Endurance 101: Zone 5 - Zone 5 VO2Max and Maximal Aerobic Power (MAP) Made Simple

What is zone 5?

As we have covered in previous posts, the "zone" system is a way of attempting to divide the range of an athlete's power output into a progressive series of numbered zones in ascending order of intensity, so as to provide one common language for both the description and prescription of training. The zone system attempts to make the zones as objectively discrete as possible by anchoring them to some underlying physiological parameter.

Most already realize that these zones represent increasing intensity, but may not be aware of precisely what the difference is between the various zones. Whereas we have looked at blood lactate levels as determining the boundaries between zones we have covered so far, zone 5 relates to a different physiological parameter - oxygen uptake.

What is oxygen uptake?

When we exercise, our body utilizes energy. Our measure of power when cycling, the watt, is a measure of energy burn, in joules, per second. Ultimately our body utilises a chemical called ATP to deliver energy for exercise but we can produce this energy from fuel via a variety of different fuel origins - that is stores - systems and pathways.

The transformation of fuel substrates into ATP can be performed both aerobically, with the utilization of oxygen, and anaerobically, without the use of oxygen. These can occur simultaneously and, in different proportions, as a response to demand. Aerobic energy production requires the body to take on oxygen through the lungs, be absorbed by diffusing between the alveoli and capillaries of the lungs, transported through the blood, delivered to its site of requirement in our working muscles by pumping oxygenated blood around the body through the heart, offloaded at its delivery site through branching capillaries feeding into muscle cells, and then consumed in oxidative energy production in concert with other chemicals and fuel substrates through oxidation in the mitochondria.

How much oxygen do we use and need for exercise?

We will not go into the detail here, but it is sufficient to know that (in general):

the proportion of energy which is produced aerobically is greater at lower intensities (and lower at higher intensities) but that ;

Thus, the overall amount of energy produced aerobically increases consistently with the demands placed upon the body until it reaches an upper limit beyond which some portion of this chain-intake, absorption, transport, delivery, diffusion, or oxidation-no longer can function at a higher capacity and levels off.

It is this second principle that is important for understanding "zone 5", as this relates to this point of maximal oxygen consumption. It is important to understand that any one part of this chain may be the limiting factor for an individual athlete at a given time.

Oxygen uptake, then, is not constrained to one part of the body but is multifarious and multi systemic: central lungs and heart and peripheral capillaries, working muscles, and mitochondria. This involves, therefore, processes all over our entire system, from the lungs to the legs. Both central and peripheral components play an important part in any athlete's capacity for aerobics and therefore constitute a potential impediment to O2 update, particularly if under-developed.

Since we are talking about oxygen uptake, it might sensibly be assumed that it would be our lungs, the primary organ of our respiratory system, which is the critical limiting factor here. Yet, this is not so. Typically, it is rather the heart that is unable to push sufficient quantities around the body sufficiently to meet demand and is our limiting factor.

Which of these then becomes the limiting factor to oxygen uptake?

In a given individual, any one of these components could be a limiting factor to maximal oxygen uptake. The reason for this is that the body is "sensibly lazy" - that is, it does not design us to be perfectly efficient exercising machines, but rather do just enough to let us survive and reproduce. While that does describe most of our history as humans, and much of the world still today, the idea is more in line with "scraping by on as little calories as possible" than "running away from tigers." Without calories, there is a danger of starvation and death. Individuals throughout evolution who could optimize resource consumption through efficient metabolism and lower caloric burn might survive periods of famine and hardship, hence outcompeting those built for optimal power output and oxygen uptake. That's why you don't look like The Rock.

In essence, to train VO2max we need to overcome that default imperative and force our body to adapt to a situation where we are in pursuit of maximal performance, not maximal efficiency. The reason we don't automatically reach our optimal performance state without training is that there's a kind of in-built "conservatism" in our physiology.

We essentially need to trick our body into believing that its number one priority in life is to run away from tigers. It is also, however, the reason why the training stimulus for such high intensity needs to be repeated and relentless. Without it, we will "detrain". That is, our body will simply revert back to a state where it prioritizes efficiency over performance, and reverses adaptations we have made for performance.

What is VO2max useful for?

Maximal aerobic powerMAP is the type of power utilised in such road riding and racing as breakaways, hill climbs, and surges. It becomes more dominant in the non-road disciplines like mountain biking, cyclocross and crit racing, where less dominance can be seen in the type of long, steady-state riding such as Time Trials TTs and ultraendurance riding.

But VO2 max is important because it also sets a cap or ceiling for our threshold, or FTP power, which can never be higher than 100% of our VO2max.

