Metric: Rate of Force Development

Rate of Force Development, commonly abbreviated as RFD, measures how quickly force is produced. It is one of the most useful force-time metrics when a professional wants to understand not only how much force a client can produce, but how fast they can produce it.

Peak force tells you the highest force reached. RFD tells you how quickly force rises.

This matters because many real-world activities happen quickly. Sprinting, jumping, landing, stepping, changing direction, recovering balance and reacting to sport demands often require force to be produced in a short time window. A client may have good peak force, but if they produce that force too slowly, it may not be useful for faster tasks.

A high RFD generally suggests the client can produce force quickly in the tested task. A low RFD may suggest slower rapid force production, but it may also reflect pain, hesitation, fatigue, low confidence, poor instructions, unfamiliarity with the test, inconsistent setup or data-processing differences.

RFD is commonly used to characterise explosive strength in athletes, older adults and clinical populations, but peer-reviewed methodological reviews emphasise that RFD is highly sensitive to testing and analysis methods, so careful protocol control is essential.

Introduction

A client can be strong but slow.

That means they may be able to produce a high peak force if given enough time, but they may struggle to access that force quickly. For some clients, that may not matter much. For others, especially athletes, runners, older adults, and clients returning to dynamic activity, the speed of force production can be highly relevant.

Rate of Force Development helps answer:

“How quickly can this client produce force?”

In Measurz, RFD can help professionals monitor explosive strength qualities, compare sides, track progress across exercise blocks, identify differences between maximal strength and rapid force production, and educate clients about why “strength” and “fast force” are not the same thing.

RFD should not be used as a diagnosis, clearance test or standalone decision-maker. It is most useful when interpreted with peak force, impulse, time to peak, symptoms, movement quality, fatigue response, task demands and the client’s goals.

Quick Summary

• Metric name: Rate of Force Development

• Common abbreviation: RFD

• What it means: How quickly force is produced

• Simple formula: Change in force ÷ change in time

• Common units: N/s or N·s⁻¹

• Other possible units: kg/s, kgf/s, lb/s, lbf/s or device-specific force-time units

• Common testing methods: Isometric force testing, force plates, load cells, handheld dynamometry, fixed dynamometry, push-pull tests and jump testing

• Best use: Explosive strength profiling, side-to-side comparison, monitoring rapid force production, tracking training response and identifying differences between peak force and fast force output

• High RFD: Usually indicates faster force production in the tested task

• Low RFD: Usually indicates slower force production, but the reason must be interpreted with context

• Major limitation: RFD is highly sensitive to start threshold, sampling rate, filtering, time window, instructions and client familiarity

What Is Rate of Force Development?

Rate of Force Development measures the speed at which force increases.

A simple way to explain it:

Peak force = how much force was eventually produced.
RFD = how quickly force was produced.

If you are looking at a force-time graph, RFD is related to the steepness of the force curve. A steep curve means force increased quickly. A flatter curve means force increased more slowly.

Simple formula

RFD = change in force ÷ change in time

For example:

• Force increases from 0 N to 600 N

• Time taken = 0.20 seconds

• RFD = 600 ÷ 0.20

• RFD = 3000 N/s

This means the client developed force at a rate of 3000 newtons per second over that time window.

How Is RFD Measured?

RFD is calculated from a force-time curve.

A force-time curve shows how force changes from the start of a contraction or movement through to the end of the test.

RFD can be measured during:

• Isometric strength testing

• Isometric mid-thigh pull

• Isometric squat

• Push or pull testing

• Handheld or fixed dynamometry

• Force plate testing

• Jump testing

• Grip testing

• Load cell testing

• Muscle Meter-style force testing

Common units include:

• N/s

• N·s⁻¹

• kg/s, when the device displays force in kilograms

• kgf/s, when the device displays kilograms-force

• lb/s or lbf/s, when the device displays force in pounds

The safest approach is to record the exact unit shown by the device and use the same device, unit, test, time window and calculation method for retesting.

Why RFD Is Used

RFD is used because many tasks happen before a client has time to reach peak force.

For example:

• A sprint ground contact may be too short for the client to reach maximum force.

• A jump take-off requires rapid force production.

• A landing or cutting task requires quick force acceptance and redirection.

