What power fade tells you
Did you ride the second half as hard as the first?
Power fade answers one question about a ride: how did your output in the second half compare with the first? Form leaves out the first 10 minutes, splits the rest in two and compares the Normalized Power of each half. If the first half averaged 200 W and the second 190 W, the fade is −5%.
A negative figure means the second half was easier. A positive figure means you finished stronger. The interactive example uses a made-up ride with a clean 200 W first half. Change the second half and watch the figure move.
Solid: power. Dashed: average of each half. Fade = 100 × (second half − first half) / first half. Illustrative ride; Form compares Normalized Power after the first 10 minutes.
Did you ride the second half as hard as the first?
Power fade answers one question about a ride: how did your output in the second half compare with the first? Form leaves out the first 10 minutes, splits the rest in two and compares the Normalized Power of each half. If the first half averaged 200 W and the second 190 W, the fade is −5%.
A negative figure means the second half was easier. A positive figure means you finished stronger. The interactive example uses a made-up ride with a clean 200 W first half. Change the second half and watch the figure move.
A fade is not always fatigue
It is tempting to read a fall in power as running out of fuel or fitness. Often it is a choice or the route. A planned easy finish, a long descent, traffic lights, a headwind that turned into a tailwind or an interval session that ended early all change the second half without any sign of fatigue.
That is why Form describes the figure as meaningful mainly on steady rides. On a ride with a lot of structure, the two halves are different sessions, and comparing them says little.
Why Normalized Power
Average power treats a surge and a coast as if they cancelled out. Normalized Power weights harder moments more, so it describes the physiological demand of a variable ride better. Form uses it for each half so a ride with a few surges in one half does not show a false fade.
How fade fits with drift and efficiency
Aerobic decoupling compares output per heartbeat across the two halves. That blends two changes: heart rate rising and power falling. Power fade pulls out the power side. If the decoupling is positive but the fade is near zero, heart rate rose against steady power. If the fade is large, the decoupling may mostly reflect that you rode the second half more easily.
Reading the two together is more informative than either alone. The drift article explains heart rate, and the efficiency article explains the ratio.
Fade is not durability
Durability looks at how your best efforts change after hours of work. Power fade is a simpler, whole-ride comparison of two halves. A ride can show no fade and still tell you little about late-race ability, and a hard final effort can sit inside a half that otherwise faded. Use fade for the pattern of a steady ride and the durability article for late efforts.
Using it well
Look at the same kind of ride over time: a steady endurance ride of similar length, ridden to the same plan. Note the terrain and how the ride was meant to go. If a planned steady ride keeps fading late, that is worth a look at pacing, fuelling and the heat on those days, but the number alone will not tell you which.