How does an increased VO2 max benefit an athlete?

By improving our VO2max, we can take on more oxygen, and hence more power delivered through increased aerobic power delivery. Given that cycling is essentially an endurance sport, this makes us faster cyclists and athletes.

Is VO2max trainable?

Yes, the absolute physiological VO2max is trainable; likewise, power at VO2max is also trainable. These factors are subtly different because power at VO2max can be further increased even when VO2max has leveled off through means like increased working efficiency.

It is believed that through acute changes, such as blood plasma volume increases, VO2max can be increased perhaps 15% in a single short block of intense training. Changes taking longer to develop, such as capillary density and heart volume, can increase our potential to raise the VO2Max around 25%.

However, as a species we are not particularly well optimised for VO2max. Whereas horses may achieve VO2max figures of around 180 mL/kg/min, for example, typical VO2max values for a relatively sedentary human may lie roughly around 35-40 mL/kg/min for men and 25-30 mL/kg/min for women. Via training this may be increased however, to between 40 to 50 and 30 to 40 .

What is a "good" VO2max?

VO2max can be trained; however, the range in which it exists and its attainable physiological maximum will be determined to a very large degree by a person's genes. Genetic expression can predispose a person to a higher percentage of slow twitch versus fast twitch fibres, for example. This, combined with hundreds of other influencing factors, ultimately determines the maximal achievable VO2max for an individual.

Elite cyclists may therefore be found with VO2max values that are simply not achievable by others, regardless of how hard they train - perhaps 85 mL/kg/min for men and 75 mL/kg/min for women.

What other factors affect VO2max?

VO2max also declines in a given individual as they age. We are all aware that a 90-year old likely cannot cycle as fast as an 18-year old, and much of this difference is attributable to VO2max declining as we age. However, the rate of decline can be much reduced greatly via an effective, and long-term (lifetime) training regime.

How do I measure my vo2max?

Something important that we haven't discussed so far is that "VO2max" is not a single number; it's modality specific. What this means in a practical sense is that a person doesn't have a single "VO2max", but many of them for one of specific activities.

This is because we utilise more oxygen whilst exercising more muscles. Thus, if we are doing star jumps we are utilising many muscles, and hence may be expected to have a much higher "star jump VO2max", than "single finger flex VO2max", no matter how quickly we wiggle that little pinkie.

This is not meant to be purely flippant - there are very real and observable differences in an individual between their "run VO2max" and "cycling VO2max" for instance, as triathletes are likely aware. That means that if we want to know our VO2max for an activity, we have to test it using that same exercise modality.

How do I measure my VO2max in a given modality?

In an exercise physiology lab, VO2Max is most commonly done by wearing a mask hooked up to a respiration analyser, which measures the percentage of oxygen in our inhaled versus exhaled breath and is able to calculate our oxygen consumption. When a ramped intensity test is performed, it is possible to observe oxygen consumption climb with an increase in the intensity of exercise until it plateausthis plateau represents our "VO2max". By noting the power output at this figure, we have a "power at VO2max" figure. This is normally referred to as our "Maximal Aerobic Power ". Notably, this is not our absolute maximal power - we could ride at a higher power than this, but any increase in power after this point comes from increased anaerobic energy production.

How can I measure my VO2max power outside of lab conditions?

Obviously most of us do not have access to physiological testing inexpensively and certainly not in an ongoing manner to be able to track our VO2max development. We therefore need to turn to proxy measures for identifying maximal aerobic uptake.

Platforms like Zwift make an attempt to do this, but often do it inaccurately. They might, for example, set an athlete's "zone5" to some assumed 120% of their recorded FTP. This might be highly inaccurate for a given individual since the percent of MAP (VO2max) power able to be sustained at threshold is highly trainable-that's essentially what threshold training is.

Ironically, the "ramp test" that Zwift use for determining FTP is actually a much better test of MAP (power at VO2max), since the average power over the last 5 minutes of the test will align more closely with an estimate for Zone5 (MAP) power.

Can I measure my VO2max using heart rate?

Although there is close correlation between heart rate and oxygen uptake, VO2max may not equate to max HR unless central - heart volume/stroke limitations are the limiting factor for an athlete's VO2 max at any particular point in training. This is not assured, since peripheral, muscular limitations may be the true limiting factor.

However, a value of around 90% of max HR is generally considered to be a reasonably "safe" estimate for the level of training exertion that indicates operating at, or just short of VO2max and hence providing an appropriate zone for training.

How can we improve our VO2Max?