• A balance recovery step may require fast lower-limb force production.

• A sport contact or reaction task may require rapid bracing or pushing.

In these situations, the ability to produce force quickly may be more relevant than maximum force alone.

A systematic review of multi-joint isometric testing found that peak force, RFD and impulse can show relationships with dynamic performance, but the strength of those relationships varies depending on the test and performance task. This supports using RFD as part of a broader assessment profile rather than as a universal performance score.

What RFD Measures

RFD measures rapid force production.

It may provide context about:

• Explosive strength

• Rapid neuromuscular output

• Early force contribution

• Fast strength expression

• Side-to-side differences

• Force-time strategy

• Fatigue-related slowing

• Response to rapid-intent training

• Performance in time-limited tasks

• Readiness to progress toward faster exercise tasks

RFD does not directly measure:

• Peak strength

• Muscle size

• Tissue status

• Pain source

• Movement quality

• Coordination

• Balance

• Overall fitness

• Injury risk on its own

• Readiness to return to sport

RFD is a useful metric, but it does not explain why the result changed. It must be interpreted with context.

Types of RFD

RFD can be calculated in several ways. These methods are not interchangeable, so the time window and calculation method must always be recorded.

Early RFD

Early RFD is calculated during the first part of the contraction, commonly within time windows such as:

• 0–50 ms

• 0–100 ms

• 0–200 ms

Early RFD may be especially relevant for tasks that require force very quickly, such as sprinting, landing, stepping or reacting. However, early RFD is also more sensitive to noise, start threshold and measurement error.

Late RFD

Late RFD is calculated over a later period, such as:

• 100–200 ms

• 100–300 ms

• 200–300 ms

Late RFD may be more influenced by maximal strength capacity, depending on the test and client population.

Peak RFD

Peak RFD is the highest rate of force rise recorded during the contraction.

It can be useful, but it is also sensitive to signal noise, filtering and device sampling rate. Methodological research shows that filtering and starting-force threshold can substantially affect RFD values and reliability.

Average RFD

Average RFD is calculated across a defined time window.

This is often easier to interpret than peak RFD because the time window is clear.

Relative RFD

Relative RFD expresses rapid force production relative to body mass or peak force.

Examples include:

• N/s/kg

• N·s⁻¹·kg⁻¹

• RFD relative to peak force

Relative RFD can help when comparing people of different body sizes or understanding how quickly a client accesses their own maximal force capacity.

RFD vs Peak Force

RFD and peak force answer different questions.

Peak force asks:
“How much force can the client produce?”

RFD asks:
“How quickly can the client produce force?”

A client can have:

• High peak force and high RFD

• High peak force and low RFD

• Low peak force and high early RFD

• Low peak force and low RFD

This is why RFD should be interpreted with peak force, not instead of it.

Practical example

A client may produce 100 kg peak force in an isometric test, but take a long time to reach it.

Another client may produce 80 kg peak force, but produce force much faster in the first 100–200 ms.

Which profile is better depends on the client’s goal. For slow strength tasks, peak force may be more important. For sprinting, jumping, landing or reacting, RFD may add useful context.

RFD vs Impulse

RFD and impulse also tell different stories.

RFD asks:
“How quickly did force rise?”

Impulse asks:
“How much force was applied across time?”

A client may have high RFD but low impulse if they produce force quickly but do not sustain it.

Another client may have lower RFD but higher impulse if they produce force more gradually and maintain it longer.

Both can be useful. RFD explains the speed of force production. Impulse explains total force-time contribution.

RFD vs Time to Peak

RFD and Time to Peak are related but different.

RFD asks:
“How steeply did force rise?”

Time to Peak asks:
“How long did it take to reach the highest force?”

A client may have:

• High RFD and short time to peak

• High peak force but long time to peak

• Low RFD and long time to peak

• High early RFD but lower peak force

Using RFD with Time to Peak can help professionals understand the client’s force-time strategy.

What Does High RFD Mean?

A high RFD usually means the client produced force quickly during the tested task.

