Aerobic and Anaerobic- What You Need to Know

Aerobic and anaerobic are two words that many in the endurance coaching world including myself bound around on a daily basis, yet for the aspiring triathlete these can cause confusion at first.

The terms refer to how the body generates energy, imagine a six year old at sports day, belting across the school field towards the finishing line. When they finish their run they will likely be breathing heavily, exhausted from the 25M sprint they have just completed. When they move into secondary school and start running the 1500 on the track and cross country they soon realise something, if they want to run longer distances they have to slow down.

Once they run longer distances at a lower intensities they are not nearly as out of breath at the end of the effort. They may be exhausted and collapse in a heap with sore legs and no energy left, but their lungs will not burn in the same way as before, they will not be recovering from what is known as an oxygen debt.

The reason you experience an oxygen debt after short efforts is due to the body relying primarily on its anaerobic system heavily for short, hard efforts, this is where your body creates energy without oxygen. I won’t go into the science of how it works here, but what you need to know is that the anaerobic system can only function for around 2 minutes before the athlete accumulates a large oxygen debt and has to slow dramatically, this is our fight or flight reaction that allows us to escape from danger. Many predators in the animal kingdom rely on their anaerobic system heavily as they sprint after prey, if the gazelle manages to slip from the cheetahs grasp or zig zag enough to tire the cheetah, it can avoid becoming lunch as the cheetah has created an enormous oxygen debt it must recover from, akin to the six year old who has sprinted full pelt over a short distance and has nothing left at the end.

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A Cheetah relies on its strong muscles and high anaerobic prowess to hunt down its prey, but if it mistimes its sprint or the animal escapes, it is unlikely to make the kill (photo credit Federico Veronesi)

On the other side of the equation we have aerobic fitness, this is energy created using oxygen. This is much more efficient and is one of the leading reasons for our dominance as a species, where our prey relied predominantly on their anaerobic system to escape danger, we were able to keep them in sight and slowly run them into exhaustion as they were unable to hold the pace that we were over longer distances.

As triathletes we are focused almost entirely upon the aerobic system, as it is very rare that we will be putting the hammer down and become predominantly anaerobic when racing even a sprint distance triathlon as we will need time to recover from this effort. The exception to this is in draft legal triathlon where you may launch an attack off the front of the pack to try to bridge to the next group, which upon joining you will be able to sit in the wheels of for a minute or so while your body recovers from the oxygen debt.

This is the reason that so much triathlon training is done at an “all day” pace, to ensure we are building and strengthening our aerobic system and not our anaerobic system. The mistake that many athletes make is doing all of their training way too fast and making very little headway on the aerobic development side of things. You may be able to run a very quick 5K, but that doesn’t necessarily translate into a great marathon experience, I can vouch for that one personally!

This is where things get confusing, I am a fairly gifted anaerobic athlete, I can push myself harder and go deeper than many others over shorter periods, but tend to suffer over especially long efforts. Normally when I mention that I have a strong anaerobic system and that 5K is my best distance to an athlete a metaphorical finger is waved in my face. “Aha! But a 5K is over 2 minutes, so it’s not an anaerobic effort”. This is of course true, but what people don’t always realise is that your body is never generating energy on a 100% aerobic or anaerobic basis. If that were the case a 100M sprinter could run with his mouth gaffer taped shut and still hit the same time as his rivals.

Anaerobic energy is created in addition to the energy that is being generated aerobically, you are using anaerobically generated energy while reading this. It is only an incredibly tiny fraction of the energy being created (think several decimal places), but is it ticking over like a pilot light, ready to leap into action at a moment’s notice.

To illustrate this more clearly here is a graph created using WKO4 (more information here) that visualises the energy systems used by an athlete at different timeframes. The data is collated using the athlete’s best performances at the time periods listed on the X axis, with the maximum power than can sustain for that period on the Y axis. I use these graphs to help athletes gain a better understanding of their individual physiologies to help us understand where we need to focus our training effort.

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Today we want to focus on the green and the blue lines, the green line represents aerobic contribution, the blue line anaerobic. If we start to the left of the chart we can see that at 1 second there is very little contribution from the aerobic system as the body has not started increasing the rate at which it pumps oxygen to the muscles yet, but using glycosides the body can create energy within the muscles and get us moving immediately. As we look closer towards the 10 second mark the aerobic system is really starting to get up to speed now, additional oxygen has been absorbed from the lungs and is being pumped to the muscles to get them fired up.

For this athlete, it is at one minute 6 seconds that the crossover occurs, and the aerobic system takes over as the primary fuel source. The aerobic system has fuel, it can continue indefinitely for as long as it has fuel, the anaerobic system making a tiny contribution that can increase on hills or when accelerating hard.

Looking at the 20 minute data point, the anaerobic system is still contributing 10W of power, which is still a respectable amount, I’m sure if this athlete saw their FTP drop by 10W they would be mortified. Remember, this is looking at the athlete’s best 20 minute effort, not all 20 minute efforts use such a proportion of the anaerobic system.

Going back to the graph, it would look very different for a track sprinter compared to a time trialist (which this athlete is classified as). In a sprinter the anaerobic system would make a much greater contribution, it would continue for much longer before the intersection with the aerobic system as sprinters need to hold maximum power for as long as possible. Their aerobic system will be very weak comparatively and they would struggle to keep up on a gentle Sunday club run as a result.

So now we’ve gone through the science, let’s have a look at the takeaway points, and how a better understanding of the two energy systems can aid your training:

-There is no benefit to developing your anaerobic system for most triathletes. I know an extremely successful athlete who has raced at Kona, yet claims he can’t sprint for toffee (never seen him sprint so can’t confirm this). He doesn’t need to train or develop his anaerobic system, he’s happy to let it fall by the wayside almost entirely to focus entirely on his aerobic system. That’s not to say that he won’t start leaning on anaerobic pathways during some sessions (such as hill reps), but the goal of these sessions is to develop muscular force, not to increase anaerobic ability although this may come as a byproduct.

-You’re never completely aerobic or anaerobic, the body is always using both, even if in very small amounts. Your anaerobic threshold is where you start to produce energy primarily from the anaerobic pathway and should be avoided for the majority of your sessions

-Avoid using large amounts of anaerobic energy in your training. It feels good as it leaves you feeling more fatigued, and changes in your anaerobic system are faster to gain and easier to track than gains in your aerobic system (“I’m 5 seconds faster up that hill!”), but are of little use to the vast majority of triathletes when it comes to race day. I know I’ve certainly fallen foul of this one in the past.

-Many fitness tests require you to use large proportions of anaerobic energy, as triathletes we are not testing you for improvements in these areas, rather trying to assess your current weighting between aerobic/anaerobic energy sources. If an athlete puts out the same amount of watts over a set period as his previous best but the anaerobic contribution is lower then the previous test, this will result in an increase in FTP when uploaded to WKO4.

I hope this has given you a better understanding of the role that aerobic and anaerobic pathways play in endurance sport, leave any questions in the comments below and I’ll do my best to answer them.

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