If you train with a power meter (highly recommended) then you probably know that setting up power training zones is best based on your Functional Threshold Power (FTP). That’s the average maximal power you are capable of sustaining for one hour. I’ve written here many times before describing how to do a field test to find your FTP. Here's one of those posts.
Athletes often ask me what their FTP should be. That’s hard to answer because (as usual) there are so many “it depends.” But here’s a quick and dirty way to estimate what your FTP based on body weight, age and gender…
Step 1. Double your body weight in pounds (1 kg = 2.2 lbs). Example: A body weight of 154 pounds (70 kg) estimates an FTP of 308 watts (154 x 2 = 308).
Step 2. Subtract 0.5% for every year beyond age 35. Example: If the above 154-pound rider is 50 years old he would subtract 7.5% from 308 (50 – 35 = 15 x 0.005 = 0.075). This would predict an FTP of 285 (308 x 0.075 = 23.1, 308 – 23.1 = 284.9).
Step 3. Women riders can subtract 10% from the estimated FTP as found in steps 1 and 2 above. Example: A 120-pound (54.5-kg) woman who is 40 years old would have an estimated FTP of 211 watts (120 x 2 = 240, 240 – 2.5% = 234, 234 – 10% = 210.6).
If your actual FTP based on testing falls short of the estimation then you may have a new training objective for this winter. If your known FTP exceeds the estimation then congratulations for doing something right in your training. (In a future post I’ll discuss how to train to raise your FTP.)
Not all of the possible variables are included in the 3-step estimation above. There could well be others. One of the most significant for some athletes is altitude. As the altitude increases above sea level aerobic capacity declines which means that one’s average max power over a one-hour period would also decline. For example, at 5,000 feet (1,516 meters) the negative effect of altitude is between 5% (acclimated to altitude) and 9% (not acclimated) according to Bassett et al, 1999. So an FTP determined in Boulder, Colorado would be roughly 5-9% greater at sea level. Example: Our 50-year-old rider from above is going from Boulder to sea level for a race and wants to know what his power zones should be at the lower altitude. Since he lives at altitude we can assume he is acclimated. So if we add 5% to his altitude FTP of 285 the new FTP is estimated at 300 watts (285 x 0.05 = 14.25, 285 + 14.25 = 299.25). He would then reconfigure his power zones based on an FTP of 300.
Of course, if you are going to altitude from sea level to race you would subtract the estimated power change to reconfigure your zones. (I’ll post an altitude adjustment table based on Bassett’s study in an upcoming blog.)
And, of course, excess fat or muscle, especially upper-body muscle, will also skew the results. If you have a considerable amount of either of these then your estimated FTP is likely to be too high. Knowing lean body mass from testing would be a better predictor if fat is an issue. That won’t help for overly muscular riders, however. (Side note: While the weight of your bike is often not included in the power-to-weight ratio it certainly is an issue. A heavy bike will diminish the effect of an otherwise high FTP estimated from body weight. This is a real issue for a small woman. A 15-pound (6.8 kg) bike is a heavy load to carry uphill for a small rider. But it’s still not included in the estimation of FTP.)
I’m certain there are even more variables, such as experience in the sport and familiarization with the testing protocol, which I have don’t know how to include in the estimation of FTP. And the above method is not scientifically proven. It’s just something I’ve come up with from coaching a small number of riders and talking with others. But give it a try to see what your FTP estimation is. I’m curious to see how far off it is from what you have found your actual FTP to be.