This may suggest:

• Faster force production

• Better explosive strength expression

• Stronger early force contribution

• Greater ability to access force quickly

• Better performance potential in time-limited tasks

• Positive adaptation to rapid-intent or explosive training

• Better neuromuscular readiness in that specific test context

High RFD may be a positive finding when:

• It improves under the same protocol

• Peak force is maintained or improved

• Symptoms remain stable or improve

• Movement quality remains acceptable

• It aligns with better jump, sprint, balance or function outcomes

• The client’s goal requires fast force production

However, high RFD is not automatically better in every setting.

A high RFD may be less meaningful if:

• The test was noisy

• Start detection was inconsistent

• The client pre-tensioned before the test

• The device sampling rate was too low

• The value came from one inconsistent trial

• The client sacrificed peak force or movement quality

• The client’s goal does not require rapid force production

Safer interpretation

“RFD was higher in this test, suggesting faster force production under this protocol. This should be interpreted with peak force, symptoms, movement quality, time window and task demands.”

What Does Low RFD Mean?

A low RFD usually means the client developed force more slowly during the tested task.

This may suggest:

• Slower rapid force production

• Reduced explosive strength expression

• Lower early force contribution

• Pain-related hesitation

• Apprehension or low confidence

• Fatigue

• Poor familiarisation

• Reduced peak force capacity

• Poor instructions

• Inconsistent start point

• Poor test setup

• Lower intent during the trial

Low RFD does not automatically mean the client is weak. A client may have good peak force but still produce force slowly.

Low RFD may be meaningful when:

• It is consistently lower than baseline

• It is lower on one side

• It occurs with slower movement performance

• It occurs with increased symptoms or apprehension

• It aligns with poor early impulse or longer time to peak

• It is relevant to the client’s sport, fitness or function goal

Low RFD may be less meaningful when:

• The test was not intended to be explosive

• The client did not understand the instruction

• Only one trial was recorded

• The start threshold changed

• The device or software changed

• The client was guarding due to discomfort

Safer interpretation

“RFD was lower in this test today, which may indicate slower force production across the measured time window. This should be interpreted with peak force, baseline, symptoms, test quality and task goals.”

How Health and Fitness Professionals Can Use RFD With Clients

1. Explain why strength speed matters

RFD is useful for helping clients understand that force production has a timing component.

You might say:

“Peak force tells us how much force you can produce. RFD tells us how quickly you can access that force.”

This can be especially helpful for clients who are strong in slow exercises but struggle with faster movement tasks.

2. Track explosive strength qualities

RFD can help monitor whether training is improving rapid force production.

This may be relevant for:

• Sprinting

• Jumping

• Landing

• Cutting

• Rapid stepping

• Balance recovery

• Sport contact

• High-speed gym-based movements

3. Compare sides

RFD may reveal side-to-side differences that peak force alone does not.

For example:

• Right and left peak force may be similar.

• One side may show slower early RFD.

This may suggest that both sides can eventually produce similar force, but one side accesses force more slowly.

4. Monitor response to training

RFD may improve after training that emphasises rapid intent, explosive actions or high-speed strength work.

However, RFD should be interpreted alongside peak force. If RFD improves but peak force drops, the meaning may be different from a case where both improve.

5. Add context to readiness and fatigue

A sudden drop in RFD may reflect fatigue, soreness, poor sleep, low motivation, pain or poor recovery. It should not automatically be treated as a problem, but it may be a useful conversation starter.

6. Support exercise progression

RFD may help guide progression from slow controlled force production toward faster tasks.

For example:

• Low peak force and low RFD may suggest a need to build base strength first.

• Good peak force but low RFD may suggest adding rapid-intent work when appropriate.

• Improving RFD with stable symptoms may support progression to faster movements, depending on movement quality and goals.

What RFD Means in Different Client Populations

General fitness clients

For general fitness clients, RFD is usually more advanced than peak force. Many clients will benefit first from tracking peak force, consistency, movement quality and exercise tolerance.

RFD becomes more useful when goals include:

• Running

• Jumping

• Agility

• Faster lifting

• Athletic development

• Dynamic exercise progression

• Improving quick reactions

Use RFD mainly as a trend metric rather than a pass/fail score.

Athletes and sport clients

For athletes, RFD is highly relevant because many sport actions occur in short time windows.

