Metric: Fatigue Index

Fatigue Index describes how much performance declines across repeated efforts or across a sustained high-intensity test. It is commonly used in repeated sprint tests, Wingate-style anaerobic testing, repeated strength tests, isokinetic endurance testing, repeated jumps, force testing and sport-performance monitoring.

A high Fatigue Index generally means there was a larger drop-off in performance. A low Fatigue Index generally means the client maintained performance more consistently. However, a low Fatigue Index is not always better. A client can have a low Fatigue Index because they maintained high output, or because they never produced a strong initial output in the first place.

This is why Fatigue Index should always be interpreted with the raw test scores, including best score, worst score, mean score, total work, peak output, symptoms, effort, rest periods and the exact formula used.

Repeated-sprint research shows that fatigue during repeated sprint exercise is usually seen as a decline in sprint speed, peak power or total work across repetitions. However, repeated-sprint ability is better described by strong average performance across sprints, not just a low fatigue score.

Introduction

Many assessments tell you what a client can do once. Fatigue Index helps show what happens when they need to repeat or sustain effort.

This matters because many real-world activities are not single-effort tasks. Field sport athletes sprint repeatedly. Fitness clients complete circuits. Older adults may need to maintain effort across daily tasks. Runners and court sport athletes need repeated acceleration, deceleration and recovery. Strength clients may need to maintain output across multiple repetitions or sets.

Fatigue Index helps answer:

“How much did performance drop off across the test?”

In Measurz, Fatigue Index can help professionals monitor repeated-effort ability, compare sessions, identify performance drop-off, track response to conditioning or strength-endurance blocks, and educate clients about the difference between peak output and repeatability.

Fatigue Index should not be used as a diagnosis, clearance tool or standalone readiness score. It is most useful when interpreted with peak performance, average performance, total work, symptoms, RPE, recovery time, protocol, client goals and related assessment results.

Quick Summary

• Metric name: Fatigue Index

• What it means: How much performance declines across repeated or sustained effort

• Common outputs: Percentage drop-off, power decline, speed decline, force decline, torque decline, work decline or time change

• Common units: Percentage, watts, newtons, kilograms, pounds, seconds, metres or device-specific units depending on the test

• Common testing methods: Repeated sprint tests, Wingate tests, repeated jump tests, repeated isometric contractions, repeated strength tests and isokinetic endurance tests

• Best use: Understanding performance drop-off, repeated-effort tolerance, fatigue resistance and conditioning response

• High Fatigue Index: Usually indicates greater decline across the test

• Low Fatigue Index: Usually indicates smaller decline across the test

• Major limitation: Fatigue Index can be misleading if interpreted without peak output, average output and the exact calculation method

What Is Fatigue Index?

Fatigue Index is a measure of performance decline.

It describes how much a client’s output drops from the beginning, best effort or peak value to a later, lower or final value, depending on the formula used.

Fatigue Index may be calculated from:

• Sprint time

• Sprint speed

• Peak power

• Mean power

• Force

• Torque

• Jump height

• Work done

• Repetition output

• Distance covered

• Time to completion

There is no single universal Fatigue Index formula. The formula depends on the assessment.

For example, in repeated sprint testing, fatigue may be expressed as a percentage decrement across sprints. In Wingate-style anaerobic testing, Fatigue Index commonly reflects the percentage decline in power from peak to minimum power during the test. A Wingate review describes Fatigue Index as representing the loss of power after maximum power is reached, usually expressed as a percentage of maximum power.

How Is Fatigue Index Measured?

Fatigue Index is measured by comparing performance across a repeated or sustained test.

Common examples include:

Repeated sprint testing

The client performs several sprints with short recovery periods. Fatigue Index may describe the drop-off in sprint performance across the test.

Common outputs include:

• Fastest sprint

• Slowest sprint

• Mean sprint time

• Total sprint time

• Percentage decrement

• Performance drop-off

Repeated-sprint ability research describes fatigue as a decline in maximal or mean sprint speed during running repeated-sprint exercise, or a decrease in peak power or total work during cycling repeated-sprint exercise.

Wingate-style testing

The client performs a 30-second maximal cycling test. Fatigue Index may describe the percentage drop from peak power to minimum power.

