Key Takeaways
- The fatigue index is the share of your peak power that you lose in a hard effort: (peak - lowest) ÷ peak × 100[1][4]. A result of 0% means no loss at all.
- A 2021 review calls it the least reliable number a Wingate test gives[1], and two other studies also rate it below peak and mean power[2][3]. Treat it as a rough guide, and compare only tests done the same way on the same equipment.
- For repeated sprints, the percentage decrement was the most valid and reliable of eight formulas compared[6]. The calculator gives you both scores.
- The sources listed on this page give no low, moderate or high bands, so the calculator shows none.
How the Fatigue Index Is Calculated
Peak power minus lowest power, as a percentage of peak power.
In a Wingate test you pedal flat out for 30 seconds against a fixed resistance[1]. The fatigue index is the loss of power from your peak to your lowest power, as a percentage of the peak[1][4].
FI (%) = (peak power - lowest power) ÷ peak power × 100
Worked example: 900 W peak, 400 W lowest
The drop is 500 W. Divided by the peak, that is 55.6%, and you held 44.4% of your peak. These numbers are made up to show the arithmetic.
FI = (900.0 - 400.0) / 900.0 x 100 = 55.6%
The peak and lowest power come from your ergometer software or a power file. Traditionally they are averaged over 5 seconds, while newer systems such as the Monark Peak Bike 894E can use one-second running averages[4]. Check which one yours uses, and keep it the same whenever you compare tests.
Researchers use more than one fatigue formula, and one comparison of repeated-sprint methods tested eight[6]. Some websites also give a watts-per-second version, but none of the sources listed here defines it, so the calculator does not use it.
What the Fatigue Index Can and Cannot Tell You
A rough guide, and weaker than peak or mean power.
A 2021 review calls the fatigue index the least reliable and valid output of the Wingate test[1]. It quotes test-retest reliability (r, where 1.0 is perfect) of 0.43 to 0.73 for the index, above 0.9 for peak power and 0.91 to 0.93 for mean power.
Two other studies agree. In 69 young adults doing a 15 second test, the index was only moderately reliable, and its measurement error of 9.6% was bigger than the smallest worthwhile change[2]. In 49 men doing 30 second tests it was rated high, but peak and mean power were very high[3].
A practice session is likely to improve the reliability of peak and mean power, but the review says the index stays unreliable even after practice[1].
The index also changes with the setup. In 43 young men it was greater over 30 seconds than over 15[2], and in 49 men it was lower on the Lode ergometer than on the Monark[3].
Sport and training matter too. Elite track cyclists who specialise in endurance had much lower fatigue rates than elite sprint specialists[1]. The same review treats the decrease in power as an indicator of muscle fibre type: people with a higher percentage of fast-contraction fibres produce higher power and a greater decrease over time[1].
Repeated Sprints: Fatigue Index or Percentage Decrement
Two scores from the same set of sprints. Research favours the second.
| Score | Power, speed or work | Sprint times |
|---|---|---|
| Fatigue index[5][6] | (best - worst) ÷ best × 100 | (slowest - fastest) ÷ fastest × 100 |
| Percentage decrement[5][6] | [1 - total ÷ (best × number of sprints)] × 100 | [total time ÷ (fastest × number of sprints) - 1] × 100 |
The two repeated-sprint scores and their formulas
The fatigue index uses only your best and worst sprint. The percentage decrement compares your total with an imagined set where every sprint matched your best. A review calls that a possible advantage, because the index is influenced more by a particularly good or bad first or last sprint[5].
Worked examples
Four sprints of 1000, 950, 900 and 850 W give a fatigue index of 15.0% and a decrement of 7.5%. Four sprint times of 5.0, 5.1, 5.2 and 5.3 seconds give a decrement of 3.0% and an index of 6.0%. These numbers are made up to show the arithmetic.
Glaister and colleagues compared eight formulas in 10 active men running 12 sprints of 30 m, on two schedules: a sprint every 35 seconds or every 65. The percentage decrement came out the most valid and reliable[6].
Even so, the authors report test-retest variability in fatigue of around 30%. They suggest that may still be small enough to evaluate the effects of interventions, and note that no gold standard exists to check any fatigue formula against[6].
