In modern times, interval training is a fundamental way to train endurance performance. I overheard my 10-year-old telling his brother, “the best way to get faster is to do intervals, that’s what my gym teacher said.” For the general public, any intervals will do, but for the highly trained endurance athlete we need to get a little more scientific.
The basic premise of interval training is that you are able to swim, bike, or run at a higher intensity if your training is intermittent versus continuous. A 5k runner, for example, could head out the door and cover 3.1 miles as fast as possible a couple times a week, but would have a hard time holding their goal pace for much more than one mile. Instead, if the training was broken into half mile intervals, a race-pace could be achieved with every 800 meter bout as long as recovery was adequate. With each repeated bout there is a cumulative effect, up to a certain point, to stimulate adaptation. Beyond a certain point, maladaptation can occur.
Types of Intervals
In an effort to simplify we will focus our discussion on the most potent type of intervals known as VO2 max intervals. VO2 max is defined as an individual’s highest rate of oxygen consumption (milliliters of oxygen per kilogram of body weight per minute). A common misconception is that interval training is strictly anaerobic. These types of intervals do have a big anaerobic component, but by definition have you operating near your peak oxygen consumption, which is the key. Most athletes can work at VO2 max for only about 5 to 9 minutes, so intervals at VO2 max need to be shorter than that. If the interval is too short, then the anaerobic contribution is big, but there is not enough time to actually get to VO2 max. Personally I like 2-3 minutes ON with about equal recovery.
Pacing is critical. Suppose you are running those 800 meter bouts and you start out by sprinting the first 200 meters and then have a gradual slow-down for the next 600 meters. Your average pace might be on target, but you have failed to reach VO2 max since you started with this huge anaerobic effort and then settled into a pace slower than your VO2 max intensity.
There is something called a slow-component to VO2 max. This means that for any pace above lactate threshold, you will eventually reach VO2 max if the exercise is continued. So you do want to go fast, but to spend the most amount of time near VO2 max, you want a pace you can sustain for 2-3 minutes. If performed correctly your oxygen consumption will approach VO2 max about half-way through each hard effort. So if your workout is 6 x 2.5 minutes, you might in reality only spend a total of 7.5 minutes at VO2 max, which is fine. If performed poorly you may only spend a few seconds of each interval at VO2 max or none at all.
Graph 1: Poor pacing strategy with high power output (purple) at the start of each bout and power dropping on each bout.
Graph 2: Better pacing strategy here. In this case, power (purple) within each bout was very consistent, but there was still a drop off in power for the last 3 bouts.
Graph 3: Power (white) fairly consistent throughout hard efforts. Notice peak heart rate (orange) is not achieved until the 6th bout. Different athletes, different software.
An ideal range for work portion is about 1.5 to 4 minutes. We like the 2-3 minute range the best with a few exceptions. Work to rest ratios are usually around 1:1. The rest interval can be adjusted to increase or decrease the intensity of the workout. If you are having a hard time keeping pace, try adding 30 seconds rest. If you are completing the workout with energy to spare, try 30 seconds less rest the next time out.
Pace or power are your best guides for this type of training. Heart rate lags so far behind that it is not the best indicator and you don’t want to try to spike your heart rate to start each effort.
If you have power on the bike, I like to use 110% FTP for 3 minute bouts, and 115% FTP for 2 minute bouts.
For running, a 5k race pace or slightly faster will get you there. A 15 minute 5k runner can just use their 5k pace, but a 25 minute 5k runner might need to increase the pace slightly.
If you are performing intervals uphill and don’t have power or pace to guide you, try this approach. Warm up to the base of consistent climb. On your first bout, hold back a fraction and note your distance at 1 minute and 2 minutes. Make a mark in the dirt. Recover on the downhill and repeat the same section of the hill attempting to at least reach the same finishing mark or go slightly further. Try to do this without going any further for that first minute.
I mentioned earlier that this is the most potent form of training. So your goal is to be able to maintain the quality for the entire workout. For most people this means 15-21 minutes of total hard work. So that is 8-10 bouts of 2 minutes, or 5-7 bouts of 3 minutes. Keep it simple. Shoot for a very similar intensity every time and if you start to slow down you have done too much.
A little bit can go a long way. I try to space out this type of training more than any other. For most people that means two quality sessions per week with one on the bike and one on the run. Training becomes more polarized during a VO2 max cycle with recovery and endurance workouts separating VO2 max bouts. Total training volume is reduced and avoid excessively long workouts during this time.
Soon Ripe, Soon Rotten
With this type of training, most people will plateau in about 6 weeks. I like to sprinkle in this type of training as key races are approaching and save heavy blocks of VO2 max interval training for the most important races of the season.
Josiah Middaugh is the reigning XTERRA Pan America Champion and 2015 XTERRA World Champion. He has a master’s degree in kinesiology and has been a certified personal trainer for 15 years (NSCA-CSCS).