Critical Pace and Power for Runners [Including CP Calculator]
Knowing your critical power or pace can be a useful way to determine and monitor your training intensity zones, track changes in fitness, and understand your individual strengths and limiters.
In this article, we’ll explain what is meant by critical pace or power, and how you can test and use it in training and racing. If you already understand how to test your critical pace/power, you can also link directly to our dedicated calculators here:
Why should I calculate my Critical Pace/Power?
Before we delve into the details of what critical power and critical pace actually are, let’s look in more detail at how they can be used. Testing your critical power and pace can provide a number of benefits. In particular you can use critical power and pace to:
Set training intensity zones. Knowing your critical power or pace gives you an anchor point to set your training zones, either based on running power or running pace. As we’ll discuss below, critical power/pace sits close to an important physiological ‘tipping point’ and essentially marks the boundary between efforts that are sustainable (e.g. for 30-mins or longer) and efforts that very quickly become unsustainable within a matter of minutes. For runners who do most of their training off-road or on hilly terrain, then setting training zones against your critical power is far more helpful than setting them by pace, and you can check out the zones we use here.
Understand strengths and limiters. When you test for critical power/pace, you also get an indication of your capacity to run above critical power/pace - essentially how much work you can do before you reach the point of exhaustion. Sometimes this is referred to as ‘anaerobic work capacity’ (though it depends very much on your aerobic fitness as well as your anaerobic abilities). Knowing your ‘anaerobic work capacity’ alongside your critical pace/power can give you an idea of the type of runner you are - are you a diesel engine who can keep running at a steady pace for hours and hours, or are you better at short races, and courses with punchy hill efforts?
Monitor progress. Testing your critical power repeatedly over a longer period of time (e.g. every 8-12 weeks) can tell you how your fitness is evolving. Is your critical power/pace improving , or is you capacity to run above this level getting stronger, for example?
Pace races or efforts. Finally, by knowing your critical power or pace, you can generate pacing estimates for any combination of distance, time, power or pace, provided the efforts are above your critical power/pace. For example, if you wanted to know the speed or power you could hold for an effort lasting 5-mins, you can calculate this. Our critical power and pace calculators linked above also include pacing calculators!
Ok, so let’s now get into the details of what critical power and pace actually are…
What is Critical Pace?
Critical pace (also known as critical speed or critical velocity) is based on the theory that, above a certain ‘critical’ running speed, there is a well defined relationship between the pace you can hold, and the length of time you can hold this for. We all know that if you sprint as hard as you can, you can only hold this speed for a short length of time. Whereas if you paced an effort at around an 8/10 level of difficulty, you could keep running at this speed for longer.
Lots of research has been done that shows that the relationship between your running speed or pace above your critical pace closely follows a well-defined hyperbolic curve as shown below:
In the graph above, there are two key ‘parameters’ (aka numbers) that define the shape of the curve. The first is the critical speed or pace, which is shown by the horizontal dashed line. The second is D’ (pronounced ‘D-prime’), which essentially defines the steepness/curvature of the red curve. We’ll explain each of these variables below.
Critical Pace
Critical pace (CP) is the speed that you’ll tend towards when running at a high intensity, as exercise duration is increased ‘indefinitely’. ‘Indefinitely’ is a mathematical construct, and not actually true in practice, which is why this critical pace model fails to hold at or below CP. In practice, people can typically only sustain CP for around 30-minutes (Vanhatalo et al., 2011).
D’
D’ (usually expressed in units of meters) gives an indication of your capacity to run above critical pace. The bigger this value, the more you will tend to excel in shorter, punchier events, whereas lower values are better suited to long-distance runners.
As mentioned above, D’ is sometimes referred to as the anaerobic work capacity, but it’s not simply an indication of your anaerobic abilities (i.e. ability to generate energy without oxygen). It also depends on physiological factors such as your VO2max, neuromuscular ability to recruit high-power muscle fibres, how much fuel is stored in the muscles (e.g. in terms of glycogen and phosphocreatine), and how well you’re able to clear fatiguing metabolites that are produced through anaerobic metabolism (Poole et al., 2016).
Top-level runners specialising in shorter endurance distances (e.g. 1500m to 5000m) might have a D’ of 150-200m or higher, whereas a relatively well-trained endurance athlete who focusses on marathon distances or longer might have a D’ more like 60-120m. A bigger D’ can indicate (i) a bigger VO2max and/or (ii) a bigger anaerobic capacity. So very good endurance runners who compete over long distances can still have a high D’ as a result of having a big VO2max, even if they aren’t anaerobically very strong. Women also have a lower D’ on average. Thus a high D’ value for a woman would generally be in the region of 130-170 (Kordi et al., 2019 and personal coaching data).
