What is Critical Power?


You may have come across this term before in relation to the threshold power. Sometimes FTP (functional threshold power) and Critical Power are used interchangeably, but this is not correct. Although both values are often close to each other, there is a clear difference. FTP is the power you can sustain for a maximum of one hour while CP is the power you can theoretically sustain for much longer. Critical Power is the lower limit (or horizontal asymptote) of someone’s power curve. To explain how this works we must first explain what a power curve is.


Mean Maximal Power (MMP) is the amount of power you can produce for a given time effort. In 10 seconds you can produce more power than you can in 1 minute. Through this model it is possible to get an indication of how much energy can be released from which system per unit of time.
To release energy the human body has 3 different energy systems available. The goal of all these systems is always to release ATP (adenosine triphosphate), because this is energy with which you can do work.


All these energy systems have advantages and disadvantages, these are briefly described below;

  1. Creatine phosphate system: also called the sprint system. This form of energy is stored in the muscle and is therefore very quickly available. This allows you to release a lot of power very quickly. After 8-15 seconds this energy system is empty and you will see a clear decrease in power. It is especially important during sprints.
  2. Anaerobic: The anaerobic system is good at releasing energy relatively quickly, it does this by burning glycogen without the intervention of oxygen. This process is very fast which makes it especially useful for short efforts. Think, for example, of a viaduct or short climb that you can ride up full speed. The disadvantage is of course that you acidify, this is due to an increase in H+ ions. These ions literally cause acidification of the muscles, which decreases the function and you will have to reduce the intensity at a given moment.
  3. Aerobic: The aerobic system releases energy by burning fats and carbohydrates with oxygen. It is a relatively slow system but can be maintained for a long time if sufficient fuel is present. The carbohydrates stored in the body can provide 1-1.5h of intensive effort, the energy you can get from the fats is enough for days of effort! This energy system is therefore for a longer duration.

Of course, all these energy systems work together and side by side. Depending on what kind of effort you make, one system is more/less active. If we therefore set all your best powers against each other, it will look something like this.


By doing a number of power tests it is possible to create your power curve. You do this for example by doing an anaerobic field test, where you test your best power over 1minute and 5 minutes. This simply means that you make a full 1 minute and 5 minute effort after a good warm-up and with sufficient rest between both efforts. Based on these values we can calculate the curve and your CP. Note that the more maximal efforts we have, for example also an 8, 12, 20 and 40 minute maximal efforts, the more accurate your power curve becomes.
Critical power is calculated by looking at where the graph flattens out, this so-called asymptote is called Critical Power. CP stands for the aerobic energy that you can release, it corresponds to FTP but is not quite the same. FTP is the actual power you can ride over 60 minutes, CP gives an indication of the size of your aerobic system and is therefore slightly lower.


The area above CP is called AWC (anaerobic work capacity), which reflects the anaerobic system. This is a tank of energy that can be drained over varying lengths of time, from 1 minute to 20 minutes. In fact, it is a tank of Joules (energy) that you can spread out over a period of time. Is the tank completely empty you can always fall back on Critical Power. Some comment should be made here. This is a highly theoretical model that describes the reality and functioning of systems well, but practice is always more stubborn. What exactly Anaerobic Work Capacity is we will explain here in another blog.


Imagine you do a 20minute test, it is 15w better than when you did it 3 months ago. Obviously you have improved over 20 minutes, but we do not know how much your FTP has improved. It may be that your anaerobic tank has become larger and that you have improved just above your threshold power. As a result, your aerobic system may have increased by only 5w. It is therefore important to create a power curve by testing multiple peak values so you can make the 2 component model. By doing this it is possible to get a better indication of which energy system has improved after a period of training or on which energy system you should focus more towards your goals.


Through the CP model it is possible to split your performance into two components; aerobic and anaerobic. This makes it easier to set and evaluate training goals. It is very important that you do this with someone who knows what he is doing and how to calculate, use and interpret the model.
This model and theoretical background can be very helpful in your training approach. Feel free to contact us about what we can do for you. Or check out our training application JOIN that creates a personalized training schedule for you.