Use critical power to guide your training
We will start this blog post with some information about what Critical Power is, and then I will discuss how I have used it to limit my athletes training.
Somewhere between your LT (Lactate Threshold) and VO2max lies perhaps the most relevant threshold, called Critical Power (CP) for cyclists or Critical Speed (CS) for runners. CP usually lies around 80-85% of VO2max and represents a clear dividing line between training you can sustain for a longer time and training that is so tough you have to stop much earlier.
The CP threshold is much clearer than the other two thresholds, lactate threshold and VO2max. Imagine the CP threshold as opening the door and entering a sauna that is 80°C. Outside the sauna (below the threshold) you can stay indefinitely, no matter how long. The environment there is comfortable. But if you enter the sauna (above the threshold), it feels nice at first. After 15-20 minutes it starts to become tough, but you still feel strong enough to stay. After half an hour you can’t stay any longer; you have to leave.
If you sit in the sauna but sit near the door, maybe on the floor where some cool air drifts in from the door frame, you can stay quite long, although it is uncomfortable. If you instead sit at the top of the sauna and keep pouring water on the heater, you will have to get out much earlier.
The same applies to your CP. You notice immediately when you cross the threshold into the heat.
Let’s say we have calculated that your CP is 350W. The difference between 345W and 355W is that you have taken the step into the sauna. At 355W you are sitting on the floor near the door. If you turn the resistance up to 400W, you sit near the heater on the lower step. If you turn it up further to 450W, you sit at the top of the sauna. There you can’t stay long. If you increase the resistance a bit more, you start pouring water on the heater. Eventually, you have to get out.
Hope my sauna analogy feels reasonable and understandable. In context, unlike the lactate threshold and VO2max, which you must measure physiologically to know where they stand (blood lactate measurement and oxygen uptake mask). If you know your body well, you can feel where your CP (Critical Power) is. Below CP, your oxygen uptake and blood lactate will be stable, as will your breathing rate and heart rate (kind of - it depends how close to your CP you work). If you ride a few watts above your CP, your physiology will go bananas and slowly but surely force you to stop pushing. The time it takes before you have to stop depends on how far above CP you are training. The reasons for these changes are many—both structural changes and biochemical.
The great thing about Critical Power is that it can be calculated using test or race results. In other words, it is a "threshold" that can be defined without any blood tests or expensive lab visits. You take your best performances over 2-4 different power efforts lasting between 2-20 minutes, and then a mathematical model calculates it. Simply put, CP is very close to the best power output you can hold for 20-40 minutes.
When training above CP, you can almost set a clock on when you will be forced to stop. It can be calculated within just a few seconds. That’s why it’s a good way to determine training intensity based on CP. Instead of struggling with percentages of VO2max, we use CP. We want to use how long we can do something before exhaustion.
You can think of it like this: VO2max tells you when your engine EXPLODES. Critical Power tells you when your engine STARTS TO OVERHEAT.
An exaptional explanation from dr. Philip Skiba in his book “Scientific training for endurance athletes” page 23.
As I wrote earlier, the difference for the example person between 345W and 355W was the step into the sauna. But that obviously doesn't mean the person can sustain 345W indefinitely. No, you get tired there too. With fatigue, CP can occur at a lower power output, while the amount of work you can perform above CP also decreases. At the end of a hard race, where you've been racing for 3 hours and expended 2500 kJ of energy, CP might occur at 320W instead of 350W. The amount of work you can do above 320W is also smaller. However, it is possible to prevent too large a drop in CP by consuming enough carbohydrates during the activity. Through sufficient carbohydrate intake, you may still expend 2500 kJ of energy, but you have ingested 270g of carbohydrates over three hours and can limit the drop in CP to 340W. Now, all these numbers are arbitrary and need not be completely accurate, but I hope you understand the principle at least. Below is an example of how the mathematical calculations may look.
Thanks to Dr. Andrew Coggan for inspiring this analysis
Here are Bradley Wiggins' numbers from various occasions, and based on these, Wiggins' CP would be 449W. Anything above 449W is quite taxing, and he can only stay above that for a limited time. But, for example, at his 2011 Time Trial World Championship, he maintained 459W for almost 54 minutes. Regular, well-trained people like you and me might manage about 10W above their CP for 20-40 minutes, but extremely fit and highly motivated individuals like Wiggins, as you can see, can stretch that limit — which is not uncommon for some very motivated and well-trained athletes.
Critical Power is a hard concept to explain relatively to the more common functional threshold power (FTP). Changing the training structure to base it on CP instead of FTP makes you understand what it is more, both understanding the concept but to understand the application as well.
We have done the majority of the structured interval sessions based on Critical Power rather than a fixed power output or time target. Since CP is above the lactate threshold but below VO2max, it provides a very intense interval training structure that is not as metabolically, biochemically, or structurally taxing on the body. We limit how hard and exhausting the intervals feel by staying at or just below CP and adjust the number, duration, and repetitions of intervals at CP rather than increasing or decreasing the power output.
An example:
Two riders have a CP of 350W and are to perform a workout of 10x2min intervals with 2 minutes rest. That is a total of 20 minutes of intervals at 350W.
Person 1: Limits themselves to their CP and performs each interval at 350W. Since person 1 does not go above their CP, they recover well during the 2 minutes between intervals to execute the same interval again with the same quality. After completing a session, the person has ridden a total of 20 minutes at their CP, feels satisfied and happy, and can repeat the same session tomorrow.
Person 2: Does not limit themselves to their CP and starts each interval at 400W, finishing the interval at 300W. If person 2 manages to get through all 10 intervals (which I would say is doubtful) with 2 minutes rest between, the average power of the intervals would still be 350W, assuming equal time is spent above and below 350W. After completing the session, the person feels nauseous and cannot perform a good training session tomorrow.
The difference between the interval sessions is that person 2 depletes their battery (W′) which is above CP, something person 1 avoids. (I will not go deeper into what W′ (W Prime) is right now). Example finished.
By knowing your CP, we can also apply that knowledge during unstructured training sessions, competitions, or other situations. We understand that our athletes deplete a very slowly recharging battery as soon as they go above CP, and that battery drains faster the further over CP they go. You could say that we take their limitations into account in a completely different way than before.
When is it worth drawing energy from the battery above CP, and when should it be avoided? How often and for how long?
It is almost impossible to compete in disciplines like XCO, XCC or XCM without ever going above CP. That is simply not the demand profile for those races. But by strategically deciding how and when I cross the threshold and when it is not worth it, I influence my performance greatly. CP can be a good measurement and limitation in longer TT races, long climbs in road races or XC Marathon or else.