Common outputs include:

• Peak power

• Mean power

• Minimum power

• Fatigue Index

• Total work

The Wingate test is commonly used to assess short-term high-intensity performance, but its outputs should be interpreted with protocol, resistance setting, body mass and client familiarity.

Isokinetic endurance testing

The client performs repeated joint actions on an isokinetic dynamometer. Fatigue Index may describe decline in torque or work across repetitions.

Research in isokinetic testing has compared peak torque and fatigue index across sex and training groups, showing that fatigue measures can differ by training status and population.

Repeated strength or force testing

The client performs repeated contractions, pulls, pushes or holds. Fatigue Index may describe decline in peak force, mean force, impulse or total work.

Repeated jump testing

The client performs repeated jumps. Fatigue Index may describe decline in jump height, peak force, flight time, impulse or power output.

Common Fatigue Index Formulas

Because formulas vary, the exact calculation should always be recorded.

Common approaches include:

1. Percentage drop from best to worst

Fatigue Index = [(Best score − Worst score) ÷ Best score] × 100

This is commonly used when higher values are better, such as power, force, jump height or speed.

2. Percentage increase in time

Fatigue Index = [(Slowest time − Fastest time) ÷ Fastest time] × 100

This may be used for repeated sprint times, where a longer time means worse performance.

3. Wingate-style power decline

Fatigue Index = [(Peak power − Minimum power) ÷ Peak power] × 100

This describes the percentage power drop from peak to minimum power.

4. Percentage decrement score

In repeated sprint testing, percentage decrement approaches may use total sprint time compared with ideal sprint time, where ideal sprint time is often fastest sprint multiplied by number of sprints.

These methods can provide different results. Research discussing repeated sprint fatigue measures notes that different mathematical approaches have different reliability and usefulness, and that fatigue scores can be problematic when performance drop-off is small.

Why Fatigue Index Is Used

Fatigue Index is used because peak performance alone does not show whether a client can repeat or sustain output.

For health and fitness professionals, Fatigue Index can help answer:

• Can the client maintain output across repeated efforts?

• Does performance drop sharply after the first few trials?

• Has repeated-effort ability improved after training?

• Is the client pacing instead of giving maximal effort?

• Is the client’s peak output high but repeatability poor?

• Is fatigue linked with symptoms, pain, confidence or movement quality?

• Does conditioning, strength endurance or recovery appear to be improving?

• Is the client’s performance profile more “high peak, high drop-off” or “moderate peak, stable output”?

Repeated-sprint ability is important in many team and racket sports because short sprints are often repeated with brief recovery periods. However, researchers have emphasised that good repeated-sprint ability is better represented by strong average performance across repeated sprints, not simply by a low Fatigue Index.

What Fatigue Index Measures

Fatigue Index measures performance decline across a defined test.

It may provide context about:

• Repeatability

• Fatigue resistance

• Performance maintenance

• Anaerobic fatigue

• Strength-endurance

• Power-endurance

• Recovery between efforts

• Pacing strategy

• Conditioning response

• Training adaptation

• Symptom-related performance drop-off

Fatigue Index does not directly measure:

• Fitness as a whole

• Aerobic capacity by itself

• Anaerobic capacity by itself

• Muscle damage

• Pain cause

• Readiness to return to sport

• Movement quality

• Motivation

• Recovery status on its own

• Why fatigue occurred

Fatigue Index is a summary of decline. It does not explain the mechanism behind that decline.

What Does a High Fatigue Index Mean?

A high Fatigue Index usually means the client showed a larger performance drop-off during the test.

This may suggest:

• Reduced ability to maintain output

• Poorer repeated-effort tolerance

• Reduced fatigue resistance

• Inadequate recovery between efforts

• High initial output followed by rapid decline

• Poor pacing strategy

• Anaerobic fatigue

• Local muscular fatigue

• Reduced conditioning for the test demand

• Pain, symptoms or apprehension affecting later efforts

• Incomplete recovery from previous training or activity

A high Fatigue Index may be meaningful when:

• The client gave a true maximal effort

• The protocol was consistent

• The drop-off is repeated across sessions

• It aligns with performance decline, symptoms or high RPE

• The client’s activity requires repeated efforts

• Peak and mean outputs are also considered

However, a high Fatigue Index is not automatically bad. A client may have a high Fatigue Index because they produced an excellent first effort and then declined. In this case, the high drop-off may reflect a high peak output rather than poor capacity overall.

