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What's so critical about critical power?

The theory behind the Critical Power

The Critical Power (CP) concept was originally introduced in the 1960s by Monod and Scherrer1 in an attempt to improve the understanding of the work capacity of muscle groups. They noted a relationship between the amount of work that could be done and the time you could hold that work for. As any athlete will appreciate, at high intensities, you can only hold an effort for short periods of time; reduce the intensity, and you can sustain it for a lot longer. The nature of this relationship is explained by Helen in the following two videos:

 

The truth!

Whilst the mathematics pinpoints a critical intensity, in practice, CP appears to be too high to be sustainable. Typically, it sits at about 85 to 90% of VO2max, and it can be maintained for 20 to 30 minutes3. In fact, exercising at CP induces a lot of physiological changes such as an increasing blood lactate, heart rate and an oxygen uptake that rises towards maximum – nothing like the steady state it claims to be.

Why is this? A lot of research interest has been generated by this very question. In short, the Critical Power you get from the calculations depends on the protocol you use: factors such as trial length / intensity used to determine the shape of the curve, the number of trials used, whether you perform all trials in the same day or on different days, and even the type of maths you use to fit through the points.

So, how IS the CP best determined?

Researchers still cannot agree in which way the CP should be computed, which makes it difficult to interpret the findings with any certainty. But, generally we would recommend:

•    At least 3 trials should be used

•    Select intensities that will allow the athlete time to achieve VO2max / maximal heart rate and to exhaust within 3 to 15 minutes

•    Can be measured in one day as long as adequate rest is given between trials (3 to 4 hours)

Some sport scientists suggest the curve fitting can be enhanced using a measure of peak power too.

Why use the Critical Power?

If you’ve read our factsheet on ‘demystifying the lactate threshold’ you will be aware of the wide choice of ‘threshold’ phenomena there are out there to help you with guiding training and measuring fitness. Why do sport scientists want to offer you another one? Well, yes, the CP does fall in a similar area to that of the ‘second threshold’ group of parameters (such as MLSS, OBLA). The key difference is that CP is a lot more user friendly to measure. Firstly, all you need is the ability to measure your work (whether that

be a power measuring device, or a distance covered) and a stop watch - oh, and don’t forget the full bag of motivation! Compare this to all the sophisticated kit needed to analyse blood and respiratory responses. Also, to the competitive athlete, performing the trials to calculate CP are a lot more akin to racing – we call that ‘ecological validity’ i.e. the test actually measures what it says it does in the most valid of conditions.

You may find having an estimate of CP useful not only for estimating your performance ability and for tracking fitness across a season, but also because it might be a useful training intensity too. Research performed at the University of Brighton over the last 3 years has looked at exactly this issue4. It appears that training at and around CP improves fitness more effectively than training at the first lactate threshold, even when total work done is kept constant (so you have to train for less time at CP). Establishing the CP allows the coach to tailor training to your individual profile.

How can you estimate your CP?

Obviously, the laboratory remains the best way to assess physiological function in the most controlled manner. However, one of the benefits with the recent accessibility of power measuring devices in cycling is that it allows the rider and coach to perform fitness testing in the same environment as race performances. And, whilst field tests such as timed hill climbs or establishing maximum heart rates have been utilised for some time, the advantage of estimating the CP probably has more application to performance: given the intensity at which the CP falls and its relationship to race intensity. Follow this method to estimate your Critical Power:

•    Plan a day where you can set aside a session to measure your fitness – treat this day like a race day i.e. be well rested, hydrated, well fed etc

•    Establishing your CP can be incorporated into a steady ride of about 1.5 hours.

•    Select a route where you know you can ride for 12 minutes without interruption from traffic, junctions etc. It doesn’t have to be flat, but it will allow a more accurate representation of your power output / time relationship.

•    After a good warm-up, perform a series of blocks where the aim is to achieve as much work (i.e. go as fast as you can!) for that time period: a 3 minute, a 7 minute, and a 12 minute block.

•    It doesn’t matter the order you do them in: in fact, you may wish to swap them around if you do this test regularly.

•    Make sure you give yourself 15 to 20 minutes of easy riding in between each block. You won’t feel completely rested, so the CP you get from this method may be a little lower than if you did each block from fresh. BUT, it will be close enough, and the well within the error of testing and measurement.

•    To calculate CP, enter the average power and the time over which you tested in our online calculator.

Beware!

The phrase ‘Critical Power’ has started to enter into coaching terminology – you may well come across some coaches who talk about a CP for a certain time period e.g. ‘CP180’ being the highest power you can sustain for 180 minutes. This is NOT the Critical Power concept! The CP is a single intensity calculated by looking at the ability to sustain different power outputs for different times.

Also, remember that the CP only uses trials causing exhaustion in 15 minutes or less. It is quite common, particularly in the running world, to take measured race times over given distances to estimate performance in another event. Whilst this uses the power (or distance in running) – time relationship, using durations above 20 minutes goes beyond the limits of the CP concept.

So, just be aware of this, and think about what it is you want to evaluate.

REFERENCES

1.   Scherrer, J. & Monod, H. Journal de Physiologie 1960, 52, 420-501.

2.   Poole, D. C. et al. Ergonomics 1988, 31, 1265-1279.

3.   Brickley, G. et al.  Eur J Appl Physiol 2002, 88, 146-151.

4.   McGawley and co-authors. Unpublished PhD work (2009).