When today's training is completed, and you breathlessly press “Save” on your , you are presented with a multitude of data. Data that can sometimes be more or less tricky to understand. Among other things, under the so-called “Training Effect,” you can see the ratio between “Aerobic” and “Anaerobic” – concepts that curious runners often encounter when reading about training. But what do these terms actually mean?
With and Without Oxygen
If you go to Wikipedia and type “aerobic,” you will read that the word derives from the Greek words “aer” meaning air and “bios” meaning life. It's about biological processes that can only occur when oxygen is present. Add an “an” in front, and the word now means processes that can occur without oxygen.
Aerobic = Biological processes requiring oxygen.
Anaerobic = Biological processes that can occur without oxygen.
And what does this mean for you?
Simply put, when you run at low intensity (slowly), your body primarily receives energy from AEROBIC processes – and when the intensity is high (you run very fast), the energy primarily comes from ANAEROBIC processes.
So what Garmin tries to tell you with this division into Aerobic and Anaerobic is where your body has sourced the energy from, and thus what benefit or training effect, you've gotten from the day's run.
This is explained further in the following sections.
Body's Energy
To run, you need energy. The muscles that swing your leg forward, absorb impact when landing, push your body forward when you accelerate, and stabilize the torso throughout the movement all require energy. In the body, this energy exists as a molecule named ATP (aka adenosine triphosphate).
While ATP is the body's fuel, it would be smart to store a lot of it in the body. However, ATP is a very heavy molecule. In fact, it's so heavy that an average adult lying in bed all day would need to store 65 kg of it just to survive. A 10 km run would thus quickly require up to 36 kg of ATP. A bit impractical to carry around when running.
The body's solution is to constantly generate new ATP. This is done from stored carbohydrates and fat, and it's here that the terms ‘aerobic’ and ‘anaerobic’ come into play again. While fat-based energy production can only happen when there's plenty of oxygen, carbohydrate-based energy production can occur both with and without oxygen.
A Matter of Intensity
To know which energy system is most active, one must look at the intensity of the activity.
One of the key differences between the aerobic and anaerobic energy systems is the speed at which energy can be produced. The anaerobic system provides energy significantly faster, and will therefore supply a larger portion of energy when the intensity is higher.
If you sprint 100 meters as fast as you can, almost all energy will come from anaerobic processes, while aerobic processes handle almost all the work when jogging a short recovery run or running long distances.
The diagram above shows the relationship between intensity and energy system. When the work is very short and intense – 5 – 60 seconds – the anaerobic system delivers most of the energy.
At around 2 minutes of work, which for fast runners corresponds roughly to an 800 m run, the energy delivery is approximately 50/50 between the two systems.
Everything beyond that will be primarily aerobic.
The Difference is Noticeable
Since the anaerobic system operates much faster than the aerobic one, one might think it's best to rely on it all the time. However, it doesn't come without drawbacks. Drawbacks that can be felt.
The anaerobic system is a less “clean” combustion than the aerobic one. When the anaerobic system is running at full speed, a lot of waste products are produced, the most well-known being lactic acid (/lactate). These waste products create an acidic environment in the body (decreasing pH), causing heavy breathing, hyperventilation, and the familiar feeling of “hitting the wall.”
Intense anaerobic work is therefore not very comfortable and cannot be maintained for long.
The Various Thresholds
In daily training, you often hear about different “thresholds.” Terms like: The aerobic threshold, the anaerobic threshold, AT, Functional Threshold Power (FTP), Critical Power, lactate threshold, ventilatory threshold, and many more are used interchangeably, leading to confusion about what these terms mean.
Physiologically, it only makes sense to talk about two thresholds: The aerobic threshold (AeT) and the anaerobic threshold (AT). These thresholds are best measured by examining how heavily one breathes during physical work with increasing intensity.
The aerobic threshold is reached when aerobic processes alone cannot supply enough energy. The anaerobic system steps in to help supply a portion of the energy. This results in a slight increase in waste products, but not so much that the level can't be maintained. Breathing intensity increases slightly, but you can still hold a strained conversation.
If pace and intensity increase further, you will eventually reach the anaerobic threshold (AT). Beyond this threshold, waste products accumulate faster than they can be cleared, resulting in a rapid buildup. Breathing increases further, hyperventilation begins, and you can no longer speak in full sentences. Continuing at this pace will eventually cause you to hit the wall and stop.
So What is Garmin Trying to Say?
When your , as shown in the image at the start, reads “Aerobic = 4.8” and “Anaerobic = 2.3,” it's trying to indicate how much you've stressed these two energy systems in the activity you've just completed.
It uses the aforementioned thresholds, which they define slightly differently, combined with heart rate data, to estimate how aerobic and anaerobic your run has been.
A steady long run, where the heart rate never goes above what has defined as the lactate threshold (the anaerobic threshold), will thus primarily show as aerobic stress on Garmin. The same goes for threshold intervals, where you run intervals at a pace just below the anaerobic threshold.
If you run classic VO2max intervals, where the intensity is higher, and you approach the anaerobic threshold, Garmin will show it as both aerobic and anaerobic stress.
If, for example, you run short and very high-intensity hill sprints, but otherwise run the rest of the session calmly, Garmin will primarily show it as anaerobic stress. The same applies if you run 400 m intervals at very high speed. Below is a chart with examples of different runs and how Garmin would assess the training effect of each run.
Your Garmin Watch Doesn't Know Everything
Data from your running watch should always be taken with a grain of salt. Even if you have the most advanced and expensive new watch, it can only measure your heart rate. These heart rate data are compared against standard values from a large number of people, but there's no real knowledge if it matches your own physiology. Your Garmin doesn't measure your lactate threshold (anaerobic threshold); it makes an educated guess.
To a large extent, the good rule of thumb about being able to hold an effortless conversation during easy training, a strained conversation during moderate training, and not being able to speak in full sentences during hard training works just as well. These three steps indeed mark the transition between the aerobic and anaerobic thresholds.
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