As an athlete, you may have heard the saying, "what you can do on your own in training, will predict what you are capable of in a race." And this is a fair statement, at least up to reasonable distances and around FTP and threshold testing.
First, it's important to understand what FTP means. FTP stands for Functional Threshold Power, which is the highest power output you can sustain for an hour. This is also why FTP is often referred to as “CP30” or Critical Power for 30 min.
I like to tell my athletes to substitute the letters “CP” for “The best effort you can do for: and the number is the duration. So CP30 = “The best effort you could do for 30 min”.
To estimate your FTP, you can do a CP30 test, which measures the best effort you can do for 30 minutes. But here's the catch: anytime you double the duration, you can predict a 5% drop off in output. So if you know your CP30, you need to subtract 5% to predict your CP60, or the highest power output you can sustain for an hour in a race.
I tend to warn against using the more common 20-minute FTP test. These tests are often done in a group environment with outside motivation, which can skew the results. And even if you subtract the 5% drop off, it may not be a clean doubling effect, leading to an inaccurate FTP prediction. In my opinion, it is also easier to “Train for a 20 min test”.
Instead, I recommend doing a solo CP30 effort, which removes any outside motivation and provides a more accurate estimate of your FTP. And because the general rule of thumb holds true, what you can do on your own in training, you can do for double the distance in a race. So if you know your CP30, you can predict your FTP for a race.
So next time you're trying to estimate your FTP, skip the 20-minute test and try a solo CP30 effort instead. With this more accurate estimate, you can better prepare for race day and achieve your goals with confidence.