Examples include:

• Sprint acceleration

• Jump take-off

• Change of direction

• Landing

• Tackling or contact preparation

• Throwing

• Kicking

• Rapid deceleration

The major review by Maffiuletti and colleagues describes RFD as a commonly used measure for characterising explosive strength in athletes, older individuals and clinical populations, while emphasising that its assessment requires careful methodological control.

For athletes, RFD should be interpreted with:

• Peak force

• Impulse

• Time to peak

• Jump or sprint performance

• Training load

• Fatigue state

• Symptoms

• Sport demands

Older adults

For older adults, rapid force production may matter because many balance and functional tasks require force to be produced quickly.

Recent research on falls and lower-limb strength notes that lower-limb power and rapid force or torque development are affected by ageing and may be relevant to maintaining postural balance, although findings about falls associations can be inconsistent across studies.

For older adults, RFD should be interpreted with:

• Balance

• Gait

• Sit-to-stand performance

• Step speed

• Confidence

• Fear of falling

• Lower-limb strength

• Symptoms

• Daily function

RFD may be useful, but it should not replace practical functional testing.

Clients with pain or persistent symptoms

For clients with pain, RFD can be affected by discomfort, apprehension or protective strategy.

Low RFD may reflect:

• Pain-related hesitation

• Guarding

• Reduced confidence

• Fear of fast effort

• Reduced force capacity

• Fatigue

• Poor familiarisation

• Test discomfort

Record symptoms and pain score every time. Avoid saying low RFD proves inhibition or damage.

Use safer wording:

“This result may suggest slower force production during this test today.”

Post-injury or return-to-performance clients

RFD can provide useful context during return-to-performance monitoring because rapid force production may remain limited even when peak force improves.

For example:

• Peak force may return before early RFD improves.

• One side may show similar peak force but slower force development.

• Time to peak may remain longer even when maximal force looks acceptable.

RFD should support monitoring, not standalone clearance.

Youth clients

For youth clients, RFD should be interpreted carefully because growth, maturation, coordination and test familiarity can strongly affect rapid force production.

Changes may reflect:

• Growth

• Maturation

• Improved coordination

• Better understanding of the test

• Increased body mass

• Training adaptation

• Better confidence

Use youth-specific reference data only when available for the exact protocol. Otherwise, use baseline and repeat testing.

Higher body mass clients

For higher body mass clients, absolute RFD may be high, but relative RFD may provide better context for bodyweight tasks.

If the client needs to move their own body quickly, such as during stepping, running, jumping or balance recovery, body-mass-normalised RFD may be useful.

However, relative values still need careful interpretation because body composition, limb length, training history and test familiarity all influence results.

What If RFD Is Reported Relative to Body Weight or Peak Force?

RFD can be reported in several relative ways.

RFD relative to body mass

This may be expressed as:

• N/s/kg

• N·s⁻¹·kg⁻¹

This helps answer:

“How quickly is the client producing force relative to their body size?”

This can be useful for bodyweight tasks such as running, jumping, stepping and balance recovery.

RFD relative to peak force

This helps answer:

“How quickly does the client access their own maximum force capacity?”

For example, two clients may have different peak force values, but one may reach a higher percentage of their maximum force earlier in the contraction.

Important limitation

Relative RFD can be useful, but it does not remove all differences related to:

• Age

• Sex

• Body composition

• Limb length

• Training history

• Pain

• Motivation

• Familiarity

• Device method

• Protocol

Use the same calculation method across sessions.

Normative Data, Reference Data and Benchmarks

Are there universal RFD norms?

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

RFD values depend on:

• Test type

• Device

• Sampling rate

• Filtering

• Start threshold

• Time window

• Body position

• Joint angle

• Instructions

• Familiarisation

• Muscle group or task

• Whether the value is peak, early, late or average RFD

• Whether the value is absolute or normalised

• Age

• Sex

• Training history

• Symptoms

Because RFD is sensitive to testing and analysis methods, universal norms are not appropriate.

What does peer-reviewed research support?

The strongest evidence supports using RFD as a protocol-specific trend and comparison metric.

Maffiuletti and colleagues emphasise that RFD is useful for characterising explosive strength, but methodological choices such as instructions, signal processing, contraction onset and calculation method strongly influence results.

