Question: Can you explain more about "critical power"?

Mark Burnley answered on 21 Jun 2011:

Critical power is what we call a “parameter of aerobic fitness”. Others include the VO2max, which is the highest rate of oxygen uptake measured during maximal whole-body exercise. You can make a guess as to your VO2max score by doing a “bleep test”, but the best way to measure it is to do it under laboratory conditions and measure oxygen uptake directly.

Critical power is different. Critical power is identified by performing a series of tests to exhaustion, usually on separate days. By noting the power output and the time to exhaustion, you can construct a curved relationship between the two (a hyperbolic curve, maths fans…). Think of it this way: if you were to run at 90% of your fastest sprinting speed and tried to maintain it, you wouldn’t last very long. Half a minute, maybe? But if you ran at 50% of this speed and tried to maintain it, you might become exhausted in, say 3 minutes. Then try 40% of maximum speed. Lets say you lasted 7 minutes here. Then, finally, you try and run at 30% of maximum speed. This might take 15 minutes to exhaust you. You can see from these very rough figures that a small reduction in running speed greatly increases your time to exhaustion. The critical power (or critical speed in this case) is the speed at which you will (theoretically) never become exhausted (for the maths fans among you, the critical power/speed is the asymptote on the power or speed axis of the graph you have constructed). If you are very aerobically fit, you will have a high critical power/running speed. My critical speed (when I was very fit) was about 17 km/h (it is rubbish now though :-)). But that doesn’t tell the whole story, because in addition to the critical power, there is another parameter which has a horrible name: the “curvature constant parameter” or, as we like to call it for no good reason at all, W’ (spoken as “double-u prime”). This tells us how much the plot curves away from critical power and it is thought to be a measure of your “anaerobic” fitness.

That’s the way we measure the critical power, but there is another special property of the critical power that is worth telling you about. If you exercise below the critical power, it is possible to reach what physiologists call a “steady state”. This is where your heart rate, breathing, oxygen uptake etc. reach stable and submaximal levels and you can continue exercising quite happily for a long time. However, above the critical power, reaching a steady state is impossible, and exercise gets harder and harder as time goes on. Eventually, your heart rate, oxygen uptake, breathing etc. reach maximal values and you are forced to slow down or stop. So critical power is not just a useful measure of fitness, but it is an important “landmark” physiologically too: if you know where the critical power is, you can predict how participants are going to react to the exercise you give them, and when and how they will fatigue.

Critical power is not just a thing we measure in humans. It (or something very like it) has also been measured in horses, rats, mice, ghost crabs and lungless salamanders. When we see such similarities in so many different species, we can be sure that what we are looking at is fundamental to our physiology (our meaning animals in general).

The last thing I have to tell you about critical power is my contribution to it. A few years ago, I was lucky enough to recruit a superb PhD student called Anni Vanhatalo. We (Anni, myself and Jo Doust – a colleague and mentor of mine) whated to find out if we could measure critical power in a single test. We thought that if you sprinted for long enough, you might be able to use up all your “anaerobic” energy and if you kept going after that, all you would have left is enough “aerobic” energy to exercise at critical power. So we got a dozen cyclists and tried it. We measured critical power the usual way (repeatedly exhausting the cyclists) and then we performed our new “3 minute all-out test”. When we compared the critical power measured in each case, they were almost exactly the same! This was very exciting, because although we predicted it from theory it was still very unexpected that humans could exercise that hard for 3 minutes AND still be producing enough power to reach critical power – but they were. I am very proud of that work. 🙂