Safer interpretation

“Fatigue Index was higher in this test, suggesting greater performance decline across the protocol. This should be interpreted with peak output, mean output, total work, symptoms, RPE, rest periods and the client’s goal.”

What Does a Low Fatigue Index Mean?

A low Fatigue Index usually means the client maintained performance more consistently across the test.

This may suggest:

• Better fatigue resistance

• Better repeated-effort tolerance

• More stable output

• Better pacing

• Improved conditioning

• Better recovery between efforts

• Lower performance decline

• Improved strength-endurance or power-endurance, depending on the test

However, a low Fatigue Index is not always better.

A low Fatigue Index may occur because the client:

• Did not give a maximal first effort

• Paced the test

• Had low peak output

• Produced consistently moderate scores

• Was unfamiliar with maximal testing

• Avoided pushing hard due to symptoms or fear

This is why Fatigue Index must be interpreted with best score, mean score and total work.

A client with a low fatigue index but poor average performance may not have good repeated-effort ability. Repeated-sprint literature specifically warns that strong repeated-sprint ability is not simply a low fatigue score; it requires high average sprint performance as well.

Safer interpretation

“Fatigue Index was lower in this test, suggesting less performance decline across the protocol. This should be interpreted with peak output, average output, total work and whether the client gave a true maximal effort.”

Fatigue Index vs Peak Output

Peak output tells you the best performance achieved.

Fatigue Index tells you how much performance declined.

A client can have:

• High peak output and high Fatigue Index

• High peak output and low Fatigue Index

• Low peak output and low Fatigue Index

• Low peak output and high Fatigue Index

Practical example

Client A:

• Sprint 1: very fast

• Later sprints: much slower

• High peak output, high fatigue

Client B:

• All sprints: moderate

• Little drop-off

• Low fatigue, but lower performance

Client A may have better top-end capacity. Client B may have better consistency. Which is more useful depends on the client’s sport, goal and role.

Fatigue Index vs Mean Output

Mean output is often just as important, and sometimes more useful, than Fatigue Index.

For repeated sprint ability, researchers have argued that average sprint performance is central. A low fatigue score without high average performance may not represent good repeated-sprint ability.

Practical interpretation

If Fatigue Index is low but mean output is poor:

• The client maintained performance, but at a low level.

If Fatigue Index is high but mean output is strong:

• The client declined, but still performed well overall.

If both mean output and Fatigue Index improve:

• The client may be producing better repeated performance with less drop-off.

Fatigue Index vs Total Work

Total work describes the full amount of output completed across the test.

In some tests, total work may provide better context than Fatigue Index alone.

For example, in repeated cycling or strength tests:

• Two clients may have the same Fatigue Index.

• One may complete much more total work.

• The higher total work result may represent better performance despite similar decline.

This is why Measurz should record both fatigue and output variables wherever possible.

How Health and Fitness Professionals Can Use Fatigue Index With Clients

1. Explain repeatability

Fatigue Index helps clients understand the difference between one strong effort and repeated performance.

You might say:

“Your best effort shows your peak. Fatigue Index shows how much your performance dropped across the test.”

2. Monitor conditioning response

If Fatigue Index decreases while peak and mean output are maintained or improved, this may suggest improved fatigue resistance.

3. Track repeated sprint ability

For field, court and racket sport clients, repeated efforts are often important. Fatigue Index can help monitor whether sprint or power output drops across repeated efforts.

4. Monitor strength-endurance

In repeated strength or isokinetic tests, Fatigue Index may show whether torque, force or work declines across repetitions.

5. Identify pacing

A very low Fatigue Index with low peak output may suggest pacing or submaximal effort. This should prompt a review of instructions and effort quality.

6. Support exercise progression

Fatigue Index can help guide progression when the client’s goal involves repeated efforts.

For example:

• High drop-off may suggest building repeat-effort capacity.

• Stable output with improved mean performance may support progression.

• Drop-off linked with symptoms may suggest modifying load or rest periods.

7. Educate using the full profile

A useful explanation might be:

“Your fatigue score improved, but your average output was also higher. That means you maintained better performance, not just paced the test.”