One big limitation of critical pace is that it doesn’t translate between different types of terrain, or gradient. It’s also affected by weather conditions - particularly wind. An alternative metric that you can consider therefore is critical power…
What is Critical Power?
First-up, let’s think briefly what we mean by running power, as not everyone is familiar with this concept. Power is the amount of energy produced (or work done) per second. In the context of running, it’s dependant on the amount of force you’re applying to the ground to propel you upwards and (more importantly) forwards with each stride, along with your stride rate.
Running power can now be estimated relatively accurately from various devices, such as Stryd foot pods, or certain models of Garmin, Polar or Coros running watch. Power is a really nice way to measure your running intensity, since it doesn’t matter what the gradient or wind direction is, your power will still be a reliable indicator of how hard you’re working in most instances. We write more about running with power here.
Critical power is based on the same theory that, above a certain ‘critical’ power, the maximum power you can hold follows a well-defined hyperbolic curve as shown:
Again, the curve can be defined by two parameters: the critical power and W’ (pronounced as ‘W prime’). We’ll take a look at each of these in turn:
Critical Power
Like critical pace, critical power is the power output that you’ll trend towards when running at a high intensity, as exercise duration is increased ‘indefinitely’. Again, people can typically only sustain CP for around 30-minutes (Vanhatalo et al., 2011).
W’
W’ (measured in kJ – i.e. units of energy) is the amount of work that can be done above critical power. Again, it depends on a range of physiological factors (as described above) and depends on both aerobic and anaerobic abilities (though it is often referred to as ‘anaerobic work capacity’).
W’ can vary widely. Endurance athletes will generally have a W’ that’s between around 9-15kJ for men, and between around 6-10kJ for women, although these values can be bigger for athletes with a very high VO2max. For more punchy running disciplines (e.g. short fell or cross-country races), higher W’ are better suited – such as between 15-18kJ for men, or 11-13kJ for women (Vanhatalo et al., 2011). For running disciplines that are very long and more steady-state, then lower W’ values are preferred.
Is there a relationship with Threshold Power/Pace?
Research (e.g. Hill et al., 1999) suggests that CP reflects the maximum intensity you can run at, while oxygen consumption remains at a stable level. At or below CP, your oxygen consumption will initially rise to match the work intensity, and then will stabilise at a level that’s below your VO2max (maximum possible rate of oxygen consumption).
However, when running above CP, oxygen consumption will never reach a steady state, and will continue trending up until it reaches VO2max, shortly after which, you will reach the point of exhaustion.
You might be familiar with the concept of the ‘lactate threshold’ or ‘anaerobic threshold’, which is the maximum pace or power you can hold while blood lactate levels remain stable. This is a very similar concept to critical power, but with a focus on blood lactate levels, rather than oxygen consumption. It’s been shown that CP occurs close to the lactate threshold (Poole et al., 2016). However, it should be recognised that it’s not a direct replacement, and CP will often come out a little higher than the lactate threshold.
Critical Power/Pace Equations
This section can be skipped if you wish, as our calculators perform all the maths behind the scenes! However, for those who are interested in the mathematical equations behind the critical power and pace estimates, these are as follows:
Power = W’/Time + Critical Power (1)
Here, power and critical power are measured in watts, time is in seconds, and W’ is in joules.
Pace = D’/Time + Critical Pace (2)
OR
Distance = Critical Pace * Time + D’ (3)
Here, pace and critical pace are measured in units of meters per second, time is in seconds, and D’ and distance are in meters. Equations (2) and (3) are just two different ways of writing the same equation.
It’s worth noting that there are also different versions of these equations available that model the relationship in slightly different ways, with varying levels of complexity. There is no accepted ‘gold standard’ approach (Maturana et al., 2018), but the equations above are the most widely used (and also some of the simplest).
How to test Critical Power/Pace
By performing a series of maximal efforts, each lasting somewhere between 3-mins and 20-mins, it’s possible to plot the relationship between critical power/pace (CP) and W’ or D’ and determine the values of these parameters.
You will need at least two maximal efforts as a minimum (Maturana et al., 2018; Simpson & Kordi., 2017). However, in our experience, using just two maximal efforts is highly sensitive to the specific test durations used, making repeat tests hard to compare. It can also result in fairly inaccurate estimates. So, we recommend using 3 maximal efforts to calculate CP accurately.
The efforts we’d recommend are:
3-mins
5-6-mins
12-mins
If you prefer to do your efforts over a fixed distance rather than time, we’d recommend:
1000m*
1500m*
3000m*
*If you are a fast runner (e.g. capable of running a sub 18-min 5km, or a sub 3-min 1000m, then you should extend these distances slightly). It’s important that all efforts sit between 3-mins and 20-mins.
