When it comes to measuring the intensity of your cycling workouts or overall cycling training program, the most reliable metric to look at is your cycling power.
But, there may be a superior alternative to measuring cycling power: critical power.
In this article, we’ll discuss critical power in cycling and look at critical power vs. FTP, covering:
- Why Is Cycling Power Important?
- What Is Functional Threshold Power?
- What is Critical Power?
- Benefits of Critical Power vs. FTP
- Advantages of FTP Over Critical Power
- How to Use Critical Power
Ready for the lowdown on critical power vs. FTP?
Let’s dive in!
Why Is Cycling Power Important?
Power is considered to be the most reliable and accurate measurement of your fitness level as well as the intensity of your training and workouts.
Whether you’re an avid cyclist, triathlete, or any other endurance athlete who does cycling workouts as part of your training, monitoring your power is the best way to get real-time and post-workout feedback on your training zones, threshold, and fitness level.
What Is Functional Threshold Power?
Functional Threshold Power (FTP), was first defined to be the maximal power output a rider can maintain for 60 minutes, meaning that true testing your FTP would require an all-out effort for a full hour.
However, since its initial description, FTP has also come to be defined as 95% of the power the athlete can maintain for 20 minutes, or 90% of the power that the athlete can maintain for 8 minutes.
The latter two options to measure FTP are clearly more appealing to the athlete who has to push through the test because it’s more digestible to go flat-out for only 8 or 20 minutes rather than a full effort.
However, it is important to note that these methods require extrapolating your data, so they’re only estimates of FTP.
What is Critical Power?
Your critical power (CP) is essentially the power output you can sustain when riding at a high intensity for around 30 to 40 minutes.
W’ (pronounced “W prime”), which can also be determined from critical power testing, is the amount of work you can perform above the critical power before reaching complete exhaustion.
W’ is measured in kJ, which are units of energy, so it’s essentially a measure of your anaerobic energy reserves above your critical power.
Critical power is determined by looking at the horizontal asymptote of your power graph, which is where the graph flattens out to the right. A power curve can be created by assimilating your results from several power tests of different durations.
For each power test, you chart the maximal peak power achieved during the duration of the power test.
For example, you can do an all-out 1-minute effort test; an all-out 5-minute effort power test; an all-out 8-minute test; and an all-out 200-minute test, and create a graph with your power output for them all.
The critical power effort level essentially represents the upper bounds of your aerobic energy system, or the maximal effort you can sustain before crossing over into anaerobic energy systems.
Therefore, your critical power is the best indicator of your aerobic fitness.
When you’re working at your critical power level, you’re mostly producing energy aerobically, which means you aren’t producing lactic acid in your muscles. In the W’ zone, you’ve moved into anaerobic energy production, so lactic acid starts to build up.
As the environment in the muscles becomes more acidic, you experience rapid fatigue and that oh-so-familiar feeling every cyclist has felt of your legs being on fire.
Critical power is just a hair below the point at which your body switches into producing a significant amount of energy through these anaerobic means.
As such, it’s a very useful measure of your aerobic fitness for long-distance cycling and endurance events, while the W’ value is your anaerobic capacity.
Benefits of Critical Power vs. FTP
Although any given athlete’s values for FTP and critical power are often close to one another, FTP and critical power are actually measuring two different things.
Despite the fact that FTP can be assessed by three different protocols and time durations, regardless of the testing method you use, the actual calculated value is said to represent the power you can sustain for a maximum of one hour of cycling.
In contrast, critical power is the power output you can theoretically sustain for 30-40 minutes.
Although FTP testing is widely used as a power measurement, there are several limitations of FTP testing relative to critical power testing:
- Limited accuracy when using the shorter test durations (8 or 20 minutes rather than 60 minutes).
- Doesn’t give insight into the specifics energy system adaptations that have contributed to changes in FTP results over time.
- Not particularly relevant measurement for shorter cycling disciplines.
Because critical power gives you separate information about your aerobic fitness and anaerobic fitness, you get a much more complete picture of your fitness.
From this, you can see where your fitness is lacking (so you can adjust your training effectively), as well as where you’ve made improvements over time with repeat tests.
Moreover, because critical power testing gives you your maximum sustainable power as well as your capacity above this power, you can apply the results to a wider variety of cycling disciplines and events. You can also use critical power to estimate the appropriate power for different interval durations.
Advantages of FTP Over Critical Power Testing
The primary benefit of FTP testing over critical power testing is that the latter requires at least two all-out tests (four is ideal) whereas FTP can be measured in a single 8 or 20-minute all-out effort.
With that said, some all-out efforts for critical power are super short (1, 5, and 8 minutes).
You also have to do calculations or use an online calculator to actually determine your crucial power and W’ from your critical power test results.
How to Use Critical Power
Once you determine your critical power, you can use the data to inform your training in a few ways:
#1. Critical Power Can Identify Your Fitness Weaknesses
Critical power provides much more insight into the size of your aerobic and anaerobic capacities than FTP.
Like FTP, critical power is an indication of the maximum power you can maintain over an extended period of time. However, FTP is the maximum power you can sustain for 60 minutes, whereas critical power effort can be sustained for about 30-40 minutes.
As such, your critical power is usually slightly higher than your FTP (anywhere from about 5-9%).
Again, like FTP, a higher critical power result indicates that you’re theoretically able to maintain a higher power or workload during endurance rides, meaning your fitness is higher.
You can use this information to inform where your fitness is lacking and which energy systems you need to target with your training.
For example, if your W’ is relatively small, you have a poorer anaerobic capacity, meaning your ability to produce and sustain power through anaerobic systems is not particularly strong.
You could then gear more workouts towards training your anaerobic energy systems, with workouts like intervals and hill sprints.
#2. Critical Power Can Be Used to Set Your Training Zones
Dr. Andrew Coggan is credited as delineating the training zones based on FTP as follows:
- Zone 1: Less than 55% of your FTP
- Zone 2: 55-74% of your FTP
- Zone 3: 75-89% of your FTP
- Zone 4: 90-104% of your FTP
- Zone 5: 105-120% of your FTP
- Zone 6: 121-125% of your FTP
- Zone 7: Greater than 125% of your FTP
Although critical power and FTP measure two different things, you can still use critical power to estimate your training zones.
If estimating your training zones using the guide above, substitute 94% of your critical power for the FTP value – bearing in mind the caveat that you need to gauge your Rate of Perceived Exertion (RPE) in zones 6-7 to dial in your exact effort.
#3. Critical Power Can Help You Gauge Your Progress
Measuring your critical power and W’ periodically allows you to monitor your progress and see if your training is working.
Testing every 3-4 months is a good place to start.
#4. Critical Power Can Be Used to Determine Your Intervals
Your critical power results can be used to help you estimate how long you can sustain a certain power.
They can also help you estimate what power you can sustain for a given duration, so long as you’re looking at power outputs above your critical power and for durations of about 3-20 minutes.
These estimates can be made by using an online critical power calculator.
Ready to make use of critical power for yourself?
Overall, critical power can be a better test of your fitness than FTP, and critical power testing can provide more insight into the relative strengths of your energy system strengths.
Though it takes a bit more work to determine your critical power than FTP, many cyclists indeed find the work is worth the reward!