What Fatigue Index Means in Different Client Populations

General fitness clients

For general fitness clients, Fatigue Index can be useful during repeated strength, circuit, cycling, running or conditioning tests.

It may help track:

• Conditioning progress

• Repeated effort tolerance

• Recovery between efforts

• Strength-endurance

• Exercise tolerance

• Client pacing

Use baseline and retesting rather than athlete norms.

Athletes and sport clients

For athletes, Fatigue Index is useful when the sport requires repeated high-intensity efforts.

Relevant sports may include:

• Football codes

• Soccer

• Basketball

• Hockey

• Tennis

• Rugby

• Netball

• Combat sports

• Court sports

Repeated-sprint ability research describes the ability to recover and reproduce sprint performance as an important fitness requirement in team and racket sports.

For athletes, Fatigue Index should be interpreted with:

• Best effort

• Mean effort

• Total work

• Recovery time

• Sprint speed or power

• Sport demands

• Training load

• Symptoms

• RPE

Older adults

For older adults, Fatigue Index may be useful in repeated sit-to-stand, step, strength or functional endurance tests. However, interpretation should focus on function, confidence, symptoms and safety.

A high Fatigue Index may indicate reduced ability to sustain repeated output, but it may also reflect unfamiliarity, pacing, pain, fear, balance concerns or general deconditioning.

Use caution with maximal repeated tests in older clients. Choose protocols appropriate to the client’s capacity and goals.

Clients with pain or persistent symptoms

For clients with pain, Fatigue Index can be affected by symptoms, guarding and confidence.

A higher Fatigue Index may reflect:

• Pain increasing across repetitions

• Protective strategy

• Fear of repeated loading

• Local fatigue

• Reduced confidence

• Symptom-related pacing

Always record pain score, symptoms and effort quality.

Avoid saying:

“This proves poor endurance.”

Use safer wording:

“This may suggest greater performance decline during this test today, which should be interpreted with pain, symptoms and effort.”

Post-injury or return-to-performance clients

Fatigue Index may help monitor whether repeated-effort capacity is improving.

For example:

• Sprint times may be good early but decline sharply.

• Repeated jump height may drop across trials.

• Force output may decline across repeated contractions.

This can help identify whether performance is sustained across repeated tasks, but it should not be used alone for return-to-performance decisions.

Youth clients

For youth clients, Fatigue Index should be interpreted carefully because motivation, growth, maturation, pacing and test familiarity can affect results.

A high or low fatigue score may reflect:

• Pacing

• Understanding of instructions

• Growth-related coordination changes

• Motivation

• Training age

• Maturation

• Fitness

Use age- and sport-specific comparison only if the protocol matches closely.

Higher body mass clients

For higher body mass clients, fatigue may appear differently depending on the task.

Bodyweight tasks may create greater mechanical demand. Absolute output may be high, but repeated performance may decline if the task requires repeatedly moving body mass.

Use both absolute and relative outputs where relevant, and interpret Fatigue Index with symptoms, RPE and movement quality.

Normative Data, Reference Data and Benchmarks

Are there universal Fatigue Index norms?

No. There are no universal Fatigue Index norms that apply across all clients, tests, devices and populations.

Fatigue Index values depend on:

• Test type

• Formula used

• Number of repetitions

• Work duration

• Rest duration

• Exercise mode

• Output variable

• Client motivation

• Pacing

• Device

• Protocol

• Population

• Sport

• Training status

• Symptoms

• Environmental conditions

Because of this, Fatigue Index should be interpreted using test-specific reference data, matched protocols and baseline comparison.

What does peer-reviewed research support?

Repeated-sprint research supports fatigue measures as useful, but also highlights important limitations. Glaister and colleagues specifically questioned whether fatigue index is a worthwhile measure of repeated sprint ability, noting reliability and interpretation issues across different fatigue calculations.

Repeated-sprint ability reviews also emphasise that good repeated-sprint ability is not simply a low fatigue score; it should involve strong average sprint performance with or without a low fatigue index.

Wingate research commonly reports Fatigue Index as one of several outputs, alongside peak power and mean power. A large dataset of male soccer players found age was related to peak and mean power, but not fatigue index, which shows that Fatigue Index may not behave the same way as other performance metrics and should be interpreted carefully.

