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.
Applying Gerschler’s Insight to Modern Training
Of course, modern interval training has evolved to target multiple fitness adaptations, not just stroke volume. Some workouts that most distance runners include in their training arsenals are:
Sprints and hill sprints, which target nervous system efficiency and muscle fiber strength for all muscle fibers.
Long hill repeats, which target strength and aerobic energy production in faster muscle fibers (especially intermediate fast-twitch fibers).
1500m-paced repetitions, which target nervous system efficiency, aerobic energy production, and anaerobic capacity in faster muscle fibers.
3K, 5K and 10K-paced repetitions, which target blood volume, stroke volume, and aerobic energy production in slow-twitch and intermediate fast-twitch fibers.
Just as with Gerschler’s intervals, all these types of training depend upon a correct pairing of the faster repetition and recovery interval to ensure maximum benefit. That means a recovery interval that both allows your body to rebound and helps keep the training stimulus centered on the targeted component(s).
Sprints and Hill Sprints
Both sprints (100m or less) and hill sprints (6-12 second bursts up a steep hill) provide an intense nervous system stimulus by recruiting all available running muscle fibers, leading to stronger fibers and a more efficient stride.
These repetitions are largely fueled by your anaerobic phosphagen system, which provides about 50 percent of the required energy. Your anaerobic glycolytic system (the system most runners associate with “anaerobic” energy) kicks in about 30-35 percent of the energy, with the remaining 15-20 percent provided by your aerobic system.
Here’s the thing: Your phosphagen system produces all that energy by using creatine phosphate (CP) stored in your muscle fibers. But your CP supply only lasts 10-15 seconds, after which you’ll need about three minutes to regenerate it. What’s more, your muscles become more dependent on CP-produced energy with each subsequent rep.
“Lots of studies over the decades have shown that with repeated bouts [of high-intensity exercise] you use less and less glycogen,” says exercise scientist Christopher B. Scott, PhD, who runs the Human Performance Lab at the University of Southern Maine. Glycogen is the major fuel source for both your glycolytic and aerobic energy systems. “What’s happening is that you start relying more on the high-energy phosphates in your muscle.”
Relying heavily on CP-produced energy is good for sprinting. The phosphagen system provides energy twice as fast as your glycolytic system and up to 200 times faster than your aerobic system. That means more energy for your muscles and faster, stronger reps. But it also means that a shorter recovery interval will cut off your supply of CP, since you can’t regenerate CP quickly enough. Shorten the interval to under 3 minutes, and you force your body to rely upon energy from your glycolytic and aerobic systems. Since these systems provide energy more slowly, you’ll experience slower reps, less speed and strength development, and less nervous system improvement. Speaking of the nervous system, it takes 3-5 minutes for that system to recover between high-intensity reps. Otherwise, it gets overloaded. Your subsequent reps will suffer, and your workouts might be flat for the next 2-4 days—or longer.
Long Hill Repeats
Long hill repeats (30-120 seconds up a moderately steep hill at approximately 3K-5K effort) are fueled by both the anaerobic glycolytic system and the aerobic system. They’re one of the best workouts there is for improving both aerobic energy production and strength in your intermediate fast-twitch fibers—fibers that are key for races from 800m-10K.
From your very first step of a hill repeat, the force required to run uphill triggers your nervous system to recruit your faster fibers, both intermediate and fast-twitch. By sustaining the stress on your intermediate fibers, you improve their aerobic energy production by stimulating the creation of more capillaries and mitochondria. That translates to more endurance at a faster pace during races.
For hill repeats, you’ll need a recovery interval that lasts 2-3 times the length in time of the repetition. If you rush the recovery, bad things happen, such as:
Too anaerobic: Your anaerobic system can contribute energy for only a few minutes at the effort level required by hill repeats. Without a full recovery between sets, you’ll overwhelm this system, and your faster fibers will shut down.
Too aerobic: If your faster fibers begin to shut down, you’ll slow down and rely more heavily upon your slow-twitch fibers. Since this isn’t an effective workout for slow-twitch aerobic development, you’ll defeat the purpose of the workout.
Nervous system overload: The high-intensity force required for hill repeats requires a full recovery between reps to allow your nervous system to reboot.
The best recovery approach between faster reps is to jog down the hill slowly, then walk at the bottom until you’re fully recovered. This keeps the stimulus on the targeted muscle fibers, ensures correct energy system contribution and development, and spares your nervous system.
Many runners are surprised to learn that the first 30-50 seconds of a repetition are the most anaerobic. That’s because it takes time to deliver an increased oxygen supply via the lungs, heart, and blood to your muscles in response to the increased energy demand that occurs at the start of your rep. Without that oxygen, your aerobic system can’t keep pace with your muscles’ energy needs, so your anaerobic system comes to the rescue.
Since the pace of 1500m repetitions limits the duration of those reps, a large percentage of each rep (that first 30-50 seconds) is fueled mostly by anaerobic energy. This makes development of your anaerobic system one of the main benefits of these reps. You’ll also improve your nervous system, allowing you to run more efficiently at faster paces, and boost aerobic energy production in your intermediate fibers.
