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Chasing Rainbows


I often get asked by athletes ‘What is my threshold power’? I also get athletes telling me ‘I think my threshold power is…’

In this blog I want to look at why your threshold power is not a fixed number and how both coaches and athletes need to embrace that.

First, let’s take a step back and understand what a ‘threshold’ is. A threshold, in the physiological sense, defines exercise intensity domains. Exercise intensity domains are simply intensities at which we can expect a certain physiological response.

4 exercise intensity domains:


The way your body responds in any given exercise intensity domain is both predictable and repeatable. When you go into the lab or perform a critical power test you are trying to test at what power the different thresholds occur and that allows your coach to define your exercise intensity domains – your training zones are based on these domains.

Moderate – this is an intensity at which blood lactate levels are not raised above baseline values

Aerobic threshold

Heavy – this is an intensity at which blood lactate levels are elevated but stable and you are breathing out more carbon dioxide compared to how much oxygen you are breathing in compared with the moderate domain

Critical Power

Severe – this is an intensity at which blood lactate levels are elevated, and will keep going up and up. Additionally, breathing rate increases and will keep increasing as you continue to exercise - as does oxygen usage! If you carry on exercising in this domain you will eventually reach VO2 max and then a little while later will have to stop!

MAP – maximal aerobic power. The highest power at which VO2 max is still achieved – this differs a lot from individual to individual but fitter athletes tend to be able to achieve VO2 max at higher power outputs

Extreme – this domains is comprised of very high intensities which you have to stop, due to fatigue, before you reach VO2 max – these are typically sprint efforts – for example a 30s sprint where at the end you aren’t breathing that hard but simply can’t sustain the power any longer!

So far so simple…

Now (and please stick with me…) let’s consider rainbows…Rainbows are made up of 7 distinct colours (red, orange, yellow, green, blue, indigo, violet). When we look at a rainbow, we can quite easily pick out these individual colours. However, what is a far more difficult is to pick the exact point at which one colour stops and the next one starts. Rather than clear bands what we actually see is a spectrum of light – one colour merges into the next.



Likewise, we can quite easily determine if an athlete is exercising in the moderate, heavy, severe or extreme domain when they are exercising bang in the middle of that intensity range. However, at the boundaries between domains things (by which I mean the physiological responses) get blurred. It is difficult to determine the exact wattage at which an athlete will move from one domain to the next. Therefore, rather than a fixed point at which one domain stops and the next one starts we instead see a spectrum of exercise intensities and responses.

There are a few reasons for this…

Threshold tests aren’t that great – when you do a threshold test and your coach tells us, ‘your new threshold is 350w’ that doesn’t mean that at 349w you will be in the heavy domain and at 351w you will be in the severe domain. What people often forget about is that thresholds tests only provide an estimation of your threshold – NOT an exact figure. As with any estimation, is it subject to a certain degree of accuracy. Some tests are better than others, but no matter how good the test is it will still never be 100% accurate. For starters, your power meter is only +/- 2% accurate so even a perfect threshold test is subject to at least +/- 2% variation. Add in a bit more for imperfect pacing, motivation levels etc and even the very best test is likely to only be +/- 5% accurate. So, when your coach tells you that your threshold is 350w what that means in reality is that your threshold is somewhere between 333w and 368w!

Fatigue – Let’s say you ride just below threshold (in the heavy domain) – what happens as you fatigue? Well, what happens is that, as you fatigue, your threshold will drop (how quickly and by how much it drops is a key determinant performance) but if you were to maintain the same power output then suddenly that power output that was in the heavy domain and therefore sustainable will, as your threshold drops, move into the severe domain and no longer be sustainable!


Good day or bad day? – we all know those days when you have great legs, and everything feels easy. We have also all (unfortunately) had those days where everything feels hard. Your threshold power will also vary on good and bad days. During periods of hard training when you are under a lot of training stress your threshold will likely be suppressed and therefore a little lower. However, on race day, when you're fresh after a good taper, your threshold might be a 1-2% higher.

Stress – stress has been shown to influence performance, training stress can make you tired but so can mental stress. Both these factors influence performance. In this category I would also consider other elements such as mental fatigue which can influence performance in the severe domain

Nutrition – what you eat can influence which fuel (carbohydrate or fat) your body uses at a given intensity. This can influence the physiological response. For example, ingesting fructose will raise blood lactate values for a given intensity – if blood lactate responses are used to determine threshold power then what you have eaten in the last 3hrs will influence the estimate.

Embrace the complexity

Does all this make thresholds useless? Of course not. A good estimate will still allow you to track your progress and adjust your training zones as you progress. It can even help with effective pacing and predicting performance. However, what coaches and athletes need to embrace is that there is an inherent variability in any threshold estimate and that when we look at the response to exercise we are looking at a spectrum rather than clear cut distinct exercise intensity domains! What coaches and athletes need to do is simply understand that a threshold, like the end of the rainbow, is a moving target. Power in a ‘threshold’ session might sometimes come out a little higher, or lower, than the threshold you have set in your online training platform. That doesn’t mean necessarily means that session was either amazing or terrible. Likewise, efforts done at the end of a hard session or during a hard block of training might be a bit down on power.

A real change?

One final note here – I often see athletes and coaches posting that a threshold has improved by a few watts! If we can only predict a threshold to +/- 5% accuracy how can we confidently say that a threshold has improved by 5w? We can’t! What we can do is shift the +/- threshold estimate up by 5w! So instead of critical power being between 333 and 368w we CAN say that threshold is now between 337 and 373w!


Remember the Rainbow


Exercise domains are like the rainbow! It’s easy to pin point a certain colour but not the exact point where one colour changes into the next! We can identify a specific domain but we can only estimate where the threshold between domains actually is! Likewise, just like the end of the rainbow, a threshold is a moving target - do enough work and your threshold will not be where it once was!

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