Training in zone 6 - VO2max intervals

Performance in endurance events is primarily limited by aerobic energy production i.e. energy being made available by the use of oxygen. Traditionally, the index of this ability is the maximal oxygen uptake, or VO2max. The VO2max is defined as the maximum rate that oxygen can be taken up from the ambient air and transported to and used by cells for respiration during physical activity. Check out our video of a VO2max ramp test to see how a sports scientist would measure this key parameter. It is the VO2max that describes the exercise intensity area of zone 6. This fact sheet explains why developing this ‘top end’ of your physiology can help your performance.

Why is ‘top end’ important?

For more information check out the following links

Training sessions for working in zone 6

VO2max test in the lab

Determinants of endurance performance

The physiological basis of the training zones

Setting your training zones

If you were to exercise at the intensity associated with your VO2max, you would typically last 4 to 8 minutes1. It would therefore make sense that in events lasting a similar duration (e.g. the 3 and 4km individual pursuit in cycling, or the 1500m in athletics), the VO2max would be the best predictor of performance. Indeed this is the case, with power outputs held in pursuits being very similar to ‘maximal minute power’ obtained in a ramp test. The time to exhaustion at maximum appears similar across different types of sports, but interestingly is inversely related to fitness2.

The relationship between VO2max and performance does extend down to less intense efforts too. Those events with durations of 30 minutes or less are still operating at high percentages of VO2max. Time trial cycling events (16km) and middle distance running events (10km) are run at ~ 90% of VO2max, and in fact, highly trained athletes appear to be quite stable at these high percentages of VO2max3,4. Some sport scientists argue that VO2max is the most important factor in dictating performance VO2, because it is the VO2max that sets the upper limit of VO2 at the race pace in events lasting one hour or so. Indeed, it is likely that a high fractional utilisation of VO2max is key because the higher you make your top end, the more likely that the rest of your profile is dragged up too. For example, consider a cyclist with a VO2max at 350W and a lactate threshold at 70% of this. If they worked specifically to improve VO2max power to 380W, they would likely also improve the LT by 20W (from 245 to 265. Furthermore, it is probable that there is some overlap in the mechanisms controlling both VO2max and high end race intensity. If a sport scientist observes an athlete with LT at a high % VO2max, they would likely suggest training to increase the ‘ceiling’.

What does limit VO2max?

Interest in the upper limit of aerobic fitness goes back to the work of Hill and Lupton in 1923, with research attempting to understand what controls VO2max ongoing ever since! Despite the amount of work performed, the exact mechanisms are still hotly debated by exercise physiologists. Essentially, the debate revolves around whether this upper limit is restricted by ‘peripheral’ or ‘central’ events in the body:

  • Peripheral: those aspects occurring at the muscle – number of capillaries, neuromuscular function, muscle fibre type, oxygen extraction capabilities. All these impact on the degree with which oxygen can be utilised by the muscle.
  • Central: factors within the central nervous system, and central circulation e.g. arousal, cardiac function, blood volume. All these impact on oxygen transport and the rate with which oxygen can be delivered to the muscle.

The figure below summarises these. It has been estimated that oxygen transport by the circulation controls 50% of the VO2max in exercise with large muscle groups5. As muscle mass decreases, peripheral factors such as the capillary blood flow and number of mitochondrial become more important as – so for the same person running and cycling, central factors might limit their running more than in cycling. It explains why cyclists often feel a VO2max test stops because of their legs!


How do we improve VO2max?

Like with any training aim, we must follow the principles of specificity. If, as the research suggests, VO2max is controlled by different mechanisms (e.g. central factors) to those in lower intensity exercise, it is important to stress the appropriate systems: in order to adapt, these systems must be stressed to a level to force re-modelling.

We know that VO2max is a product of the highest cardiac output (the amount of blood pumped around the body each minute) and the highest extraction of oxygen from this blood volume. Training to enhance either or both of these should lead to increases in VO2max. In a series of papers, Daussin and colleagues6, 7 looked at how these two products could be changed by training. In their first study, they found that interval training improved VO2max and cardiac output, more so than continuous, steady training of a similar total work. They concluded that interval training improves both central and peripheral components whereas continuous training was mainly associated with greater oxygen extraction. In their second study, they found the interval training more successful in changing the capacity of the muscle mitochondria to burn oxygen, yet capillary density was improved in interval work AND continuous training (in fact, to a greater extent in the latter mode).

Muscle fibre type changes do not appear to play a major role in VO2max enhancement in well trained individuals. This is mainly because the oxidative capacity of muscles far exceeds the cardiovascular systems’ ability to deliver the oxygen (i.e. increased rates of oxygen utilisation can always be instigated). However, working at VO2max intensities in training might recruit the fast twitch fibres preferentially. This would force them to become more oxidative, and add them to the muscle fibres usable in sustained, endurance events.

When would I choose to train the VO2max?

