Human physiology is full of thresholds. In this context, a threshold is simply some exercise intensity where your physiology shows a marked change. For example, if you went out for an easy run at a constant speed, we could use a gas analyzer to show your oxygen use was low, stable, and proportionate to your effort. However, if I told you to run a 3k as hard as you could, we would find that your oxygen use is unstable. Running at a constant speed would result in a continual rise in oxygen use until it reached the maximum you were capable of and you were forced to quit. In the early part of the 20th century, physiologists first noted this change in physiology, but did not understand the cause. (One of the scientists famously suggested that the difference was due to a rock in the athlete’s shoe during the harder effort!)
Today, most athletes are familiar with this particular threshold. Even without knowing any physiology, you can feel that there is a level of effort beyond which you get tired much more quickly. This level of effort has been called many things over the years, and there are many ways to estimate it. The most accurate way to find the speed or power where your physiology becomes unstable involves using the Critical Power (CP) or Critical Speed (CS) models. These models can be set up in a spreadsheet program, and give you two important pieces of information.
Calculating Critical Power and Critical Speed
First, let’s look at CP or CS. When you exceed this speed or power, you begin to tap into an energy storage system called the W-prime (W’, cycling) or D-prime (D’, running or swimming).
Both the CP / CS and the W’ / D’ are important. You can visualize the model like a mobile phone with a charger that is a bit too small. If you are sending emails, the charger (i.e., your metabolism and oxygen transport systems) can keep up, and the battery stays fully charged. However, if you start watching HD video (e.g., sprinting away from a group), the charger can’t fully meet the power requirements and the battery will run down a bit. If you do this too frequently, the battery will go dead (i.e., you will stop and puke in the bushes). It’s important to carefully pace yourself when using the battery, because it takes a long time to recharge, even if you sit down and rest.
How do you discover what your CP or CS is? You need to do a series of tests that take between about 2 and 20 minutes. For example, you can get on your trainer and go as hard as you can for 3 minutes and record the average power. On another day, do the same thing for 12 minutes. On a third day, do it for 5 minutes.
Now, you multiply each test duration in seconds by the average power, which gives you the energy expenditure. Finally, go to a spreadsheet and graph the energy on the vertical axis and the seconds on the horizontal axis. Click “add trendline”and “show equation.” The equation will be of the form Y=MX+B, where the M is your CP (the threshold) and the B is the W” (the battery). You can do the same thing with running or swimming. Just graph the meters run on the vertical axis and the seconds on the horizontal.
Does this sound like AP Calculus all over again? Don’t worry, there are a multitude of programs and devices that can help you perform these tests and calculate the values you need automatically, but there are caveats, of course.
The Problem with FTP
There are some pitfalls here you need to be aware of. As we said, there are many ways of estimating threshold, and they are not all created equal. Some people like the term functional threshold power (FTP). This isn’t a proper physiological threshold, and in fact most ways of estimating it give you a number considerably lower than the CP. It also tells you nothing about the size of the battery, which is a crucial piece of information.
Threshold can also be estimated by lactate testing, but this is not as accurate in most circumstances and involves sharp objects and blood. People will sometimes criticize the CP model by saying it changes based upon the test durations used, and that is true. However, this is a bit like complaining that your car breaks down if you put diesel in the tank instead of unleaded. Your car isn’t built for diesel. To find the point at which your physiology becomes unstable, you need to feed the model the correct test durations.
Putting it Into Practice
You now know how to properly estimate your threshold power or speed, and you know how large or small your battery is for the times you need to climb a hill or break away from a group. How do you train to improve these things? That answer is a little complicated, and has been a source of much confusion. You see, when you train to increase the CP / CS, you usually decease the size of the W’ / D’. You become more endurance oriented, and less sprint oriented. Don’t panic! Sure, the battery got a little smaller. However, the higher CP / CS means you can now recharge it faster. Also, you will be able to go harder before you need to tap into the battery. This combination of changes allows you to go faster for longer, and is really important.
Typical workouts targeting CP / CS might include 20-minute intervals on the bike, or 1k repeats while running. You approach the target effort from below, sit there for a while, and then take some short recovery (2-5 min) before repeating the effort. These can be done a couple times a week if you are highly motivated, but once a week is usually enough because these intervals can beat you up quickly. Perhaps more crucially, there are other factors that are equally important if you want to be able to leverage your threshold power to its fullest potential.
Variables Affecting Threshold
It turns out that the threshold changes depending upon how long your event is, and how good your nutrition is. A team of my colleagues led by Ida Clarke demonstrated that if you ride below CP for a couple of hours, and then test CP, the CP is lower than expected. This drop in threshold can be rescued by consuming carbohydrates during the ride. No surprise there.
However, it isn’t all about carbohydrates. My colleagues James Spragg and Peter Leo recently showed that top-tier cyclists and junior cyclists typically have similar physiology when rested, but that the top-tier cyclists are more durable. In other words, if you “pre-fatigued” the cyclists the way Ida did, you would find that top-tier cyclists would maintain their threshold better than the juniors. This is likely a residual effect of all the extra miles those top tier riders have put in over the course of their careers. Again, this makes sense, intuitively.
The take-home message is that your threshold training is important, but it must be supported by a foundation of lots of endurance training if you are to reach your fullest potential. The best results usually come from a combination of volume and intensity, not one or the other in isolation.
Dr. Philip Skiba is the Director of Sports Medicine for Advocate Medical Group in Chicago, and is an honorary Associate Professor at The University of Exeter. He serves on the medical board for USA Cycling. Dr. Skiba has trained dozens of elite, world champion and Olympic athletes, and was a consultant to the Nike Breaking2 Project. His new book, Scientific Training for Endurance Athletes is available in the USA on Amazon, and worldwide from physfarm.com.