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Fatigue Index Calculator

Track your fatigue index with our free sports calculator. Get personalized stats, rankings, and performance comparisons.

Reviewed by Sher, Sports Science & Nutrition Specialist

Reviewed by Sher, Sports Science & Nutrition Specialist

Formula

Fatigue Index (%) = (Peak Power - Minimum Power) / Peak Power x 100

Where Peak Power is the highest power output achieved during the test (typically in the first 5 seconds), and Minimum Power is the lowest power output (typically in the final 5 seconds). A lower fatigue index indicates better anaerobic endurance and ability to maintain power output during maximal effort. Power drop rate is calculated as (Peak - Min) / (Duration - Time to Peak).

Worked Examples

Example 1: 30-Second Wingate Test Result

Problem:A 78 kg cyclist achieves a peak power of 950W at second 4 and minimum power of 520W at second 28 during a 30-second Wingate test. Calculate the fatigue index and related metrics.

Solution:Fatigue Index = (950 - 520) / 950 x 100 = 45.3%\nPower drop rate = (950 - 520) / (30 - 4) = 430 / 26 = 16.5 W/s\nRelative peak power = 950 / 78 = 12.18 W/kg\nRelative min power = 520 / 78 = 6.67 W/kg\nAverage power = (950 + 520) / 2 = 735 W\nTotal work = 735 x 30 = 22,050 J = 22.05 kJ\nPower maintenance = 520 / 950 x 100 = 54.7%

Result:Fatigue Index: 45.3% (Below Average) | Power Drop: 16.5 W/s | Avg Power: 735W (9.42 W/kg)

Example 2: Team Sport Athlete Comparison

Problem:A 70 kg soccer midfielder achieves peak power of 820W and minimum power of 600W in a 30-second test (peak at second 5). Calculate their fatigue profile.

Solution:Fatigue Index = (820 - 600) / 820 x 100 = 26.8%\nPower drop rate = (820 - 600) / (30 - 5) = 220 / 25 = 8.8 W/s\nRelative peak power = 820 / 70 = 11.71 W/kg\nRelative min power = 600 / 70 = 8.57 W/kg\nAverage power = (820 + 600) / 2 = 710 W\nTotal work = 710 x 30 = 21,300 J = 21.30 kJ\nPower maintenance = 600 / 820 x 100 = 73.2%

Result:Fatigue Index: 26.8% (Good) | Power Drop: 8.8 W/s | Power Maintenance: 73.2%

Frequently Asked Questions

What is the Fatigue Index and what does it measure?

The Fatigue Index (FI) is a quantitative measure of the rate at which an athlete loses power output over the course of a maximal anaerobic effort, typically assessed using the Wingate Anaerobic Test (WAnT). It is calculated as the percentage difference between peak power and minimum power relative to peak power: FI = (Peak Power - Minimum Power) / Peak Power x 100. A lower fatigue index indicates better ability to maintain power output, which reflects superior anaerobic endurance and buffering capacity. The metric was originally developed for 30-second all-out cycling tests but has been adapted for repeated sprint protocols, rowing ergometer tests, and other maximal effort assessments across various sports disciplines.

What is considered a good fatigue index score?

Fatigue index scores vary by sport, position, and training focus, but general classifications have been established through decades of Wingate test research. Scores below 20% are considered excellent and are typically seen in trained endurance athletes who have developed superior lactate buffering capacity. Scores of 20-30% are good and common among well-conditioned team sport athletes. Scores of 30-40% are average for recreationally active individuals. Scores of 40-50% are below average and may indicate poor anaerobic endurance or insufficient conditioning. Scores above 50% are considered poor and suggest significant limitations in sustaining high-intensity effort. Sprint-focused athletes like 100m sprinters may have higher fatigue indices because their training prioritizes peak power over power maintenance.

What physiological factors determine the fatigue index?

Several interconnected physiological systems determine an individual fatigue index score. The phosphocreatine (PCr) system provides immediate energy for the first 5-10 seconds of maximal effort, and its depletion rate directly affects how quickly peak power drops. Glycolytic capacity determines how effectively the body can maintain high power output through anaerobic glycolysis during seconds 10-30. Lactate buffering capacity, primarily through muscle carnosine content and bicarbonate buffering, influences how well the body handles the hydrogen ion accumulation that inhibits muscle contraction. Muscle fiber type composition plays a major role because Type II (fast-twitch) fibers produce more power but fatigue faster than Type I (slow-twitch) fibers. Neural factors including motor unit recruitment patterns and firing rate maintenance also contribute significantly to the fatigue profile.

How can athletes improve their fatigue index?

Improving the fatigue index requires a targeted training approach that addresses anaerobic endurance rather than just peak power. High-intensity interval training (HIIT) with work intervals of 15-30 seconds and short recovery periods (1:2 to 1:3 work-to-rest ratio) specifically targets the glycolytic energy system. Repeated sprint training (RST) with 6-10 sprints of 5-10 seconds with 20-30 second recovery periods improves the ability to maintain power across repeated efforts. Beta-alanine supplementation has been shown to increase muscle carnosine levels, improving hydrogen ion buffering capacity and reducing fatigue by 3-5% in meta-analyses. Sodium bicarbonate loading before testing can acutely improve buffering capacity. Training at altitude or using blood flow restriction can also enhance anaerobic endurance adaptations over time.

References

Reviewed by Sher, Sports Science & Nutrition Specialist · Editorial policy