The main adaptation that training in "Zone 5" -that is, riding at our VO2max power-elicits is an increase in the heart's stroke volume: that is, how much blood it pumps in a given contraction, or the difference between fully expanded and contracted heart chamber extents. That's right; the goal of high intensity training in zone 5 is to repeatedly stretch the heart muscle -that is, fill it with lots of blood and then expel it.

It is like any other muscle in as much as it is "trainable", so by repeatedly performing this exercise-so to say, like lifting weights in the gym-we are able to place a stress on it such that it is forced to adapt by increasing both its contractile strength and size. The hearts of professional athletes can be substantially larger than those of sedentary individuals, enabling them to deliver more blood with each heartbeat.

Once we become more highly trained via VO2max training, the heart's stroke volume reaches, or approaches its maximum possible value.

How can we continue to improve our VO2max at this point?

Once changes that induce cardiac output begin to plateau, then further adaptations need to be sought via peripheral factors instead. These adaptations can take longer to deliver and are generally elicited by long-term, high-volume training at a lower intensity-the "zone2" training that we covered in our earlier post.

Given that we breathe heavily when working at high intensity, the obvious question is, does this ever limit exercise performance, and if so, should the lungs be specifically and independently trained in much the same way as we individually train the heart? Various devices for training the lungs do exist and would no doubt be of benefit if lung capacity were limiting; however, the science would appear to suggest that our lungs are an "over-engineered" component, and very seldom a limiting factor. This is likely especially true in cycling in which the only working muscles are our legs compared to whole-body exercise modalities.

Are there any safety concerns over VO2max training?

Yes. More than perhaps any other exercise intensity or zone there is a "risk" from zone5 or VO2max training. This is because VO2max training relies on specifically and deliberately placing unprecedented stress upon the heart.

While it is necessary to place stress upon the heart in order to elicit adaptation, it does also carry an element of risk. It is possible to overstate this risk as the health outcomes from high-intensity exercise are almost always lower than those of remaining sedentary, but they do exist and you should always consult a physician if you have any concerns.

Risks I am aware of include, but are not limited to:

• The possible scarring of cardiac tissue via repeated maximal intensity training;

• An acute trigger for pre-existing cardiac conditions;

• The development of heart arrhythmias; and

• Problems relating to over-developed heart size-the risk of which, again, is relatively low for all but professional athletes.

All of these risks partially overlap. The biggest proportion of fatal cardiac events that have suspected links to high-intensity exercise, from what I have read, pertains to hypertrophic myocardial fibrosis: an underlying genetic condition occurring in about 1 in 50 but aggravated by adaptations made by the heart because of high-intensity exercises, wherein thickening of chamber walls reduces electrical signalling strength, leading to arrhythmias.

I am not a doctor, and this is no specialty of mine. I don't want to alarm people either, but on the other hand it's irresponsible not to say anything either about the risks. Consult a physician if in doubt or if you have any known underlying conditions.

What sessions can I use to develop VO2max?

We can see that training in any aerobic zone, zones 1-5 improves VO2max to some extent. Training in zone 5, at or around our point of VO2max is necessary, however, in order to optimise our VO2max. It is not possible to continuously train at VO2max for any extended duration within a single session. Hence, High-Intensity Interval training - HIIT - is the protocol generally used.

Default Workout protocol The default workout protocol consists of "long blocks" of continuous work in zone 5, interspersed with period of recovery.

An example workout may consist of 4-5 rounds of intervals. In each interval we may ride in zone 5 for between 2-6 minutes at our zone5 power, before recovering in a very low power, and then repeating, for a total of around 20 minutes session-TiZ.

In general, we want to maximize aerobic contribution to the "on" intervals, which we do by making certain that we rapidly deplete the anaerobic systems and then prevent them from recharging to contribute to power output in subsequent intervals. For this reason, we keep the recovery intervals relatively short and do work in a 2:1 work:rest ratio.

What Power Should I Ride For Each "On" Interval?

There are often estimates provided such as "ride at 90% HR" or "ride at 120% threshold power", but if we are being honest we'd recommend instead to simply ride at maximum power you can sustain in order to get through the interval. Obviously it may take a few trial and error efforts for you to initially establish this.

Don't be worried if each interval is at a slightly lower power than that preceding it-that is just the way it is because you are exhausting more of your anaerobic reserve each time. So long as your average power per intervals is comfortably above your threshold power-say 105% or greater-and you are panting like a fish, then you are delivering an appropriate stimulus.

You should feel your heart rate increase on each successive interval.

How do you progress VO2max training in a block?

When we are training VO2max in a block, we may decide to train it twice per week. Progressive overload can be achieved via either pushing power up, or pushing interval duration out.

