Why Aerobic Threshold Is Important and How to Use It to Get Fitter
Perhaps the most effective thing you can do to improve your ability to run or cycle faster for longer distances, longer than 2 hours up to several days, is to improve your speed or power at your aerobic threshold. I thought it would be a good idea to explain why this is and how you can use the knowledge to get faster, so I wrote this article.
So, why is aerobic threshold important? Your aerobic threshold dictates how fast your can go for durations of more than around 2 hours. So if you improve the speed that you can go at your aerobic threshold, you will improve the speed that you can go for all events longer than 2 hours, particularly where you will be going at a reasonably steady level of effort like running a marathon or cycling a sportive, gravel race or ultra-endurance bike packing event.
Knowing a little more detail about what your aerobic threshold is and how you can improve it will help you be more focused with your workouts and perform to the best of your potential.
What is Aerobic Threshold (AeT)?
Your aerobic threshold is the point at which your body starts to use the anaerobic metabolism to provide energy to your muscles.
When you are going slowly, your body works completely aerobically, using oxygen to break down fatty acids to provide fuel to your muscles. As you run, cycle or swim faster the amount of energy that you produce in this way increases until it reaches a maximum, at your aerobic threshold.
As work harder, beyond your aerobic threshold, your body starts to use carbohydrates in the form of glycogen that is stored in your muscles and liver to help your aerobic system and fuel your muscles as you get faster and faster. Eventually, you get to a point known as your Anaerobic Threshold (AT), after which point you can only last a few minutes before you have to slow down or stop and recover.
It was thought that the body changed from aerobic to anaerobic energy sources at the Anaerobic Threshold but a paper by Seiler and Tønnessen, published in 2009 found that this isn’t the case and fuelling beyond the Anaerobic Threshold is still predominantly aerobic. You can read all about it in the paper, which is very interesting and has a lot of additional practical information: https://sportsci.org/2009/ss.htm.
It is the anaerobic threshold that is most commonly talked about and used as to determine training intensities. Terms such as Functional Threshold Power or Pace (FTP), Critical Power or Pace (CP) or threshold heart rate are common terms used to represent performance at Anaerobic Threshold but your performance at this higher intensity isn’t likely to be the most important determinant of how well you perform in longer events.
You don’t need to know the details of how blood lactate accumulates but because it is a common way of measuring the AeT and AT, it is useful to reference it. Reference to a paper by Andrew Jones, published in 2006, describing physiological testing of the then World Marathon Record Holder (Paula Radcliffe), is also a very convenient way of demonstrating the trainability of the Aerobic Threshold. You can read the paper here: https://www.semanticscholar.org/paper/The-Physiology-of-the-World-Record-Holder-for-the-Jones/925092caeba7349449504d9ce878c5ad5115c3f5.
To illustrate the improvements over time I have typed in the blood lactates, versus running speed for various annual measurements to create a coloured graph that clearly shows how the aerobic threshold was improved over years of training. The aerobic threshold is taken as the point at which blood lactate levels increase above resting levels, when he curves start to rise. You can see that with training, running speeds at all lactate levels improve but the marathon world best was run at aerobic threshold pace, therefore this is the most relevant metric for race performance. Here is the graph, which you should take as for illustration only and read the paper if you want the real details.
Comparing heart rate values from the same chart shows that both AeT and AT occurred at increasing heart rates, varying from 170 to 175 bpm, showing that there is some shift in heart rate at AeT with training, although for most practical purposes, using standardised heart rate based training zones will be more than adequate for all but the most elite performers.
How to find your Aerobic Threshold
It is difficult to accurately determine your Aerobic Threshold without measuring blood lactate accumulation or inspired and expired gas analysis, which usually requires a visit to a sports performance laboratory, or at least access to specialised equipment.
You can estimate your AeT by gradually increasing your exercise intensity until you noticeably increase your respiration rate, when you have to breathe a bit harder.
There are two ways to do this but the method is much the same. Spend 10 to 15 minutes warming up with some easy exercise and then start cycling or running slowly, either on a good flat surface like a road, or on a gradual uphill section that has a reasonably constant incline.
For the first method, breathe only through your nose and gradually increase your effort by going faster until you can’t sustain the effort without breathing through your mouth. Slow down a bit and adjust your speed until you can keep going and breathe through your nose, this is your aerobic threshold.
The second method is the same but instead of breathing through your nose you speak a story, recite the alphabet or something similar. Your aerobic threshold is the highest speed/heart rate at which you can continue to speak in phrases of around 30 seconds.
Basically, you are training below your aerobic threshold if you can hold a conversation, which is nice because you can make your longer runs and rides more sociable by going out with a friend or friends. Just be careful not to make it competitive and don’t get drawn into going too fast and compromising your training so that you become too tired.
If you train and record your data with a GPS, speed, heart rate or power monitor, it can be convenient to use these tests as a check and combine them with heart rate, pace, or power based training zones.
