent. The recovery interval lasted until an athlete’s heart rate dropped to 120 beats per minute, at which point he or she launched into the next repetition. When an athlete’s heart rate took longer than 90 seconds to slow to 120 beats, the workout was ended.
Interval-trained athletes soon shattered world records at 400 and 800 meters. But your main takeaway should be that it was the correct pairing of repetition and recovery interval that resulted in the desired training adaptation.