VO₂ max: The Limiting Factors of Maximum Oxygen Uptake

VO₂ max, commonly known as maximum oxygen uptake, is defined as the highest rate that oxygen can be inspired and utilized in the body during severe exercise at sea level. Consequently, VO₂ max represents the maximal rate of aerobic respiration in the mitochondria and is a measure of an individual's maximal capacity to work aerobically. When exercising at work rates above VO₂ max the energy for this additional work is met entirely through anaerobic metabolism i.e. anaerobic glycolysis. Its current concept originated with the work of Hill et al. in 1923, hypothesising that¹:

•           An upper limit to oxygen uptake exists.
•           There are inter individual differences in VO₂ max.
•           A high VO₂ max is a prerequisite for success in middle- and long-distance running.
•           VO₂ max is limited by the ability of the cardiorespiratory system to transport O₂.

Although there are various inherent physiological limitations to VO₂ max, which will be discussed in turn later in this article, there are other mediums by which VO₂ is also influenced by. The first of which is the fitness level of the individual in question, typically the highest VO₂ max values are seen in athletes which participant in whole body endurance activities such cross-country skiing or long distance running. Another consideration is age, after the age of 25 VO₂ max tends to decline by 1% each year with muscle sarcopenia (the degenerative loss of skeletal muscle mass) accounting for much of the major decrease in values observed at old age. Lastly, gender contributes to a great deal of variation seen in VO₂ max values. Males possess greater amounts of muscle mass along with a larger heart and lungs, all of which allow for superior oxygen transportation. On the other hand, women vary in body composition by increased fat percentage, less muscle mass and smaller heart/lungs.

Perhaps the principal rule to consider when discussing the maximum oxygen uptake is the Fick equation, which is as follows:  

·         VO₂ = CO x a-v O₂ difference
So, VO₂ max = Maximal CO x max a-v O₂ difference.

In this equation, CO represents the cardiac output – in other words central oxygen delivery. On the other hand, a-v O₂ difference relates to the peripheral oxygen utilization - the difference in the oxygen content of the blood between the arterial blood and the venous blood.  Ultimately, maximal utilization of oxygen at the tissues requires an effective O₂ transport cascade and it is the limitations of this pathway which will determine VO₂ max:

Air    -    Alveolar    -    Arterial    -    Capillary    -    Myoglobin    -    Mitochondria.

The first of the limiting physiological factors to maximum oxygen uptake is the pulmonary system, this includes the lungs and the muscles of breathing and is responsible for the delivery of oxygen/removal of carbon dioxide from the blood. Surprisingly, with regards to this system it has been seen in highly trained athletes that arterial O₂ desaturation during maximal work occurs². Although trained individuals have a far superior maximal cardiac output than their sedentary counterparts, this leads to decreased transit time of the red blood cell in the pulmonary capillary. Consequently, there is not sufficient time to saturate the blood with oxygen before it exits the pulmonary capillary, thus significantly reducing the potential quantity of oxygen available to the working skeletal muscle.

In the field of sports science we know that the normal range of VO₂ max values among sedentary and untrained individuals is mainly due to variation in maximal stroke volume, since considerably less variation exists in maximum heart rate and systemic oxygen extraction. Throughout maximal exercise almost all of the available oxygen is extracted from the blood that circulates the active muscles³, the approximate oxygen content of arterial blood is 200mL O₂∙L⁻¹ and in venous blood draining maximally this falls to about 20-30mL O₂∙L⁻¹. Showing there is little oxygen remaining for extraction during severe exercise. As a result the principal method for increasing the VO₂ max with relevant training must be an increase in blood flow, it is estimated that 70-85% of maximum oxygen uptake is linked to maximal cardiac output.⁴
A variety of longitudinal studies have displayed the notion that a training induced VO₂ max increase results from a greater cardiac output, as opposed to a widening of the a-vO₂ difference.⁵  

Within the active skeletal muscle fibres, the mitochondria act as the site where oxygen is consumed in the final step of the electron transport chain during aerobic respiration. It would be reasonable to suggest that an increase in the number of mitochondria would result in a higher maximum oxygen uptake. However, one study observed a modest 20-40% increase in VO₂ max despite a 2.2 fold increase in mitochondrial enzymes⁶. This finding is constant with the view that maximal oxygen uptake during a bout of whole body exercise is limited by oxygen delivery, not it’s utilization at the skeletal muscle.  

