The Rapid Spike of RFD.

Paweł Krotki-Borowicki
Sep 23, 2025
11 min read

This time without intros and quotes.

If an athlete’s RFD amounts, say, to 6000 N/s, this does not at all mean, although it may seem so, that in a given trial the athlete will reach the 6000 N after the lapse of one second. It will not happen for one basic reason: the athlete is limited by the “ceiling” of maximal force, which prevents it from further develompnent. These circumstances explain the significance of one of the most frequently discussed and at the same time still not fully understood and exceptionally dynamic performance metrics, which is the ‘rate of force development’ (RFD).

Rate of Force Development (RFD).

Generation, acceleration and deceleration of dynamic forces of different magnitudes and velocities constitute one of the secrets of performance in sport. In sprinting, ground contact time of the foot amounts to <100 ms ¹, and in tasks such as early acceleration, cutting during change of direction or jumps it falls within the range of 120–300 ms ². Peak strain and rupture of the anterior cruciate ligament (ACL) occur already within the first 50 ms from the moment of ground contact ³ and a typical sprint–related hamstring injury occurs at approx. 120–150 ms before heel strike ⁴. That is precisely why the rate of force development (RFD), or, as I prefer to call it in my own way: “sudden force bursts”, are of such enormous significance in performance training and injury prevention.

RFD naturally scales to the available maximal force and in elite athletes rises about 20–30% more abruptly ⁵. This is expressed as an almost “instantaneous” intensification of the contraction action in relation to the “ceiling” of peak forces (Diagram 1). At the same time, RFD is strongly correlated with the state of freshness of the nervous system, which determines how steep the slope of force increase over time will be ⁶. The metaphor of an “explosion” of force, popular in practice, aptly reflects the character of ultra–fast behaviors that play a critical role in sport. For example: angular velocities in the hip and knee joint of elite sprinters may reach angular velocities on the order of 700–900°/s ⁷. This means that in peak moments, if the hip and knee behaved like free ball–and–socket joints, their rotational movements of 360° would theoretically last merely 0.4 s.

This description, although devoid of a vivid picture of dynamic complex system theory, is at present one of the best, precise explanations of what the dynamics of strenuous athletic movements under high–intensity conditions are. Thanks to this, it is easier to understand the basic argument that RFD in sports science practice is measured in short, limited “time windows,” falling within the range of 0–200 ms ⁸ (Diagram 1)—appropriate for majority of the sporting efforts, which “takes place” precisely in such smaller fragments of a second.

Diagram 1. A graph of force development over time with a marked window that is particularly sensitive to differences in its rate of increase (RFD). Three cases of force expression are presented: (1) resistance training aimed at reaching peak force (PF), (2) explosive–ballistic training oriented toward generating fast forces (scaling to PF), and (3) characteristic of untrained individuals.

Measurement Methods

Testing RFD goes beyond the subjective observation of movement—the detection of differences in RFD values requires that the expert (coach or physiotherapist) possess appropriate lab setup. In practice, dynamometers enabling the most rigid possible fixation relative to an immovable anchoring point are most often used (therefore, hand–held dynamometers are not suitable for this purpose), as well as force plates that are becoming increasingly affordable, such as the Force Decks by Vald Performance. A prerequisite for the reliability of measurement is sampling at the level of 1000 Hz, which makes it possible to precisely reproduce and linearly estimate the course of the force–time function in the critical 0–200 ms windows ⁹.

Testing schemes usually include the countermovement jump (CMJ), selected plyometric tests such as the hop jump or single–leg drop jump, as well as various isometric trials, most often oriented toward muscle groups in which force deficits are suspected. The selection of tests should be meticulous, so that in the given context of sport and/or injury the focus remains on the most important aspects, instead of testing everything without clear necessity ¹⁰.

Examples of training methods and typical tests enabling the assessment of RFD variables. Scroll through the slides by clicking on the arrow and play each exercise by pressing the video field. The presentation introduces a new template of interactive and practice–centered slides that I am creating for the future e–learning platform.

Note that the above methods are arranged according to the step–by–step scaling principle: they begin with isolated and isometric muscle testing, then include the effectiveness of jumps under conditions of slow and fast SSC (at the threshold of ≤250 ms ground contact time). They lack measurement of RFD outcomes in running conditions and change–of–direction tasks, which may seem valuable, however everyday S&C practice, apart from simple qualitative video analysis, doesn't usually include such tests due to tech limitations—they are also rarely the subject of scientific research.