Best evidence-based approach for Measurz

For most professional settings, interpret Fatigue Index using:

• Client baseline

• Same test and same protocol

• Same formula

• Best score

• Mean score

• Worst score

• Total work or total output

• RPE

• Pain or symptoms

• Rest periods

• Environmental context

• Published reference data only when protocol and population match

When published reference data is useful

Published fatigue data may be useful only when it matches:

• Same test

• Same formula

• Same number of repetitions

• Same work-to-rest ratio

• Same output variable

• Same device

• Same population

• Same sport or activity level

If these details do not match, published values should be treated as broad context rather than strict benchmarks.

Misconceptions About Fatigue Index

Misconception 1: Low Fatigue Index always means good fitness

Not always. A low Fatigue Index may mean the client maintained performance well, or it may mean they paced the test or had low peak output.

Misconception 2: High Fatigue Index always means poor fitness

Not always. A high Fatigue Index may occur because the client produced a very strong first effort and then declined.

Misconception 3: Fatigue Index is the same as endurance

No. It is one fatigue-related metric from one specific protocol. Endurance is broader.

Misconception 4: Fatigue Index can be interpreted without the raw scores

No. Always review peak, mean, worst and total output.

Misconception 5: One formula works for all tests

No. Different tests use different fatigue calculations.

Misconception 6: Fatigue Index explains why fatigue occurred

No. It only describes the decline. It does not identify the cause.

Limitations of Fatigue Index Testing

Fatigue Index can be affected by:

• Formula selection

• Test type

• Number of efforts

• Rest periods

• Pacing

• Motivation

• Pain

• Symptoms

• Familiarisation

• Warm-up

• Sleep

• Training load

• Hydration

• Heat

• Surface

• Device

• Timing accuracy

• Trial validity

• Whether the first effort was truly maximal

A major limitation is that Fatigue Index can look “good” when the client underperforms from the start. This is why the raw performance data matter.

How to Improve Fatigue Index Testing Quality

To improve Fatigue Index data quality:

• Use the same test each time.

• Use the same formula each time.

• Record the formula.

• Standardise warm-up.

• Standardise work and rest periods.

• Provide clear maximal-effort instructions.

• Record best, worst, mean and total output.

• Record RPE.

• Record pain and symptoms.

• Record environmental conditions.

• Record whether pacing was observed.

• Use familiarisation when appropriate.

• Avoid comparing different protocols.

• Interpret fatigue with performance, not separately from it.

How to Record Fatigue Index in Measurz

Record:

• Metric: Fatigue Index

• Score/result: fatigue value

• Units: percentage, watts, seconds, kg, lb, N, N·m, metres, repetitions or device-specific unit

• Formula used: exact calculation

• Test name: repeated sprint, Wingate, repeated jump, repeated push/pull, isokinetic endurance or other test

• Output variable: time, speed, force, torque, power, work, jump height or repetitions

• Number of repetitions or efforts

• Work duration

• Rest duration

• Best score

• Worst score

• Mean score

• Total work or total output

• Side: left, right or bilateral if relevant

• Dominance: if relevant

• Device used: force plate, load cell, dynamometer, ergometer, timing gates, Muscle Meter or other device

• Trial validity: completed, paced, stopped, invalid or modified

• Pain score: before, during or after testing

• Symptoms: pain, breathlessness, dizziness, cramping, fatigue, apprehension or none

• RPE: perceived effort

• Environmental context: heat, surface, footwear or other relevant factors

• Baseline comparison: previous result

• Retest date: planned follow-up

• Progress note: contextual factors that may explain the result

Measurz should be used to support measurement, comparison, monitoring, education and progress tracking. Fatigue Index should not be positioned as diagnosing a condition, confirming fitness or clearing participation on its own.

Practical Examples

Example 1: High peak, high fatigue

A client produces an excellent first sprint but slows substantially across later sprints.

This may suggest strong peak speed but reduced repeatability.

Example 2: Low fatigue, low output

A client has very little drop-off, but all efforts are slow.

This may suggest consistent performance, but not necessarily strong repeated-effort ability.

Example 3: Improved fatigue and improved mean output

A client improves mean sprint time and shows less drop-off.

This is a stronger sign of improved repeated performance than Fatigue Index alone.

Example 4: Pain-related fatigue

A client’s force output drops across repeated contractions and pain increases.

