Functional threshold power is the maximum power that you are able to sustain over a one hour period.
During heavy exertion, lactic acid is produced in the muscles. When you begin to produce lactate faster than your body can remove it, you’ve reached your lactate threshold. If you go beyond your lactate threshold, your body will no longer be able to sustain the work load and eventually force you to slow down.
While it is possible to raise your lactate threshold to maintain a higher intensity level for a longer period of time, you’ll need to establish your training zones to focus your workouts.
Your true lactate threshold can only be determined through blood tests using expensive laboratory equipment.
However, a more practical way to determine functional threshold power is to do a one hour ride as steady and as fast as you possibly can. It is unlikely that you will have the will to go as fast as you are able. You probably will not be as rested as you should be, you will not be racing anyone, and no one will be judging you if you go slow or congratulating you if you go quickly.
If you choose to ride for an hour and it is not during a race you should take the average power output and add 5% – 8%. So if your average watts were 250 then your functional threshold power would be between 262.5 and 270 watts.
The easier and more practical test can be done in 20 minutes. It is easier to start out too fast in a 20 minute test, so ensure you get up to speed quickly, but do not push it too hard as you will tire too quickly and it would result in an inaccurate result.
Once you have completed the 20 minutes reduce the average wattage by 5% – 8%. So if your average watts were 250 then your functional threshold power would be between 230 and 237.5 watts.