Decoding HIIT& VO2max TESTING: A Guide for Endurance Athletes

High-Intensity Interval Training (HIIT) isn't just for gym-goers looking for a quick calorie burn. For endurance athletes, strategically incorporating HIIT can be a powerful tool to shatter plateaus, boost performance, and add a sharp edge to your hard-earned aerobic base. Understanding your current fitness level, particularly your power at VO2 Max, is key to making these high-intensity efforts truly effective.

Let's break down how you can estimate your power at VO2 Max and then explore some common types of high-intensity intervals and how they can benefit athletes like the cyclists I coach.

Estimating Your Power at VO2 Max (pVO2 Max)

While I'd suggest taking the INSCYD metabolic test to get an accurate measurement of VO2 Max, a cyclist with a power meter and heart rate monitor can perform a reliable self-administered test to estimate their power output at this crucial intensity. This metric, often referred to as pVO2 Max, is essential for setting accurate power targets for your high-intensity intervals. You should not use a % of FTP for the interval intensity.

Self-Administered VO2 Max Test Protocol (5-Minute Maximal Test):This protocol uses a maximal 5-minute effort as a strong indicator of your power at VO2 Max.

Preparation:

• Ensure you are well-rested (no hard training in the 24-48 hours prior).

  
  

• Choose a safe, uninterrupted stretch of road or use a smart trainer.

  
  

• Ensure your power meter and heart rate monitor are calibrated and working correctly.

Protocol:

• Warm-up (20-30 minutes): Start with 10-15 minutes of easy spinning (Zone 1-2).

  
  

• Include a few short bursts (30-60 seconds) at moderate intensity (Zone 3) to activate muscles.

  
  

• Perform 2-3 short, sharp efforts (15-20 seconds) at high intensity (Zone 5) with full recovery (2-3 minutes easy spinning) to open up the legs.

  
  

• Finish with a few minutes of easy spinning.

The Test Effort (5 minutes):

• Start the 5-minute effort. Your goal is to ride as hard as you possibly can for the entire 5 minutes, aiming for a consistent, maximal power output.

  
  

• Pacing is key: Don't go all-out in the first minute and fade dramatically. Try to find a pace you can sustain maximally for the full duration. It should feel very hard from the start and become excruciating by the end.

  
  

• Focus on maintaining good form despite the intensity.

Cool Down (10-15 minutes):

• Spin easily in Zone 1 to recover.

Determining Your Estimated Power at VO2 Max:

After completing the test, analyse the data from your power meter. The average power maintained during the 5-minute maximal effort is your estimated Power at VO2 Max (pVO2 Max).

Also, note your peak heart rate reached during the final minutes of the effort. This can serve as a reference point, though power is the primary metric for structuring the intervals that follow.

Why HIIT for Endurance?

While your long Zone 2 rides build the essential aerobic engine, HIIT sessions push your upper limits. By working at or above your pVO2 Max, they improve your actual VO2 Max (the maximum amount of oxygen your body can use), increase your anaerobic capacity, enhance your ability to clear lactate, and recruit more muscle fibres, ultimately making you faster and more efficient at higher intensities.

Types of HIIT Intervals Based on Your pVO2 Max

The goal of many effective VO2 Max workouts is to accumulate as much time as possible with your oxygen consumption (and thus, your physiological stimulus) at or above 90% of your VO2 Max. While you can't directly measure oxygen consumption during training, specific interval structures using your pVO2 Max can help you achieve this.

Here are some common structures you might encounter or program, with intensities referenced to your estimated pVO2 Max:

1. Long Intervals (Targeting Sustained Time at VO2 Max)

What they are: Efforts typically lasting from 3 to 8 minutes. Intensity is usually around 95-105% of your estimated pVO2 Max. Recovery periods are often equal to or slightly shorter than the work interval duration.

Why they work: These intervals spend significant time at or near your VO2 Max, making them highly effective at increasing your aerobic ceiling. They also improve lactate tolerance and your ability to sustain hard efforts.

Example for Cyclists: 4 x 5 minutes at 98-103% of your estimated pVO2 Max with 5 minutes of easy spinning recovery between intervals.

Why it works: These longer intervals, performed slightly below or right at your pVO2 Max, allow your oxygen consumption to climb and stabilise at a high level (often >90% VO2 Max) for a significant portion of each interval. The recovery allows you to be ready for the next hard effort.

2. Short Intervals (Targeting Repeated Spikes to VO2 Max)

What they are: Efforts typically lasting from 30 seconds to 2 minutes. Intensity is usually supramaximal, around 105-120% of your estimated pVO2 Max. Recovery periods are often equal to or slightly longer than the work interval (e.g., 1 minute work, 1 minute easy recovery). The total number of intervals is higher than in long interval sessions.

