Your Critical Power maps to a duration that may range from 30-70 minutes. In order to accurately predict performance at another duration, a fatigue factor is required.

Different runners have different fatigue factors. For example, a fast-twitch fiber dominant runner would have a larger fatigue factor compared to a slow-twitch fiber dominant runner.

Our Race Power Calculator determines your fatigue factor based on your age, gender, and the characteristics of your Model Curve on the Power Duration Curve. If your Model Curve has a very steep downward slope, you will be assigned a larger fatigue factor that corresponds with the “speed demon” classification in the previous generation Race Power Calculator. Someone who has a more gentle slope on their Model Curve will have a smaller fatigue factor that corresponds with the “aerobic monster” classification.

If you have an accurate fatigue factor, the Race Power Calculator prediction will be accurate across all distances. Otherwise, you may receive overly optimistic or pessimistic results. For example, if you are an “aerobic monster” with a small fatigue factor but your Model Curve has a steep drop off because you lack a max effort in the 10-20 minute range, you will be incorrectly assigned a larger fatigue factor. This will lead to an overly optimistic prediction for a short distance race. You should log a max effort in the 10 to 20-minute range in order to correct your fatigue factor.