VO2max centred training is hard work, and will put great stress on the athlete’s system, so it is wise to consider when and how this training will be incorporated in the training year. You may use VO2max training in the following instances:

  • When looking to progressively increase VO2max to its maximum trainable limit over the many years of a runner’s competitive career. Several authors have suggested that athletes approaching their trainable limit for VO2max may even need to attain and maintain VO2max to elicit further increments8
  • After scheduled periods of low-intensity training and relatively low total training loads which have caused a transient decrease in VO2max (i.e. as part of a periodised training programme)
  • Peaking prior to competition when all physiological capacities are maximised to their trainable limit
  • After an absence from training due to a scheduled (holiday) or unscheduled lay-off (e.g. due to illness or injury). This training will lead to rapid improvements in form (if adequate base training has already been performed)
  • When decreasing the total amount of training time while still stimulating or maintaining a high level of cardiorespiratory fitness. Short bouts of training at and near VO2max may be effective in maintaining training during low training loads such as during tapering9

It is not recommended to start VO2max training without care and planning. Preparatory training should include several months of base training at intensities of 65–70% VO2max (zones 2 and 3) followed by transition training at 85% VO2max (zone 4)10.

What is the best training to do?

At the beginning of this factsheet, it was mentioned how the time to exhaustion (TTE) at VO2max is typically 4 to 8 minutes. This parameter has been used to optimise interval training sessions11, 12 – the idea being that athletes aim to increase the total time spent at VO2max in order to stress the appropriate systems as much as possible. When athletes perform high intensity intervals at the power associated with VO2max for 60% of the time to exhaustion, 40km TT performance and VO2peak increased (5% and 1% respectively)11. Using 60% of their own time to exhaustion gives an athlete enough time for the VO2 to ride to maximum in each interval – this is highly individual, as the speed of the oxygen uptake rise towards maximum levels may be longer or shorter than this: there is a need to decipher the time to reach VO2max for each athlete. Indeed, using a longer time to exhaustion (74%) is needed for some athletes to reach VO2max13.

So which is the best duration to use? Yes, using a higher % the TTE ensures VO2max is attained, BUT does it accumulate fatigue too rapidly? Perhaps it is better to extend the time spent at VO2max in the whole training session14 (i.e. the athlete can do more reps).


Intermittent protocols have been found to be more effective than continuous protocols for maximising the time spent at VO2max – that’s pretty obvious, just try to as much work as you can at your maximal minute power in one go, compared to splitting the work into 3-minute blocks! Research points to the work interval intensity ideally being between 90% and 105% of the VO2max, enabling ~3 min. in each interval12. There has been a lot of interest generated in using smaller durations than this. Comparison of 15s on/15s off interval running and 4 x 4 min of interval running (at 90-95% HRmax) found similar increases in VO2max over 8 weeks, both being better than moderate intensities15.  Recent research has even begun to consider the use of more intense, short duration repetitions.

What about the recovery?

As soon as you mention interval training, we have to factor not only the intensity and the duration of the effort, but also the recovery. Intervals to boost VO2max require the emphasis of ‘aerobic’ functioning:

  • Recovery needs to be long enough to facilitate recovery and enable greater accumulation of time spent at VO2max
  • Recovery duration too long: yes, a higher intensity could be attained BUT, bigger contribution to energy supply from anaerobic energy metabolism.

Most research suggests an equal work rest ratio to be optimal. In a recent study by Rozenek and colleagues16 selected physiological responses to short-duration (< or = 60 seconds) interval work performed at velocities corresponding to 100% of VO2max were characterised. The researchers compared 15s on/15 s off (15/15); 30/15; 60/15; and a TTE trial at 100% of VO2max. They found high intensity, short-duration 2:1 W/R intervals to produce responses that may benefit both aerobic and anaerobic energy system development, and thus recommended these for training.

Again, in keeping with the fact that the recovery period is to facilitate recovery, you need to make sure the intensity of the recovery is kept low enough. Some continuation of exercise rather than purely passive rest is recommended though, as this keeps blood flow higher.



1.   Billat & Koralsztein. Sports Med 1996, 22, 90-108.

2.   Billat et al. Ergonomics 1996, 39, 267-277.

3.   Perrey et al. Int J Sports Med 2003, 24, 138-143.

4.   Billat et al. Arch Physiol Biochem 1998, 106, 38-45.

5.   di Prampero. Eur J Appl Physiol 2003, 90, 420-429.

6.   Daussin et al. Am J Physiol Regul Integr Comp Physiol 2008, 295, R264-272.

7.   Daussin et al. Eur J Appl Physiol 2007, 101, 377-383.

8.   Billat. Sports Med 2001, 31, 75-90.

9.   Hickson et al. J Appl Physiol 1985, 58, 492-499.

10. Midgley et al. Sports Med 2006, 36, 117-132.

11. Laursen et al. Med Sci Sports Exerc 2002, 34, 1801-1807.

12. Billat et al. Eur J Appl Physiol 2000, 81, 188-196.

13. Laursen et al. Res Q Exerc Sport 2004, 75, 423-428.

14. Smith et al. Eur J Appl Physiol 2003, 89, 337-343.

15. Helgerud et al. Med Sci Sports Exerc 2007, 39, 665-671.

16. Rozenek et al. J Strength Cond Res 2007, 21, 188-192.

For more information check out the following links

Training sessions for working in zone 6

VO2max test in the lab

Determinants of endurance performance

The physiological basis of the training zones

Setting your training zones