An opinion piece goes further. It argues that calculating fatigue from a small drop-off in performance will always give an unreliable measure, so using a fatigue index or percentage decrement may be questioned[7]. That is a reason to read small differences between two tests with caution.
A review of repeated-sprint research notes that decrement scores have typically been 10 to 25% in cycling studies and 5 to 15% in running studies[5]. Those are values reported in studies, not targets for you.
The same review says less fatigue does not always mean better performance. In its example, people tested in the evening had a stronger first sprint but the same later sprints, which gave a bigger decrement. It advises reading fatigue scores alongside total or mean output[5].
"Muscle Fatigue Index" Means More Than One Thing
Same words, different measures. Here is which is which.
In the Wingate and repeated-sprint research, the fatigue index is the performance-drop measure calculated above. Questionnaires use the word for how tired you feel.
One example is the 20-item Multidimensional Fatigue Inventory, tested in cancer patients, people with chronic fatigue syndrome, students, army recruits and junior physicians[8]. It is copyrighted in the names of its authors, free for academic use and charged for commercial use[9], so this page does not reproduce it.
Related Tools
For questions this page does not answer.
- How hard a session felt: RPE calculator.
- Heart rate recovery after exercise, and session load: Recovery Time calculator.
- Hours since your workout, against what studies measured: Muscle Recovery calculator.
- A drop in performance that lasts weeks: Overtraining Syndrome calculator.
- Another power test: Vertical Jump calculator.
Limits
What these numbers cannot tell you.
- The index describes one effort. It says nothing about how recovered you are today or whether you are overtrained.
- The result is only as good as the readings. The Wingate review says the sampling rate should be as high as possible[1].
- When you paste readings, the lowest value is taken at or after the peak, so a low reading from a standing start does not count. Mean power is the plain average, which is only right if the readings are equally spaced. Both are the calculator's own choices.
- A 30 second all-out effort is hard. The review calls the acidosis it causes, and the after-effects, one of the biggest disadvantages of the test[1].
- The input limits, such as 5,000 W or 3 to 60 sprints, and the "short set" note below 5 sprints are the calculator's own settings. They are not physiological rules.
Frequently Asked Questions
Sources & References
- The Wingate Anaerobic Test, a narrative review of the protocol variables that affect the results obtained. Castañeda-Babarro A. Applied Sciences 11(16):7417 (2021)
- Test-retest reliability, criterion-related validity, and minimal detectable change of score on an abbreviated Wingate test for field sport participants (abstract). Hachana Y, Attia A, Nassib S, Shephard RJ, Chelly MS. Journal of Strength and Conditioning Research 26(5):1324-1330 (2012)
- Validity and reliability of the Lode Excalibur Sport cycle ergometer for the Wingate Anaerobic Test (abstract). Lunn WR, Axtell RS. Journal of Strength and Conditioning Research 35(10):2894-2901 (2021)
- The test-retest reliability of new generation power indices of Wingate all-out test. Ozkaya O, Balci GA, As H, Vardarli E. Sports 6(2):31 (2018)
- Repeated-sprint ability, part I: factors contributing to fatigue. Girard O, Mendez-Villanueva A, Bishop D. Sports Medicine 41(8):673-694 (2011)
- The reliability and validity of fatigue measures during multiple-sprint work: an issue revisited. Glaister M, Howatson G, Pattison JR, McInnes G. Journal of Strength and Conditioning Research 22(5):1597-1601 (2008)
- Is a fatigue index a worthwhile measure of repeated sprint ability? (opinion piece, abstract). Oliver JL. Journal of Science and Medicine in Sport 12(1):20-23 (2009)
- The Multidimensional Fatigue Inventory (MFI): psychometric qualities of an instrument to assess fatigue (abstract). Smets EM, Garssen B, Bonke B, De Haes JC. Journal of Psychosomatic Research 39(3):315-325 (1995)
- Fatigue: questionnaires used as outcomes in pulmonary rehabilitation (web page; MFI-20 cost and licence). Van Herck M, Goërtz Y. American Thoracic Society, MFI entry last changed May 2022
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Disclaimer: This calculator is for informational and personal journaling purposes only. It logs measurements and compares them to published reference values. It is not health, medical, or nutrition advice.