1985 saw the definitive research experiment showing that maximal oxygen uptake is limited by oxygen delivery due to restricted blood flow. Saltin et al⁷ observed the effects of maximal exercise using only a small amount of muscle mass, allowing a great amount of cardiac output to be directed onto a concentrated area. Under such conditions the measure oxygen uptake at the quadriceps was 2-3 times than in the same area during a whole body maximal bout of activity. It was therefore concluded that skeletal muscle has a remarkable capacity for blood flow and thus VO₂, however this simply cannot be matched by maximal cardiac output of the heart during whole body exercise. Proving that VO₂ max is bound by the delivery of oxygen and not by the ability of the mitochondria to utilize oxygen.

From the wide range of studies and research discussed throughout this article, it is clear that each step in the pathway of oxygen transport contributes to the determination of VO₂ max. A reduction in this transportation network will predictably result in a lower maximal oxygen uptake. The evidence also demonstrates that it is primarily the ability of the cardiorespiratory system to transport oxygen to the skeletal muscle and not the muscle mitochondria’s ability to consume it that limits and athlete’s VO₂ max value.

References

1.  Hill, D.K. and Lupton, H. (1923) Muscular exercise, lactic acid and the supply and utilisation of oxygen. Quarterly Journal of Medicine. 16: 135-171.
2.    Dempsey, J.A., Hanson, P.G., and Henderson, K.S. (1984). Exercise-induced arterial hypoxaemia in healthy human subjects at sea level. Journal of Physiology. 355: 161-175
3.   Shephard, R. (1977) Endurance Fitness. Toronto and Buffalo: Univ. of Toronto Press. 2:64–103.
4.  Cerretelli P, Di Prampero PE. (1987). Gas exchange in exercise. Handbook of Physiology. The Respiratory System. 4: 297–339.
5.   Ekblom, B., Åstrand, P.O., Saltin, B., Stenburg, J., and Wallstrom, B. (1968) Effect of training on circulatory response to exercise. Journal of Applied Physiology. 24:518–528
6.  Saltin, B., Henriksson, J., Nygaard, E. and Andersen, P. (1977) Fiber types and metabolic potentials of skeletal muscles in sedentary man and endurance runners. Annals of the New York Academy of Sciences. 301:3–29.
7.  Saltin, B. (1985) Hemodynamic adaptations to exercise. American Journal of Cardiology. 55:42-47

Exercise Euphoria: The runner’s high

Many avid runners will already know what I mean by the “Runner’s High” and the feelings that it provokes, often it is completely unanticipated yet feels as though it’s the most natural feeling in the world. Frequently when on longer training runs that are up to six to ten miles in length, I have slipped into the running high, finding myself in another world for between one and two miles. The run seems effortless, I’m almost gliding with every pace and a sense of fulfilment encapsulates me. One definition from Sach and Berger states the runner’s high is a “euphoric sensation experienced during running, usually unexpected, in which the runner feels a heightened sense of wellbeing, enhanced appreciation of nature and a transcendence of time and space”.

It has been reported that among runners who have previously experienced the high up to 30% of those encounter it on their daily runs, claiming to feel a sense of mental awareness, liberation, exhilaration and pain suppression. Additionally, in an interview of 60 runners it was revealed that the high brought about by running cannot be reliably predicted but can be facilitated by the absence of distraction and cool weather conditions. Runs should be ≥6 miles in length at a comfortable pace and it is also vital there is no concern with regards to timing or pacing.

Several theories exist regarding how the running high is brought about with perhaps the most famous being that of endorphin release, the body’s natural painkiller. However this theory has several problems, fundamentally endorphins are simply too large to pass over the blood-brain barrier. Consequently, although endorphin concentrations do increase within the circulation during exercise, without reaching the brain they cannot be held accountable for the high exercisers experience. Another key hypothesis is that of the opioid system, opioids are psychoactive chemicals that resemble morphine in their pharmaceutical effects. Research has revealed that release of endogenous opioids occurs following prolonged exercise and that this release is closely correlated with perceived euphoria among runners. However, it is also known that the opioid system is accountable for responses such as respiratory depression and other effects that are detrimental to running performance.

Perhaps the most feasible alternative to the endorphin theory is the ‘endocannabinoid hypothesis’’. Cannabinoids, an active ingredient found in marijuana binds with the nervous system to reduce pain and aniexty, producing a profound sense of wellbeing. Our body has the ability to create its own cannabinoids (endocannabinoids), these are composed of lipid molecules small enough to pass over the blood-brain barrier to provoke an affect in the brain. Research findings have shown that exercise increases the concentrations of these endocannabinoids, producing psychological effects closely resembling those associated with the runner’s high. The endocannabinoid systems activation is also thought to elicit a reduction in attentional span, time estimation difficulties, memory impairment and a sense of wellbeing. All of which characteristics are often included in the reported psychological profiles of long distance runners. These findings may be as a consequence of decreased metabolism in the prefrontal regions of the brain with increased endocannabinoid concentrations, whilst also demonstrating disadvantageous affects to cognitive functioning.

