cycling training

Decoding Interval Workouts: How to determine the number and duration of intervals

Creating interval workouts with the appropriate amount of time-at-intensity and recovery between efforts is an essential skill for any athlete or coach designing training plans. Individual workouts are the small building blocks that build upon one another to create training stress that leads to physiological adaptations and improved performance. Workouts don’t have to be complicated to be effective, and the most effective are usually the simplest. But they still have to be designed correctly.

A training plan critique is one of the most important practices we have in the CTS Coaching College. A plan is put on the screen, the coach provides background information. This includes the athlete’s training history, some personal history, and goals (no names). Then the coach describes the rationale for the presented plan. At that point, the coaching staff critiques the plan and the rationale. They challenge the coach to defend their decisions and they provide constructive criticism. It’s challenging, and a crucial component of professional development.

One of the most important lessons for coaches is that there are many scientifically sound and effective paths to improve athletic performance. For instance, there are several ways to increase “time-at-intensity” or “time-in-zone” as an athlete gets stronger. When it’s time to decide whether to make intervals longer or add more intervals, here are some of the principles we use and recommend.

The fundamentals of interval duration

Interval duration determines the intensity of the effort and the physiology being stressed, not the other way around. Thinking about intervals in this manner, they are at the maximum intensity you can maintain for a given time. The maximum time an athlete can sustain an effort at VO2 max is about 8 minutes. An effort that’s 20 minutes long will not be a VO2 max effort because after about 8 minutes (or less depending on fitness) components of the energy production necessary to sustain a VO2 max effort are depleted. All efforts eventually become aerobic because that is the most basic and sustainable way we produce energy.

Similarly, the highest intensity achievable for intervals that are between 8-20 minutes in duration will be at or slightly above lactate threshold power output. This is because it takes that long for reliance on anaerobic glycolysis to drop so you reach a steady state condition. When people make lactate threshold intervals too short, too much of the energy for the effort is coming from anaerobic glycolysis (i.e. the effort is not aerobic enough).

Generally speaking, the range of durations for individual intervals are:

  • 20-60 seconds: Anaerobic Capacity
  • 2-5 minutes: VO2 max
  • 8-30 minutes: Lactate threshold
  • 20-60 minutes: Tempo & Sweet Spot Tempo
  • 60+ minutes: EnduranceMiles (Zone 2, aerobic endurance)

Why we start with higher number of shorter intervals?

Newer athletes or those inexperienced at a particular type of interval need to learn how to pace their efforts. Jumping straight into a 20-minute lactate threshold interval is difficult. Athletes don’t know how to predict what they’ll feel like or be able to sustain 20 minutes later. Eight-minute efforts are more manageable (we’ll sometimes start with six-minute efforts, even). Don’t worry if your pacing is off in the beginning. Athletes using short threshold intervals have tremendous room for improvement. As a result, they make gains in lactate threshold power even if the pacing is a bit off.

Intervals at the shorter end of the range are also good because new athletes can achieve higher quality efforts. That increased quality comes from more effective pacing of short intervals, and having the fitness to ride powerfully for the full duration. To increase peak power at a VO2 max, I’d rather see 5 strong 2-minute efforts than 2 mediocre 5-minute efforts. The goal is “time-at-intensity”, so if two minutes is as long as you can maintain an effort that elicits VO2 max right now, more 2-minute intervals is the way to accumulate maximum time-at-intensity.

Add intervals or make each interval longer?

OK, so after a block of training and some recovery, you’ve adapted and improved. It’s time to add more training stress to your workouts, but how? Keep the intervals the same duration and add more efforts? Or, keep the number of intervals constant and make them longer? Generally, the harder the effort, the more we start by keeping interval duration constant and increasing the number of efforts. For instance, with VO2 max intervals for moderately fit cyclists, the amount of time-at-intensity within a single workout will max out at around 15-20 minutes. Early on we use small chunks of time to get to the point where you can do 15-20 minutes time in zone. Then we may reduce the interval number and increase the duration of each, in an effort to reach the point where you can do 5-minute VO2 max efforts.

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With lactate threshold and challenging aerobic intervals (Tempo and Sweet Spot Tempo), the individual intervals start out longer. As a result, adding more intervals can represent a big jump in stress. Because of this, it is harder to make rule of thumb recommendations for interval progression at this intensity. If you start with three 6- to 8-minute efforts, add time to each interval to get to 10- to 12-minute efforts. Then, depending on the athlete, go to 4 efforts but knock them back to 8-10 minutes each. Alternatively, stick with 3 intervals and add time to get closer to the goal of 20-minute individual intervals.

Maximum Time-at-Intensity

We’ve established there are limits to how long athletes can sustain a single effort at given intensity. Cumulative time-at-intensity during a single training session is (or should be) also limited, too.  Even once you can do effective 5-minute VO2 max intervals, you can’t do an endless number of them. So, as we build individual workouts for moderately fit cyclists using variations of interval number and duration, the maximum times-at-intensities we’re looking for are:

  • 3-5 minutes: Anaerobic Capacity
  • 15-20 minutes: VO2 max
  • 45-60 minutes: Lactate threshold
  • 75-90 minutes: Tempo and Sweet Spot Tempo
  • 3+ hours: EnduranceMiles (Zone 2, aerobic endurance)

Perspective, Not Prescription

Knowing how intensities and durations work together to generate training load is good for gaining perspective on the principles behind the workouts on your training plan. However, it isn’t enough to create plans that account for training history, athlete phenotype, and strengths and weaknesses. Some parts of your plan and workouts also must be specific to the demands of your goal event. This is where working with a professional coach shows its value, especially a coach who has to defend your training plan before the entire coaching staff!