I would always recommend attempting to extend time in zone first via interval duration extension, certainly if interval duration is less than 5 minutes. After this point you could then repeat the VO2 block, dropping the duration down to 2-3 minutes but at a higher power, and then attempt to extend duration per workout again.

How should VO2max training be periodised?

Typically, we can expect to max out gains from VO2max-zone5 training within 4-6 weeks' time, maximum 8 weeks. At this point, we should absorb the training and work either on threshold training, zone 4, or train extensively in zone 2, bringing on peripheral adaptations to support further VO2 max increase. You would likely want to do VO2max "maintenance" sessions in this time, once a week, maybe, without looking for the progression in order to avoid de-adaptation.

A number of research papers recently have addressed "VLAmax", the parallel for VO2max in the glycolytic (anaerobic) energy systems. There is some convincing suggestion that there may be some kind of interference effect between the two, such that you should avoid aggressive training of the glycolytic system if you want to maximise your aerobic power at any given time.

How do I know if the VO2max interval or session was effective?

The training effect we want to deliver is to make sure we are stretching the heart muscle by filling it full of blood and then forcing that blood out under pressure. But we do not commonly have access to field-based "systolic pressure" monitors during training, we commonly only have access to "heart rate" or beat frequency monitors, and the two are not completely aligned.

One example is that one may have a very high heart rate, while the heart is not actually under significant systolic pressure, because it's possible for the heart rate to be very high while it is not working hard, since heart rate lags.

Therefore, measures such as "percentage of time with heartrate over 90% max" may give us a false indication. What we want is the heart specifically to be working hard, and this must take precedence over a simple attainment of high heart rate.

Are "micro-intervals" effective?

Contrasting with the more traditional "long block" VO2 max intervals, there has been growing interest in recent times in work from researchers like Billat and Rønnestad, variously labeled under the umbrella of "micro-intervals," where the work is parceled out in very short, oscillatory blocks. This might take the form of 30-seconds at zone5 power, followed by 15seconds coasting, for 13 repetitions total. These collectively represent a single work block, which in turn can be done multiple times with longer intervening rests.

Research does indeed show these micro-intervals can elicit the same high heart rates as long-block VO2 work. However, we need to be cautious: much of that time apparently 'in-zone' is when the heart rate is elevated but the heart muscle is not working under systolic pressure. The jury is out, but my suspicion is that these are not as effective a protocol.

Anything to optimize VO2max sessions?

Well, yes, VO2max sessions can be optimised using "priming". This means doing a few, short efforts at high intensity-lets say 2x30s at VO2max power-prior to our working sets, as part of our workout. What we are trying to do here is to turn on the so-called "slow component" of VO2max in order to optimize aerobic contribution to the following work.

Second, be sure to have adequate active cooling, particularly while training indoors on Zwift or other virtual training platforms. If there is no cooling, your body may throttle back your effort to protect you. The goal here is not to sweat, nor is it an indicator of any effective workout.

Third, fueling is important. Like with all threshold or above threshold training, VO2max works places a high demand on glycogen as a fuel source. Because zone 5 intensity is so high, time in zone-and therefore total caloric burn-may be relatively short when compared to a longer threshold session, for example. Thus, it's unlikely your body's glycogen stores are depleted so long as you eat a reasonable amount of carbohydrates off the bike prior to your ride. On the other hand, having only a few sips of sugary drink can "fool" the brain into thinking glycogen fuelling is optimised and reduces RPE for session.

Finally, fatigue monitoring: training above threshold, as in zone 5, is very stressful. Generally speaking, one should perform high intensity riding no more than 2-3 times a week maximum. The remainder of your training volume should be comprised of recovery rides, rest days, or low-intensity endurance riding. Remember that you might reduce your capability for exercise by life stress. A depressed heart rate can often be an indication of too much fatigue, over training, or excess life event stress.

Fifth, and related, sleep is very important both to help absorb prior training, and also support optimal future training. Evidence seems to show that overall pattern of sleep in the week prior to a training session is more important than that of the immediately prior night alone - put simply, you can get by on one night's bad sleep, but not a string of them.

What kind of issues may I encounter while doing the VO2max workouts?

The most common issue besides simple fatigue or mental failure is "stitches" or abdominal pain. These you can somewhat avoid by ensuring you do not eat immediately prior to your VO2max session. Also, make sure you're posture is good - even if you're training for a TT, VO2max training can be optimized by sitting relatively upright when you ride, to open up the lungs and diaphragm. It may also help to focus on taking deep breaths where possible, rather than short, sharp intakes of breath.