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Smoothed Power

When you ride with a power meter, you'll notice how your power jumps all over the place based on the terrain, grade, wind, and other factors. When you stop pedalling power drops to zero immediately. Your heart rate may take 30 seconds or so to recover.

In truth, although the power meter says zero watts when you stop, the body's physiological response continues for roughly 30 seconds, as HR drops, breathing recovers and more complex energy system processes continue.

This means that if we want to use power output as a measure of training stress we will also need to translate those simplistic power readings into something that reflects the associated physiological processes and their half-lives.

This is what Dr. Andrew Coggan's Normalised Power, Dr. Phil Skiba's xPower and Velo Hero's Smoothed Power are doing; they "smooth out" the power data to reflect the underlying physiological processes.

The underlying mathematics is slightly different. However, all formulas are much more accurately than just taking a simple average.