As yet there is no reference to a cyclist’s or swimmer’s high, it is likely this is due to endocannabinoid receptors residing in the skin and so as runner’s make contact with ground endocannabinoid release is stimulated. Another key point worth noting is that low level skills such as running are highly controlled by the basal ganglia which are responsible for cognition and habitual behaviours. The net result of this is that they more readily activate the endocannabinoid system than high skilled activities such as hockey or basketball.

Furthermore, endocannabinoids interaction with the neurotransmitter dopamine suggest that they play a role in the brain’s rewarding system, possibly contributing to exercise/running addiction. This many result in detrimental health affects among athletes who continue to train despite a chronic overuse injury. Lastly, it has be observed that the endocannabinoid system also attributes peripheral effects including bronchodilation and vasodilation. Such physiological changes can facilitate endurance performance by allowing for more efficient oxygen transportation, thereby promoting feelings of ease and effortlessness.

This article intended to provide an overview of the “Runner’s High” phenomenon, there is still much room for further research however it is clear that endurance runners frequently encounter many of the mood components mentioned whilst training. The most promising theory is that of endocannabinoids, providing both a physiological and psychological explanation for the exercise high. Although these feelings are subject to great individual variation and it is still unclear how age, sex and exercise intensity can affect the feeling of exercise euphoria.

"I always loved running... it was something you could do by yourself, and under your own power. You could go in any direction, fast or slow as you wanted, fighting the wind if you felt like it, seeking out new sights just on the strength of your feet and the courage of your lungs." - Paula Radcliffe

The Fat Burning Zone: Fact or Myth?

It's that time of year again, for many the festive bulge is protruding over their favourite pair of jeans, New Year gym deals have been snapped up and already several fad diets have been tried and tested.

A common misconception for exercisers aiming to decrease body fat is to jump onto an aerobic gym machine such as a treadmill or cross trainer and begin exercising within the 'Fat Burning Zone' of 50 - 60% VO₂ max that is so frequently advertised. However, could training at such an intensity actually be doing exercise participants more harm than good? This article will discuss key factors to consider when training to reduce body fat and banish the notion of the elusive 'Fat Burning Zone'.

One statement that is true is that as exercise intensity increases we metabolise greater amounts of our carbohydrate (CHO) reserves as oppose to fat. This is due to fat requiring approximately 15% more oxygen in order for its utilization, as we train more intensely this oxygen cannot be taken in, diffused into the blood and transported to the performing  muscles rapidly enough for its effective use. We are able to determine the percentage values of carbohydrate and fat metabolism through our respiratory exchange ratio (RER), this is the ratio between the amount of carbon dioxide (CO₂) and oxygen (O₂) consumed in one breath. When this RER value is low (0.75), optimal fat utilisation is  achieved, as this value increases to 1.0 predominantly more carbohydrate is being used as an exercise fuel.  There is one problem with this, when at rest our RER is at its lowest and so predominantly fat is being burnt, implying our optimal fat burning zone is achieved when sat in front of the TV or asleep. Awesome!

However, when at rest our total calorific expenditure is very low and so in truth we are only burning a very tiny amount of fat. The data below is for two 30 minute exercise periods, one at a low aerobic intensity (50% VO₂ max) and the other at a moderate aerobic intensity (70% VO₂ max).

	50% VO₂ max	70% VO₂ max
RER	0.84	0.88
Fat : CHO	50 : 50	40 : 60
CHO Kcal	73 Kcal	124 Kcal
Fat Kcal	73 Kcal	82 Kcal
Total Kcal	146 Kcal	206 Kcal

The data shows that at a moderate exercise intensity the percentage of fat utilized is lower, but although less fat is metabolised as a percentage, the absolute value is greater as the total calorific expenditure is increased and so consequently more fat is broken down as the exercise fuel. Not only does this moderate intensity of aerobic training allow a greater number of total fat calories to be burnt but it also gains physiological adaptations to optimize the substrates utilization in following training sessions. Capillary and mitochondrial density within the muscles will be elevated, respiratory muscles strengthened and oxidative enzymes increased. All of which optimise aerobic endurance and fat oxidisation during physical activity by improving the transportation and subsequent utilization of oxygen.

Consequently,  in order to reduce body fat it is of greater benefit to achieve a larger total calorific expenditure through exercising at moderate intensities between 70 - 80% VO₂ max. This does not mean to say that training above this level does not gain these mentioned muscular adaptations, however the duration of such an intense workout is frequently too short to gain the optimal level of physiological change.  Not only will this equate to more fat calories being burnt as an absolute value but it also ensures appropriate adaptations are made to optimise fat utilisation in future training sessions.