Lum and colleagues reviewed multi-joint isometric force-time characteristics and found that RFD, peak force and impulse can relate to dynamic performance, but results vary by task and protocol. This supports using RFD with matched testing methods rather than generic norms.

Moir and colleagues showed that filtering and starting-force thresholds can change RFD values and reliability during isometric back squat testing, reinforcing that RFD interpretation depends heavily on processing method.

Best evidence-based approach for Measurz

For most professional settings, interpret RFD using:

• Client baseline

• Side-to-side comparison

• Same test, same device and same protocol

• Time-window-specific comparison

• Body-mass-normalised values where relevant

• Published reference data only when the protocol and population match

• Related measures such as peak force, impulse, time to peak and functional performance

When published reference values are useful

Published RFD data may be useful only when they match:

• Same test

• Same device

• Same time window

• Same start threshold

• Same filtering method

• Same population

• Same units

• Same contraction instruction

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

Misconceptions About RFD

Misconception 1: RFD is the same as strength

No. RFD measures how quickly force is produced. Peak force measures how much force is produced.

Misconception 2: High RFD always means better performance

Not always. High RFD is useful when the task requires rapid force production. It may be less relevant for slower strength or endurance tasks.

Misconception 3: Low RFD always means weakness

No. Low RFD may reflect slow force production, but it can also reflect pain, hesitation, fatigue, low confidence, poor familiarisation or measurement differences.

Misconception 4: RFD can be compared across devices

Not safely unless the device, sampling rate, filtering and calculation method are comparable.

Misconception 5: Early RFD and peak RFD are the same

They are not. Early RFD, late RFD, average RFD and peak RFD may tell different stories.

Misconception 6: RFD has universal norms

No. RFD is highly protocol-specific and should be interpreted using matched reference data or repeated testing.

Limitations of RFD Testing

RFD is useful but sensitive.

It can be affected by:

• Device type

• Sampling rate

• Filtering

• Start threshold

• Contraction onset detection

• Time window

• Joint angle

• Body position

• Stabilisation

• Instructions

• Pre-tension

• Countermovement

• Familiarisation

• Pain

• Fatigue

• Motivation

• Effort

• Test anxiety

• Assessor technique

The most important limitation is that small methodological changes can meaningfully change RFD. This makes RFD less forgiving than peak force.

Where possible, use RFD as a trend metric under tightly standardised conditions.

How to Improve RFD Testing Quality

To improve RFD data quality:

• Use the same device each time.

• Use the same body position.

• Use the same joint angle where relevant.

• Use the same instructions.

• Use clear explosive intent cues.

• Avoid pre-tension unless it is part of the protocol.

• Use familiarisation trials.

• Record multiple trials.

• Use the same time window.

• Use the same start threshold.

• Use the same filtering method.

• Use the same scoring method.

• Record symptoms and pain.

• Record body mass if normalising.

• Interpret RFD with peak force and task demands.

A practical instruction is:

“Push as fast and as hard as possible.”

This is different from:

“Push as hard as possible.”

The first instruction emphasises rapid force production. The second may produce a slower ramp and reduce RFD relevance.

How to Record RFD in Measurz

Record:

• Metric: Rate of Force Development

• Score/result: RFD value

• Units: N/s, kg/s, kgf/s, lb/s, lbf/s or device-specific unit

• RFD type: early RFD, late RFD, peak RFD, average RFD or relative RFD

• Time window: for example, 0–50 ms, 0–100 ms, 0–200 ms or 100–200 ms

• Test name: isometric push, pull, squat, grip, mid-thigh pull, jump or other force test

• Side: left, right or bilateral

• Dominance: dominant or non-dominant side

• Position: seated, standing, supine, prone, side-lying or sport-specific position

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

• Contraction type: usually isometric unless otherwise defined

• Instruction: “fast and hard”, “as quickly as possible”, or other exact cue

• Trial number: trial 1, trial 2, trial 3

• Final score method: best score, average score or selected trial

• Body mass: if normalising RFD

• Pain score: before, during or after testing

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

• Effort quality: explosive, hesitant, submaximal or unclear

• Related metrics: peak force, impulse, time to peak, fatigue index, torque or functional test

• 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. RFD should not be positioned as diagnosing a condition or confirming readiness on its own.