But here’s the tricky part: The duration of your recovery interval will depend on what specific component of running fitness you’re attempting to improve.
If your goal is nervous system efficiency (i.e., improving your ability to run comfortably at 1500m pace), you’ll want a recovery interval that lets your nervous system rebound completely between reps. Depending on the length of the interval, this can require between 2-5 minutes.
If you’re attempting to improve your anaerobic system by increasing anaerobic enzymes, lactate clearance, buffering, etc., you’ll need recovery intervals that are at least twice the length in time of the repetition. This allows your system to clear anaerobic by-products from the previous rep, which in turn makes it possible to run a greater number of repetitions.
If you’re trying to improve your speed endurance (your capacity to maintain the repetition pace over a longer distance), you might consider cutting your recovery interval to an amount of time equal to the length of the repetition. If you choose this latter option, however, you’ll have to closely monitor your body’s feedback, as you run the risk of overloading both your anaerobic and nervous systems.
3K, 5K & 10K-paced Repetitions
These repetitions improve aerobic energy production in your slow-twitch and intermediate fast-twitch muscle fibers, as well as developing nervous system efficiency and running economy at these paces. All three paces allow you to train at or near your maximum aerobic potential.
For fit, experienced runners:
— 3K pace is approximately 100 percent VO2 max
— 5K pace is approximately 95 percent VO2 max
— 10K pace is approximately 90 percent VO2 max
Your “VO2 max” represents the maximum amount of oxygen you can consume, or utilize for creating aerobic energy, in one minute. This makes repetitions at-or-near VO2 max a great stimulus for improving red blood cell count (the better to carry oxygen), stroke volume (the better to push oxygen via blood through your body), capillary density (the better to deliver oxygen to working muscles), and mitochondrial density (the better to use that oxygen to create more aerobic energy). The benefits gained from repetitions at any of these paces include increased aerobic energy production at all race paces—from 800 meters to an ultramarathon.
Your recovery interval will vary depending on where you are in your training cycle. Early in your cycle, you’ll need to worry about basic muscle, connective tissue and nervous system fatigue. At any of these paces, you’ll accumulate a large volume of repetition running at an impact force and intensity for which your body might not be prepared. For this reason, it’s wise to schedule recovery intervals that are equal to the length in time of the repetition for shorter reps (e.g., a 2-minute recovery interval for a 2-minute repetition) and only slightly reduced for longer repetitions (e.g., a 3-minute recovery interval for a 4-minute repetition). This allows your body to recover enough to weather impact forces, rebuild energy supplies and reboot the nervous system.
Later in your training cycle, both your repetition and recovery interval can be altered to create a better training stimulus. As you become stronger, protecting your body with long recovery intervals becomes less important; instead, accumulating more work at-or-near VO2 max becomes your primary goal. Longer repetitions get in the way of this goal.
“I could put you on a treadmill with a mouthpiece and a nose clip,” says Dr. Scott, “and have you start [running]. And it would take about two or three minutes before your oxygen uptake would rise, level off, and plateau. In that two or three minutes, you’re not supplying your muscles with enough oxygen.”
In other words, you’re not getting maximum benefit for your effort for at least the first two minutes of your repetition. By this standard, this is the amount of max-quality work you get during longer repetitions:
2-minute repetition: 0 minutes
3-minute repetition: 1 minute
4-minute repetition: 2 minutes
5-minute repetition: 3 minutes
The only way to circumvent this two-minute delay is to take advantage of another physiological process: When you finish a repetition, your VO2 max remains elevated for the first 15-20 seconds of your recovery interval before decreasing to match the requirements of your recovery activity (e.g., jogging). So if you limit your recovery interval to 30-45 seconds, you can begin your next repetition close (in terms of VO2 max) to where you left off with the last one.
This is why Billat Intervals, which alternate 30 seconds at VO2 max pace with 30 seconds of recovery, provide a greater accumulation of VO2 max stimulus than 2-3 minute repetitions at the same pace. With Billat Intervals, you’ll get 45-50 seconds of VO2 max work per minute. With a 3-minute repetition and a 3-minute recovery interval, you’ll only log a little over one minute of VO2 max work per six minutes.
Or, for a slightly less intense workout, you can run multiple 400m repetitions at 5K pace, with recovery intervals of 100 meters. Again, the short recovery allows you to return quickly to the targeted 95 percent VO2 max, and you can accumulate a huge amount of work at that effort level.
The key to running an effective interval/repetition workout is twofold:
1. Choose the correct pace and length for your repetition.
2. Choose a recovery interval that maximizes the targeted stimulus of the workout.
Choosing the wrong recovery interval can lead to tapping the wrong energy systems, recruiting the wrong muscle fibers and overloading your nervous system. Choosing the correct recovery interval, on the other hand, allows you to reap the benefits of your training.
About The Author:
Pete Magill, five-time USA Masters Cross Country Runner of the Year, is the fastest-ever American distance runner over age 50 in the 5K and 10K. He is the lead author of Build Your Running Body and author of The Born Again Runner (Spring, 2016).