What is Functional Threshold Power and Critical Power?

Posted Wed, 30th Apr 2014

Functional Threshold Power (FTP)

By Thomas Murray,MD

Functional threshold power (FTP) is the maximum average wattage that a cyclist can sustain for a one-hour time trial.

As such, FTP is akin to threshold heart rate over a similar period of time, though FTP is more consistently reliable than threshold heart rate. Whereas hydration, fatigue, nervous energy, caffeine levels and daily rhythm changes can affect heart rate, FTP remains a more consistent measure of work output.

One sure way to establish functional threshold power is to do a one-hour time trial and measure your average wattage over this time period. Given the pain associated with such an effort another testing method to determine an accurate estimate of FTP: the 20-minute Critical Power (CP) test. This can be done on an indoor trainer or on the road, however to get the best readings, it is always prescribed to come in to an indoor training facility to get this done.

The 20-minute Critical Power test is conducted as follows:

  • On a trainer or on a relatively flat section of road, warm-up easy for 15-20 minutes;
  • Over the next 5 minutes, do five (5) hard 30-second efforts, followed by 30 seconds of soft-pedaling. The purpose of this drill is to open up the blood and oxygen flow and to increase the heart rate prior to the 20-minute effort, so don’t go too hard. Push a wattage you think you can sustain for 10 to 20 minutes.
  • Pedal easy for 5-minutes and prepare yourself mentally for the 20-minute test, as it’s going to hurt!
  • Start the 20-minute test by selecting a wattage you think you can sustain for the full-20 minutes. The cardinal rule of time trialing applies here: don’t start out too hard. Keep in mind that the best cyclists in the world can only sustain 400-500 watts over a 1-hour period of time, so if you find yourself starting out at 500 watts, you know you are likely going much too hard. It’s best to start out easy for the first two minutes, and then build progressively to a wattage level you think can sustain.
  • Hold that level for the first 15 minutes, and then give it your best effort during the final five minutes. (If you find yourself fading in the last five minutes instead of holding steady or building, then you may have gone out too hard. Keep this in mind for your next test).
  • Take your average wattage for the 20-minute period of time and multiply by .95.
  • Your functional threshold power = 20-minute CP test x 0.95.

If your average power for the 20-minute test was 210 watts, then your FTP is estimated at 200 watts. If your average was 300 watts, then your FTP is estimated at 285 watts etc.

The objective of the triathlete / cyclist is to benchmark and progressively increase functional threshold power over time – peaking for goal events throughout the year. As such FTP not only serves as a benchmark for training zones, but it also can serve as a "cruise control" target to help avoid over or under performing on the bike. With a focused training plan, diet and rest you should see an increase in power.

Dr. Thomas Murray, an avid cyclist, is board certified by the American Board of Orthopedic Surgeons and an active member of the American Academy of Orthopedic Surgeons and the American Orthopedic Society for Sports Medicine. He is President of the Maine Society of Orthopedic Surgeons.

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