Developed by Peter Riegel, a runner’s fatigue factor can help predict finish times based on a known race time or critical power. It measures how much a runner fatigues over increased distances and is a key part of good race power planning.
Assume two runners with identical 5k PRs run the same marathon. Everything else equal, the runner with the lower fatigue factor will have the faster marathon time.
In other words, the lower the fatigue factor, the less a runner will slow down over increased distances.
While there are various calculators/spreadsheets online that calculate a runner-specific fatigue factor, I’ve found them inaccurate for race planning. Most likely due to their accuracy requiring a lot of data points (which most runners don’t have).
However, thanks to the work of Steve Palladino (The Palladino Power Project) and Steve Bateman (from1runner2another), fatigue factor is easily estimated within the SuperPower Calculator. Rather than try to calculate a runner’s specific fatigue factor, it provides a most likely value (and range) using recent race times.
That approach has worked extremely well for race planning across a wide range of runners.