Why they work: Short intervals allow you to accumulate a significant amount of time at very high intensities (VO2 Max and above) with less overall fatigue per interval than long intervals. They improve VO2 Max, anaerobic capacity, and the ability to handle repeated bursts of speed. The short recovery helps keep the physiological stimulus high across the set.

Example for Cyclists: 12 x 1 minute at 110% of your estimated pVO2 Max with 1 minute easy spinning recovery between intervals.

Why it works: While the intervals are short, the high intensity quickly drives your oxygen consumption up. The relatively short recovery prevents full recovery, meaning subsequent intervals start from a higher physiological state, allowing you to accumulate valuable time at >90% VO2 Max across the series of intervals.

3. Sprint Interval Training (SIT)

What they are: Very short, maximal or supramaximal efforts (all-out sprints), typically 10-30 seconds in duration. Intensity is well above pVO2 Max, limited only by your peak power. Crucially, SIT protocols are characterised by very long recovery periods, often 5 to 10 times the duration of the sprint (e.g., 10-20 seconds sprint followed by 60-200 seconds recovery). The total number of sprints is usually low (e.g., 4-7 sprints).

Why they work: SIT places an extreme demand on your anaerobic systems and neuromuscular power. It can lead to rapid improvements in VO2 Max, anaerobic capacity, and peak power output, often with a relatively low training volume. The long recovery allows for near-full replenishment of energy stores, enabling maximal effort on each sprint.

Example for Cyclists: 6 x 20-second maximal sprints from a low speed, with 3 minutes of easy spinning recovery between sprints.

4. Repeated Sprint Training (RST)

What they are: Multiple short sprints (typically <10 seconds) with very brief recovery periods (<60 seconds). Intensity is maximal or supramaximal. Unlike SIT, the recovery is insufficient for full recovery, leading to a progressive decline in power output across the set of sprints. A set might involve 10-20 sprints, and a workout might include multiple sets.

Why they work: RST focuses on improving your ability to repeatedly produce high power outputs and, importantly, to recover quickly between sprints. It trains your anaerobic capacity and your body's ability to buffer and clear lactate under repeated stress. This is highly relevant for sports with repeated short bursts of activity, like criteriums or many team sports.

Example for Cyclists: 2 sets of 10 x 6-second maximal sprints with 30 seconds of easy spinning recovery between sprints, and 5 minutes recovery between sets.

5. Fartlek Training

What they are: Fartlek, meaning "speed play" in Swedish, is a less structured form of interval training. Instead of fixed work and recovery durations, you vary the intensity and speed based on how you feel, the terrain, or landmarks. For cyclists, this means incorporating spontaneous bursts of speed, short climbs at higher intensity, or periods of harder riding interspersed with easier pedaling. Intensities will vary, often hitting Zone 4, 5, or even higher for short durations.

Why they work: Fartlek training is excellent for developing the ability to change pace and respond to unpredictable demands, mimicking the dynamic nature of group rides or races. It improves aerobic and anaerobic capacity, teaches your body to recover while still moving, and adds a fun, less rigid element to training. It's also effective for building mental toughness and learning to read your body's signals.

Example for Cyclists: During a steady endurance ride, pick a landmark (e.g., the next climb, a specific tree, a town sign) and ride hard to it (e.g., 1-3 minutes at Zone 4-5). Then, recover at an easy pace (Zone 1-2) until you feel ready for the next effort, which could be a short sprint (SIT/RST style) or a sustained push. Strava live segments can enhance your interval performance by boosting motivation and providing positive reinforcement, which in turn increases your power output. A "Strava-Val" involves pushing yourself to the limit, similar to an interval, but using the live Strava segment as motivation. Real-time feedback on whether you're ahead or behind your personal record can add 10-20 extra watts of motivation. It's like a solo group ride. You can perform VO2 Max intervals, anaerobic intervals, climbing intervals, breakaway intervals, time trial intervals, and sweet spot intervals with this training approach. The options are limitless, and during mid-summer, variety adds excitement, while motivation to push hard is crucial for maintaining peak performance.

Incorporating HIIT into Your Training

The key to using HIIT effectively is strategic placement within your training plan. It should complement, not replace, your foundational endurance work. Recovery after HIIT is crucial due to the high stress placed on the body.

By performing a regular 5-minute maximal test to update your estimated pVO2 Max and using that value to guide your interval intensities, you can effectively structure your HIIT sessions to maximise the time spent at the high intensities needed for significant aerobic performance gains.

Happy Training!