This may reflect symptom-related performance decline, not just physical fatigue.

Example 5: Athlete

A field sport athlete shows strong peak sprint speed but a large percentage decrement.

This may suggest a need to improve repeated-effort tolerance, depending on their sport demands.

Example 6: General fitness client

A client completes repeated push tests. Their best score stays similar, but their later repetitions improve.

This may suggest improved force repeatability.

FAQs

What is Fatigue Index?

Fatigue Index measures how much performance declines across a repeated or sustained test.

What does a high Fatigue Index mean?

A high Fatigue Index usually means greater performance drop-off. It may suggest reduced repeatability, fatigue resistance or recovery between efforts, but it must be interpreted with peak and average output.

What does a low Fatigue Index mean?

A low Fatigue Index usually means less performance drop-off. This may suggest better maintenance of output, but it may also occur if the client paced or underperformed early.

Is a lower Fatigue Index always better?

No. A low Fatigue Index is only useful if the client also produced strong peak and mean performance.

Are there universal Fatigue Index norms?

No. Fatigue Index depends on the test, formula, work-to-rest ratio, output variable, device and population.

What is the best way to interpret Fatigue Index?

Use the client’s baseline, repeat testing under the same protocol, best score, mean score, total work, symptoms and RPE.

Can Fatigue Index diagnose poor fitness?

No. It can support assessment and monitoring, but it does not diagnose fitness level or explain why fatigue occurred.

What formula should I use?

Use the formula appropriate for the test and record it clearly. Do not compare results from different formulas.

Why can Fatigue Index be misleading?

Because a client can have a low Fatigue Index by pacing or starting with low output. Always review raw performance scores.

Should Fatigue Index be used alone?

No. It should be interpreted with peak output, mean output, total work, symptoms, RPE and task demands.

Key Takeaways

• Fatigue Index measures performance decline across repeated or sustained effort.

• High Fatigue Index usually means greater drop-off.

• Low Fatigue Index usually means better maintenance, but only if output is genuinely high.

• Fatigue Index is formula-dependent.

• There are no universal Fatigue Index norms.

• Fatigue Index should always be interpreted with peak, mean, worst and total output.

• Measurz should record the formula, protocol, raw scores, symptoms, RPE and related findings.

References

Bar-Or, O. (1987). The Wingate anaerobic test: An update on methodology, reliability and validity. Sports Medicine, 4(6), 381–394. https://doi.org/10.2165/00007256-198704060-00001

Costa, G. A., Freire, B., Oliveira, A. S., de Azevedo, P. H. S. M., & Ritti-Dias, R. M. (2017). Comparison of elbow flexor isokinetic peak torque and fatigue index between men and women of different training levels. European Journal of Translational Myology, 27(4), 7070. https://doi.org/10.4081/ejtm.2017.7070

Girard, O., Mendez-Villanueva, A., & Bishop, D. (2011). Repeated-sprint ability—Part I: Factors contributing to fatigue. Sports Medicine, 41(8), 673–694. https://doi.org/10.2165/11590550-000000000-00000

Glaister, M., Howatson, G., Pattison, J. R., & McInnes, G. (2008). The reliability and validity of fatigue measures during multiple-sprint work: An issue revisited. Journal of Strength and Conditioning Research, 22(5), 1597–1601. https://doi.org/10.1519/JSC.0b013e318181ab80

Glaister, M., Stone, M. H., Stewart, A. M., Hughes, M. G., & Moir, G. L. (2008). Is a fatigue index a worthwhile measure of repeated sprint ability? Journal of Science and Medicine in Sport, 11(1), 20–23. https://doi.org/10.1016/j.jsams.2007.02.013

Nikolaidis, P. T., Matos, B., Clemente, F. M., & Bezerra, P. (2018). Normative data of the Wingate anaerobic test in 1 year age groups of male soccer players. Frontiers in Physiology, 9, Article 1619. https://doi.org/10.3389/fphys.2018.01619

Vandewalle, H., Pérès, G., Heller, J., Panel, J., & Monod, H. (1987). Force-velocity relationship and maximal power on a cycle ergometer: Correlation with the height of a vertical jump. European Journal of Applied Physiology and Occupational Physiology, 56(6), 650–656. https://doi.org/10.1007/BF00424805