At a training talk, a keen young athlete asked me what the best way to train Max VO2 was. He said he was told by his coach to “go and do a few 1000m reps at your best 3000m race pace, with about equal time jogging recovery in between hard efforts”. Somehow his coach had communicated that Max VO2, or maximal oxygen uptake, was still an aerobic exercise, but at a very high percentage of aerobic capacity.

I’m sorry to say that either the coach didn’t understand the basic physiology, or the athlete didn’t fully understand what was being said. They are not alone. There is a short answer to this one, with two correct answers.

However, to make things absolutely clear and cut through the confusion, we need to look at what really happens in the body when we are exercising.

As we start to jog, and break into a steady state of running, our heart rate rises from its resting pulse rate (zero intensity) to a higher constant pulse rate (or steady state) as the nervous system recruits more and more slow twitch muscle fibres and initially burns fats for energy production.

Energy production is achieved in the muscle cells by the rapid breakdown of fuels into molecules of high-energy ATP (Adenosine tri-phosphate), and this phosphate-rich molecule can be considered the currency of energy production at the cellular level. The first and vastly more accessible fuel is FAT. After a few minutes, a comfortable steady state is reached where the athlete can run continuously without running low on muscle fuel or running out of breath. If the intensity is easy enough for the athlete to hold a conversation, then he or she is probably exercising in the true aerobic zones, where the main fuel source is still fat.

As the pace steadily picks up more, a higher pulse rate and bigger stroke volume is required to supply the muscles with oxygen and fuel, until finally a steady pulse rate is reached where the amount of oxygen breathed in does not quite meet the exercise requirements. The body has now switched to burning glucose aerobically in the fast twitch IIA muscle fibres, and this in turn makes the lungs blow off more carbon dioxide than when burning fat aerobically in the Type I slow twitch muscle fibres.

FAT is by far the most effective fuel for energy production in the human; whereas the body is very limited in how much glucose it can store as long chains of glycogen in the liver or in the muscle, even the lowest body fat individual will have many grams of available body fats that can be metabolized at low exercise intensities in the mitochondria (microscopic aerobic energy production organelles) of the slow twitch muscle fibres.

Full aerobic breakdown or aerobic lipolysis of the typical 16-carbon human fatty acid palmitic acid can generate 130 ATP molecules in energy yield, whereas full aerobic breakdown of a 6-carbon glucose molecule will yield a net 36 ATP. Even more promisingly, for endurance athletes prepared to patiently train SLOWLY enough to get the required adaptations, over 90% of the body’s fatty acids are stored as triglycerides, which can deliver over three times more energy than ‘long-chain’ fats like palmitic acid.

Pretty soon, diminishing oxygen delivery to the muscle cells and increasing acid levels switch glucose breakdown (glycolysis) from a super-efficient ATP yield of 36 in the presence of oxygen, to cellular energy shutdown and a net ATP yield of only 2 ATP without oxygen (anaerobic glycolysis)

Note that a triglyceride (glycerol tri-palmitate) can deliver over three times the ATP energy yield of a regular single-chain fat.


These concepts are laid out more in the diagram above, where the biochemical pathways and net energy yields are shown :

The exercise intensity where the muscle metabolism switches from aerobic glycolysis to anaerobic glycolysis is known as the anaerobic threshold. Blood acid levels when measured are usually double those measured at rest. (4mmol/litre compared to 2mmol/litre)

The anaerobic threshold is characterized by huffing and puffing, and an inability to easily hold a conversation while exercising. It occurs where the rapidly ascending heart rate levels off to near its maximum, despite increasing effort. If one were to measure and graph the amount of carbon dioxide being exhaled against oxygen being inhaled, the CO2 levels would start rising at around the same time as the increased breathing rate, and if we measured and graphed acid levels they’d start climbing steeply at the same deflection point.

Even though it is mildly uncomfortable, this level of intensity can be maintained for up to an hour by a well-conditioned athlete, but technically, it’s ever-so-borderline anaerobic and if sustained will result in an increased level of acidosis in the body. The human body functions best physiologically in a mildly alkali state. For a good club athlete, anaerobic threshold could be reached at 15 km road race pace, and for a world-class elite racer it could be nearer half-marathon pace.

By the end of this hour or so, the athlete will not be able to keep up the high rate of work for much longer because his or her glycogen stores will be depleted, and the acidic bloodstream created by extended running in a mildly oxygen-depleted state will shut down the ability of the nervous system to fire off at the neuromuscular junction, where the muscle recruitment signal from the brain goes awry at the effector muscle synapse.

This next diagram shows how the various intensity zones relate to the anaerobic threshold and the far more intense anaerobic intensity of VO2Max.

Strictly speaking, our physiology starts to enter the anaerobic zones as soon as the switch from running comfortably to running with mild discomfort occurs, and this acid load increases steadily as we increase pace, until the acid load is so great it prevents normal muscle function and coordination, full-stop!

Everything functions better physiologically when the majority of exercise time is spent training at steady aerobic levels. Training at steady aerobic levels will train the body to preferentially use fats for fuel instead of stored glucose or glycogen, and keep the blood system nicely alkali.

There are two contributions to achieving VO2Max. One is the aerobic capacity, which takes years of patient low-intensity work to develop, and the other is the maximal anaerobic capacity or oxygen debt that can be incurred on top of that: this only takes about 4 to 6 weeks of judicious high-intensity anaerobic work to develop to its maximal level.Too much training of this system can turn an athlete’s form from excellent to poor overnight, and experience tells us that people who train the highest-intensity systems without a large and well-developed aerobic foundation to support that intensity will often get injured, or come down with mild infections  because the blood system is mildly acidic, which suppresses the immune system and all of the energy systems of the body.

There are therefore two distinct ways of increasing VO2 Max. By far the most sustainable and safe is the steady increase in our maximal aerobic capacity by steady training in the fat-burning aerobic zones. The one that appears to get the quickest and most spectacular increases in maximal performance is VO2 Max Interval Training, where short workbouts of up to 3 minutes in the maximal anaerobic zone of 100% VO2 Max, which is much more intense than the anaerobic threshold, are alternated with equal or shorter low-intensity aerobic recoveries.

However, experience reveals less improvement in absolute VO2 max with this more intense interval  training.

For some more excellent information on these concepts, follow this link:


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