Your goal in the main portion of the test (the 20-minute segment) is to produce the highest average watts possible over the entire
time. The test doesnt work if you start out too hard and suddenly run out of energy, because you wont be able to produce your true maximal, steady-state power. Its always better to start out in the first two minutes a little under what you believe to be your FTP, build up along the way, and then ride at your maximum level in the last three minutes.
Now that youve done the test and downloaded your data, find your average power from the entire 20-minute effort. Take this number and subtract 5% percent from it. The result is your functional threshold wattage value. For example, if you averaged 300 watts during the 20-minute time trial, 5% of 300 (300 x 0.05) is 15, and 300 minus 15 is 285. Your FTP is 285 watts.
The reason for subtracting 5% from your average watts during the 20-minute test is that your true FTP is the highest average power you can maintain for sixty minutes. Most athletes have a hard time putting out maximal effort for sixty minutes, however, and those who can learn very quickly that a sixty-minute time trial is not much fun. Ive found that twenty minutes is a more realistic time frame. Its obviously a shorter time period, however, and it incorporates more of the athletes anaerobic capacity, which skews the wattage data by about 5% over a sixty-minute effort. By subtracting that 5%, you end up with a wattage number that should be very close to your true FTP.
Ready? Go! Whats your FTP?