By Chris Carmichael, with Coach Adam Pulford

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Comments 16

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  2. 3+ Endurance or zone 2 looks open ended. Is the intent 3 hours of cumulative time at endurance pace is ideally the minimum prescribed with upper end undefined? I remember reading at one time anything over 6 hours didn’t have any benefit, just adds fatigue and doesn’t contribute to forcing adaptation. Thoughts?

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  4. I agree with Gkuche. That is one of the very best CTS articles I have ever read. Those basic concepts and fundamentals of interval training are presented very well in your book The Ultimate Ride, but this explains the logic and reasoning of the various training schedules. As a very old man, I have been successfully following them since about 2004. :<)

  5. To increase peak power at a VO2 max, I’d rather see 5 strong 2-minute efforts than 2 mediocre 5-minute efforts.

    How long does one rest between 5 2 min efforts at % effort compared to the 2 5 min efforts to get the max benefits.
    For VO2max efforts should one recover at 40% FTP for 1/2 the interval length?
    For threshold efforts should one recover at 60% FTP for 1/2 the interval length, etc?
    There is much info on what intervals to do for adaptations, but I’ve found little info on the best recovery method between intervals.

  6. I’m confused by how to determine what my VO2 Max is. I’ve done lots of structured training including VO2 max intervals but I would like to better understand how to determine my VO2 max. I’ve got two years of power data informing my power duration curve. How about using my best peak 5 or 6 minute power over the last six weeks from my PDC and using that number? Or, I could do a set of 4×5 intervals at the maximum effort I can sustain just based on my RPE and then, assuming I was able to keep the power close to the same level for all four intervals, use that number as my VO2 max.

  7. Thank you, very helpful. One thing confuses me. It’s the sentence that reads “The maximum time an athlete can sustain an effort at VO2 max is about 8 minutes.” If so, why do we take our average power output over the better 8-min stretch of an FTP test and multiply it by 0.9? Based on the sentence above, we would take that wattage—sans scaling—as our FTP. Your insight is most appreciated, and thanks again!

    1. Post

      Thanks for the question. A few things to point out. FTP is Functional Threshold Power, another way of defining maximum steady state or lactate threshold. That’s not the same thing as power at VO2 max, which is the power you can produce at your maximum aerobic capacity. Power at VO2 max will be higher than power at lactate threshold, and one of the goals of training is to increase power at threshold as a percentage of power at VO2 max. For instance, you may start with LT power or FTP at 75% of your power at VO2 max. We want to see VO2 max power increase, but it has less potential to increase than your power at FTP. Ideally, with training you’d increase VO2 max and your FTP as a percentage of VO2 max fro 75% to 80-85%.

      The second part of your question deals with the 8-minute test and the statement that the maximum time a person can maintain a VO2 max effort is about 8 minutes. There are very few people who can actually sustain an 8-minute effort at VO2 max. And when starting from a standstill, a portion of any interval targeting VO2 max is spent getting up to the appropriate intensity. So, the reason we typically prescribe 2-5 minute efforts when training VO2 max is because that’s the range of time most athletes can effectively reach and then sustain a quality effort. We know people are operating above FTP for the 8-minute test, which is why the conversion factor of .9 is used (similar to the .95 used for people doing a 20-minute test). But because most athletes are not able to sustain VO2 max intensity for 8 minutes, the test typically yields an average power about 10% above threshold.

      Jim Rutberg, Co-author “The Time-Crunched Cyclist” and “Training Essentials for Ultrarunning”

  8. but i keep seeing article after article reporting on studies showing the benefits of super short interval training, like this one:

    where does this fit in?

    1. sorry about the url. it’s this:

      1. Post

        Yes, very short, very high intensity intervals can be useful in training. We’ve talked about them here: and here: These are different than anaerobic capacity intervals, which are similar in duration (20-60 seconds) but have longer recovery periods of 3-5 minutes between efforts.

        Jim Rutberg, Co-author “The Time-Crunched Cyclist” and “Training Essentials for Ultrarunning”

        1. I agree with the conclusions in the Ultra articles that Jim cites – the key is quality/effort during the interval, combined with shorter rest periods for incomplete recovery. For example: Tabata (20sec on, 10 sec off for 4 minutes) is effectively a VO2 max workout, but I think easier for many people to manage (all out, then catch your breath while you’re still moving) than a specific pace target.

          When I was running cross country, one workout we did early in the season was 20 x 400m a bit faster than mile race pace – a little over a minute per lap, with only 30 sec rest. I didn’t think about it this way at the time, but that was essentially 20+ minutes of very high quality VO2 max training. As hard as I remember it being, maintaining the pace/effort for a minute at a time was much easier mentally than mile repeats – the other brutally hard workout.

  9. So is a reasonable general fitness approach to go at max effort for eg 30 mins (ie try not to pace) and so get anaerobic and VO2 for “free” in what is primarily an LT ride?

    1. Post

      You are producing energy through aerobic metabolism and anaerobic glycolysis all the time. The relative contributions from each change with intensity. So, during an effort you’re describing – starting out at maximum and gradually slowing to a more aerobic and sustainable pace, which will end up at an average power around lactate threshold – you might spend a little time near VO2 max at the beginning, but not enough time-at-intensity to stimulate an adaptation. The reason we use shorter intervals with shorter recovery periods for VO2 max is so you can repeat them several times and increase the total time at or above 90% of VO2 max.

      The effort you’re describing might have some utility for lactate tolerance, however, as the initial maximum effort will produce a lot of lactate and you will be processing that lactate and integrating it into normal aerobic metabolism as the interval proceeds.

      Jim Rutberg, Co-author “The Time-Crunched Cyclist” and “Training Essentials for Ultrarunning”

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