Most people have heard of training zones and many of us use them to determine how hard we are training so that we can work towards specific goals. Since there are only two measurable physiological markers for aerobic activity, Aerobic and Anaerobic Thresholds, albeit with some slight variations according to how they are measured, training zones that are based on these values make the most sense.
Since it is difficult to accurately determine the Aerobic Threshold without a lab test measuring blood lactate or gas analysis, training zones are often determined based on Anaerobic Threshold power, pace or heart rate with various percentages used to estimate the various zones. This method works well and because the most important factor for aerobic training is to stay below the AeT value and avoid the accumulation of blood lactate and associated onset of fatigue, these estimates provide a safe upper bound.
I like to use the Andy Coggan model for most of the athletes that I work with using either heart rate or power. For pace, you can use an alternative approach and verify the values using a heart rate monitor.
When working with power, for either cycling or running, I keep an eye on heart rate to make sure the zones are consistent with external work (power/pace) matching physiological response (heart rate) within reasonable margins.
Here are the Coggan Zones for power and heart rate and here is a link to how you can determine your thresholds to calculate them.
Training zones 1 and 2 are below the Aerobic Threshold, which can be safely estimated at around 83% of AT heart rate, as can be seen in the table.
How to improve
Mixing both ‘easy’ training, below AeT and harder training, above AeT is the best way to improve both AT and AeT and balancing this mix takes a bit of trial an error.
To be effective, training must be progressive and put your body under enough stress to make it change and adapt to the stresses imposed on it. However, if you impose too much stress, your body will break down and you will get slower, very tired and possibly very ill. For this reason, it is best to stay on the safe side and aim to be a little undertrained rather than overtrained.
Stephen Seiler, and now others, has done a lot of work on how the best endurance athletes in the world train and developed what has become known as Polarised Training. He found that top endurance athletes appear to do a very large amount of training below their aerobic threshold, a small amount of training above the anaerobic threshold with almost no training between the two thresholds. This is despite, some of them competing in the zones between the two thresholds, commonly known as the tempo zone.
In top level athletes, it was found that around 80% or more of training was below AeT.
Further studies showed that a similar format and proportion of above AT and below AeT training appears to be optimal for recreational level athletes as well as elite.
The video at this link https://vimeo.com/98353863 shows Stephen Seiler giving a really useful summary of much of his work.
This, combined with my personal experience both as an athlete and coach that has trained and worked with a wide range of runners and cyclists, leads me to believe that the polarised approach can be generally applicable.
Harder, Interval Sessions
The logic for the Polarised approach is that the higher level of fatigue incurred by training above AeT is similar for training in all zones above AeT but the gains are greater for training above AT. In Seiler’s research, it was found that doing efforts a little above AT was the most effective, particularly, 3 sessions per week of 4 x 8 minutes with 2 minutes recoveries. He compared these to 3 sessions per week of 4 x 4 minutes and 4 x 16 minutes, both also with 2 minutes recoveries. The 8 minutes efforts were shown to be significantly the best.
Further study showed that it is the intensity of effort and relatively shorter recoveries that appear to be important, so efforts as short as 30 seconds, with 15s recoveries can work, as well as longer efforts, up to the 8 minutes used in Seiler’s original studies.
You might find my article on using Frank Horwill’s 5 Pace System to workout your interval sessions useful. You can find it here: https://endurancebikeandrun.com/blog/2020/1/27/get-fit-for-cycling-using-the-5-pace-system. The article is about cycling but the method is just as applicable for running, for which it was originally derived, or any endurance sports.
Recovery and Training Slower than Aerobic Threshold
The benefit of training below AeT is that evidence suggests that it doesn’t significantly slow down recovery from harder, interval sessions and therefore you can pack in the miles, or hours, and further improve your speed at AeT without compromising the amount of harder work you can do.
Therefore, within reason, the more training you can do at a pace slower than your Aerobic Threshold, the fitter you will be. Obviously there is a limit and you need to make sure that you can still do the harder sessions hard because mediocre training leads to mediocre fitness.
Remember the simple breathing guides we talked about earlier. You should be able to hold a conversation during your aerobic training workouts.
Getting the mix right
The best approach is to plan your harder sessions and then fit your easier training around them. Fitting in easier days and rest days to make sure that you are properly recovered to get the most out of your harder sessions. There is plenty of evidence to show that higher intensity training is most effective when you are properly rested. You will be able to work harder and recover more effectively afterwards if you plan things properly.
In my experience, 3 hard interval sessions is a lot to cope with, both physically and mentally. I find that two really good sessions a week is as much as most people can manage, although some can do 3 and some people find 1 session to be enough.
You need to work out what is best for you and if doing 1 good hard session a week is enough for you, with some longer, slower training, then that is what you should do.
If you are starting out, it is a good idea to do a few weeks of just training below your Aerobic Threshold to give yourself a base of fitness to work with. Maybe 4 weeks of building up some volume until you are able to train for at least 40 minutes, 4 to 6 times per week.
Once you have build up some base fitness you can do a test to workout some training zones, or you can just go by how you feel. Working to feel is surprisingly accurate and if you are measuring heart rate and pace or power, you can gradually collect data to see how things work. Many training applications will estimate your threshold values and training zones for you, but it is best to do a test, once you feel confident to do so.