Practical Examples

Example 1: High peak force, low RFD

A client has strong peak force but low early RFD.

This may suggest they can produce high force eventually, but not quickly. This may matter for sprinting, jumping, landing or rapid stepping.

Example 2: Improved RFD with stable peak force

A client’s peak force remains similar, but RFD improves after a training block.

This may suggest they are accessing their existing force capacity more quickly.

Example 3: Low RFD with pain

A client shows low RFD during a push test and reports apprehension.

This may reflect pain, guarding or confidence, not just reduced strength.

Example 4: Older adult balance context

An older adult shows low lower-limb RFD and also demonstrates slow stepping responses.

This may provide useful context for balance and function, but should be interpreted with gait, strength and functional testing.

Example 5: Athlete

A field sport athlete has good peak force but lower RFD on one side.

This may suggest a rapid force production difference that could be relevant to sprinting or change-of-direction tasks, depending on broader findings.

Example 6: Post-injury monitoring

A client restores peak force symmetry but still shows lower RFD on the previously affected side.

This may indicate that maximal force has improved, but rapid force production remains different.

FAQs

What is Rate of Force Development?

Rate of Force Development measures how quickly force is produced. It shows the speed of force production during a contraction or movement.

What units is RFD measured in?

RFD is commonly measured in N/s. It may also be reported in kg/s, kgf/s, lb/s, lbf/s or body-mass-normalised units.

Is RFD the same as peak force?

No. Peak force measures how much force is produced. RFD measures how quickly force is produced.

What does high RFD mean?

High RFD usually means the client produced force quickly in the tested task. This may be useful when the task requires rapid force production.

What does low RFD mean?

Low RFD usually means force was produced more slowly. It may reflect reduced explosive strength, pain, hesitation, fatigue, poor familiarisation or testing differences.

Are there universal RFD norms?

No. RFD is highly dependent on test, device, time window, sampling rate, filtering, start threshold and protocol. Use baseline, side-to-side comparison and matched reference data instead.

Is RFD useful for general fitness clients?

It can be, but it is usually more advanced than peak force. RFD is most useful when the client’s goals involve speed, agility, power, balance recovery or dynamic movement.

Is RFD useful for older adults?

It may be useful because rapid force production can be relevant to balance and functional tasks, but it should be interpreted with gait, strength, confidence and functional measures.

Can RFD diagnose a problem?

No. RFD can support assessment and monitoring, but it does not diagnose a condition or explain symptoms by itself.

Should RFD be used alone?

No. RFD should be interpreted with peak force, impulse, time to peak, symptoms, movement quality, baseline, function and client goals.

Key Takeaways

• RFD measures how quickly force is produced.

• It is useful for understanding rapid force production and explosive strength.

• High RFD generally suggests faster force production.

• Low RFD generally suggests slower force production, but context matters.

• RFD is especially relevant when tasks require force to be produced quickly.

• There are no universal RFD norms.

• RFD is highly sensitive to device, time window, start threshold, filtering and instructions.

• Measurz should record RFD with the exact protocol, unit, time window, symptoms and related metrics.

References

Lum, D., Haff, G. G., & Barbosa, T. M. (2020). The relationship between isometric force-time characteristics and dynamic performance: A systematic review. Sports, 8(5), Article 63. https://doi.org/10.3390/sports8050063

Maffiuletti, N. A., Aagaard, P., Blazevich, A. J., Folland, J., Tillin, N., & Duchateau, J. (2016). Rate of force development: Physiological and methodological considerations. European Journal of Applied Physiology, 116(6), 1091–1116. https://doi.org/10.1007/s00421-016-3346-6

Moir, G. L., Getz, A., Davis, S. E., Marques, M., & Witmer, C. A. (2019). The inter-session reliability of isometric force-time variables and the effects of filtering and starting force. Journal of Human Kinetics, 67, 139–151. https://doi.org/10.2478/hukin-2018-0049

Sato, S., Yoshida, N., Akagi, R., Numaguchi, S., & Takahashi, H. (2025). Association of lower-limb strength with different fall histories or physical performance in community-dwelling older adults. BMC Geriatrics, 25, Article 137. https://doi.org/10.1186/s12877-025-05685-3