The concept of Functional Threshold Power (FTPw) forms the basis for power based training. Training zones are set relative to FTP. Functional Threshold Power output represents the exercise intensity at which lactate begins to accumulate in a person's blood (i.e. lactate threshold).
The metabolic strain experienced when paddling is dependent upon the athletes intensity relative to their lactate threshold intensity. For this reason, an athlete's Functional Threshold Power output provides a sound reference point around which a training program and training zones can be structured.
The following test outlines a method that closely evaluates an athlete's Functional Threshold Power. With a FTP test, we're trying to establish the power number that corresponds to the lactate threshold of the paddler. The test involves doing an all-out 20 minute time trial effort.
20 minutes easy warm up
3 x 1-minute efforts with a minute rest between (80-90 spm)
5 minutes easy warm up continued
5 minutes hard (but not to the point that you can't complete the effort)
10 minutes easy warm up continued
20 minute time trial effort (all out effort but keep in control. Hard but steady, you don't want to die before the end)
10 to 15 minute cool down
After performing the effort, calculate the average power over the 20 minute time trial. Then subtract 5% from the average power value. The result is Functional Threshold Power as shown in the equation below:
FTP = AveragePower∗0.95
"The reason for subtracting 5 percent of the watts from your 20-minute test is that FTP is defined as the highest average wattage or power that you can maintain for 60 minutes. Since 20 minutes is a shorter time period, it incorporates more of the athlete's anaerobic capacity, however, and this skews the wattage data by about 5 percent over a 60-minute effort. By subtracting that 5 percent, you will come up with a wattage number that would be very close to your 60-minute power measure. "
- Training and Racing with a Power Meter, 2nd Edition (Hunter Allen and Andrew Coggan PhD)