Start to include some harder training with one session of 2 x 8 minutes and see how that goes. You don’t need to worry too much about training zones for this type of interval training, just do them as hard as you can but aiming to do the efforts at a constant speed or power. If you use heart rate, you will need to allow a bit of time for your heart rate to rise, it won’t go up instantly, so you should be in your heart rate zone 4 when you get 3 or 4 minutes into the effort. If you feel okay after 2 efforts you can try a 3rd and then build up over each week.
Try 2 hard sessions each week at first and if you find that you are full of energy during your easier training sessions you may be able to manage 3 sessions a week. If not, stick with 2 for a while and see how it goes. I am confident that you will improve with 2 good hard sessions a week and some easier miles.
Remember, being a bit undertrained is far better than being overtrained and making yourself ill.
It is also a good idea to have a much easier week every 3 or 4 weeks to let yourself recover and reset before another 2 or 3 weeks of harder work. During this easier week you need to reduce the amount of training you are doing and make your harder sessions either shorter, perhaps just doing 1 harder session during your easier weeks. A good interval session for easier weeks is something with shorter efforts that keeps everything moving but doesn’t tire you out, something like 8 x 1 minute with 30s or 1 minute recoveries.
A weekly plan
I find it best to work with a weekly plan and if you work a conventional working week, you might want to schedule your longer workouts at the weekend, when you may have a bit more time.
If you do the same, or similar things on the same days each week, you can quickly workout how much to do to make sure you are properly rested and ready for your harder sessions and keep control of your training volume as you build up.
A good weekly plan can be to do your hardest sessions on Tuesdays or Wednesdays and Saturdays (making Saturday a bit longer and Tuesday a bit higher intensity). You can then fit in longer workouts around that and have easier or rest days on Mondays, Tuesdays and Fridays to make sure you are ready for your harder sessions.
A good weekly plan could be:
Monday: rest or something easy after the weekend;
Tuesday: 40 to 90 minutes below your AeT;
Wednesday: Hard interval session such as 3 or 4 x 8 minute with 2 minutes recoveries (remember to warm up properly and cool down afterwards;
Thursday: 1 to 3 hours below your AeT;
Friday: rest or something easy;
Saturday: Hard interval session such as 6 x 5 minutes with 1 minute recoveries or 3 sets of 15 x 30s hard/15s easy with 5 minutes between sets;
Sunday: Long workout of 90 minutes to 5 hours below your AeT.
Obviously you need to adjust things so that you build up to that if you aren’t used to doing so much training, take extra rest days when you need them and build up the number of efforts you do in the interval sessions. You can’t rush getting fit, your body needs to adapt and it you push too hard you will get slower and make yourself ill.
Also, remember it is a good idea to have an easier week every 3 or 4 weeks. Most people I work with, including professional full time athletes, find that 2 harder weeks and 1 easier week is about right but a few can manage 3 hard and 1 week easier.
Bear in mind you are aiming for perfect training that gets you fit, not just hard training. If you aim for hard training, all you are likely to do is make yourself tired and slow your progress or make yourself ill.
Watch your improvement
As you build your fitness week after week, month after month, you will notice that you are getting faster at your usual routes or segments during your easier rides, or if you are using pace or power zones, you will notice that your heart rate is getting lower for the same power or pace.
This is exactly what you want and you can be confident that you are doing the right thing….. and of course, if you are doing the right thing, keep doing it.
I hope you like what I have written and find it interesting and useful. Please let me know if I can do things better and/or if there are other things you would like to see. Of course, if you want to contact me directly, I would love that and if I can help either informally or formally please let me know.
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Is it worth having a lactate profile test to determine my thresholds? Unless you are particularly interested in the numbers, there isn’t a need to have a lactate profile test to accurately determine your Aerobic and Anaerobic Thresholds. Doing the tests described in this article, or other similar tests to determine your training levels or zones and then measuring your improvement will meet your needs. Even the laboratory tests have inaccuracies and are only valid on the day of the tests, so you need to commit to regular testing and continue to work with your measured training data to make them work effectively. Even the professional endurance athletes I work with no longer have blood lactate profiles regularly, we work with measured data and regular testing and various durations and intensities to plan training and keep things on track.
What if my Aerobic Threshold increases, do I need to increase the speed of my longer endurance workouts? Hopefully, your speed/power at your aerobic threshold will increase but you can still use the same training zones and guidelines, such as staying in zones 1 and 2, and making sure you are keeping to a pace you can hold a conversation. As long as you aren’t incurring excessive fatigue during your aerobic threshold workouts, you are building your aerobic fitness, so there is no need to go harder. After a while you will develop a feel for what is too hard and start to notice your breathing getting a bit harder when you are going too fast. Learn to pay attention to this and also to make sure you take it easier when you are tired. Sometimes you should be well below your AeT. Remember to aim for perfect training, which is sometimes easy training and sometimes hard training but always do the right training, be disciplined and you will see improvements.
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