Each maximal effort should be paced as consistently as possible. So, don’t start too hard, and then fade towards the end.
The tests should be performed on different days, with at least 24H rest between, so that your results aren’t impacted by fatigue. Completing multiple tests in the same day will skew your results, because the W’/D’ may not be fully ‘reconstituted’ (i.e. recovered) by the time you begin the second test effort.
Make sure you warm up well before each maximal effort. This is important because one of the assumptions of the critical power model is that the aerobic systems kicks in instantaneously to provide energy during each maximal effort. In practice this isn’t true, and there’s a lag between the effort starting, and the aerobic systems ramping up their energy contribution. However, this lag can be minimised by warming up well beforehand. Research looking at oxygen uptake after a warm-up (Jones et al., 2003) suggests a good warm-up should include:
a gradual increase in exercise intensity from around a 2/10 to a 4/10 effort level.
then some hard efforts above your expected critical power/pace, where you should notice your breathing rate increase to the point you can only speak single words. These efforts should only feel moderately hard, not all-out.
then finally a short period (e.g. 4-5 mins) of gentle running around a 2-3/10 effort level to allow lactate levels to reduce.
If testing critical pace, this will need to be done on the flat, on a hard-packed surface (e.g. tarmac or firm, non-technical trail). Ideally you should choose a day with minimal wind.
Simply record the results from you test efforts (i.e. the speed/power you held over each test duration) and input these results into one of the calculators below!
Testing Tips
Keep things consistent. Both our own experience and data from scientific studies shows that the length of the testing intervals can have a notable impact on your CP and W’/D’ estimates (Maturana et al., 2018). So, if you’re trying to track changes over time, try to keep the length of the test intervals consistent each time you test.
Be cautious when interpreting your results the first two times you determine your CP and W’/D’, as these results will likely be less reliable due to sub-optimal pacing. Research shows that results become more reliable once you’ve completed the testing protocol at least three times (Simpson & Kordi, 2017).
Potential Limitations
Finally, to wrap-up this article, we should consider whether there are any limitations to critical power and pace testing.
It’s worth mentioning firstly that like any field-based testing that demands maximal efforts to be produced, the results are highly sensitive to factors such as fatigue, motivation, nutrition and environmental conditions, which can influence performance on the day. There will ALWAYS be day-to-day differences in performance and your associated CP and W’/D’ values. Try your best to control these confounding factors as much as possible.
We’ve already mentioned above that the test results are sensitive to testing durations and will be less reliable if you’re inexperienced with these test durations. But again, this is true of other field-based testing methods, such as the various protocols for determining FTP.
References
Ferguson, C., Wilson, J., Birch, K. M., & Kemi, O. J. (2013). Application of the speed-duration relationship to normalize the intensity of high-intensity interval training. PloS one, 8(11), e76420.
Hill, D. W., & Ferguson, C. S. (1999). A physiological description of critical velocity. European journal of applied physiology and occupational physiology, 79, 290-293.
Jones, A. M., Koppo, K., & Burnley, M. (2003). Effects of prior exercise on metabolic and gas exchange responses to exercise. Sports Medicine, 33(13), 949-971.
Karsten, B., Petrigna, L., Klose, A., Bianco, A., Townsend, N., & Triska, C. (2020). Relationship Between the Critical Power Test and a 20-min Functional Threshold Power Test in Cycling. Frontiers in Physiology, 11.
Kordi, M., Menzies, C., & Galbraith, A. (2019). Comparison of critical speed and D′ derived from 2 or 3 maximal tests. International Journal of Sports Physiology and Performance, 14(5), 685-688.
Maturana, F. M., Fontana, F. Y., Pogliaghi, S., Passfield, L., & Murias, J. M. (2018). Critical power: How different protocols and models affect its determination. Journal of Science and Medicine in Sport, 21(7), 742-747.
Pettitt, R. W. (2016). Applying the critical speed concept to racing strategy and interval training prescription. International journal of sports physiology and performance, 11(7), 842-847. Chicago
Poole, D. C., Burnley, M., Vanhatalo, A., Rossiter, H. B., & Jones, A. M. (2016). Critical power: an important fatigue threshold in exercise physiology. Medicine and science in sports and exercise, 48(11), 2320.
Simpson, L., & Kordi, M. (2017). Comparison of Critical Power and W′ Derived From 2 or 3 Maximal Tests, International Journal of Sports Physiology and Performance, 12(6), 825-830.
Vanhatalo, A., Jones, A. M., & Burnley, M. (2011). Application of critical power in sport. International journal of sports physiology and performance, 6(1), 128-136.