Peculiar Variability

RFD is a parameter extremely susceptible to variability—both biological and measurement–related. Its reliable assessment requires an understanding of the sources of fluctuations and precise standardisation of the research protocol. This variability between trials may reach as much as 20–40%, particularly in early and short time windows (0–50 ms), where the reliability index (ICC) is often moderate (approx. 0.60–0.70) ¹¹. In later intervals (50–200 ms), this reliability increases significantly and may reach ICC values even above 0.90, provided that the applied testing protocol is appropriately coached ¹². This is why “familiarisation” with the test, a clear explanation of its purpose and athlete's motivation are so important ¹³.

Fatigue and micro–muscle damage affect the reduction of RFD faster than maximal strength, which makes RFD considered a much more sensitive indicator of neuromuscular fatigue ¹⁴. It is observed that after intensive eccentric exercises, decreases in RFD persist longer than decreases in peak force, which may be significant when assessing readiness for subsequent training sessions ¹⁵. The type of execution instruction: ‘ballistic’ or ‘sustained’ has a considerable impact on RFD values—contractions performed with the intention of the fastest possible force increase are characterised by higher RFD values in the 100–200 ms range. This effect is particularly visible in the case of short, dynamic trials of the SHORT type (~1 s), compared to the “traditional” and longer isometric trials TRAD (~5 s), designed for easier achievement of PF ¹².

In rehabilitation after anterior cruciate ligament reconstruction (ACLR), it is observed that despite improvements in PF, jump height or force impulse, early RFD <100 ms may remain reduced even 6–12 months after surgery. Such a deficit may mask the real risk of reinjury and not be visible in classic maximal strength tests ¹⁶. Therefore, in monitoring the progress of rehab or physical preparation, the sub–metric of RFD₀₋₁₀₀ ₘₛ may be a better indicator of neuromuscular performance than max values alone—it is precisely in the early phase of RFD that the nervous system has the greatest influence on the “burst” of firing rate and the synchronisation of motor units sudden recruitment ¹⁷. Later intervals of 150–200 ms may be associated to a greater extent with muscle architecture and the pennation angle of its fibers ¹⁸.

Importantly, studies indicate that the smallest worthwhile change for RFD may amount to as much as 15%, whereas for maximal strength this value often does not exceed 5%. This means that small changes in RFD, observed for example after a training intervention, may fall within the bounds of natural variability and should not be automatically interpreted as an adaptive effect ¹⁹. This raises the following question: does a large part of RFD variability result from biological determinants, i.e., fatigue, transient fluctuations of neural excitation and motivation or does the limitation remain the (quite substantial) difficulty of standardisation and repeatability of tests?

Normative values.

At present we do not possess RFD results that could be regarded as fully reliable normative data. Establishing such norms would require complete normalisation of tests, identical time windows, consideration of the specificity of a given sport type and target population, as well as a consistent manner of presenting results. Consequently, each team of S&C coaches and “sport scientists” collects RFD data according to its own unique protocol, whereby the final results—using the terminology of the theory of science—are not fully “replicable” across different worldwide settings (contexts) ⁸.

Tolerance of forces also appears to have a highly specific character. It is assumed that eccentric RFD exceeds concentric values, with athletes performing frequent and intense decelerations, i.e. basketball players or combat sport athletes, typically displaying even higher values ²⁰. Since the simplest point of reference remains body mass, RFD values are increasingly expressed in units of [N·s⁻¹·kg⁻¹] or presented as relational indices, comparing RFD with another strength parameter in a given task, i.e max force (PF) ²¹. Nevertheless, reliable norms derived from field tests—such as sprints, change of direction or situational drills—are still lacking, which limits the practical usefulness of current RFD assessment standards.

The results of selected isometric tests may serve as a guideline. In a study involving IMTP, RFD₀₋₅₀ ₘₛ in healthy individuals reached 3300–4300 N·s⁻¹, and RFD₀₋₁₀₀ ₘₛ 3200–3700 N·s⁻¹ ²². A well–prepared knee extension test at 60° flexion and a one–second SHORT trial offers RFD₀₋₂₀₀ ₘₛ results >6000 N·s⁻¹ ²³. The commonly used ballistic CMJ is characterized by high variability; however, a certain point of reference may be mean RFDCON values close to ~4424 ±2293 N·s⁻¹ and RFDECC ~7217 ±2845 N·s⁻¹ ²⁴. This confirms one of the universal principles of sports performance mentioned earlier: rapid deceleration is associated with higher force values ²⁵. Does RFD therefore possess its own “ceiling”? Indeed, the maximal observed values in CMJ studies reach ~7000–8000 N·s⁻¹ for CON and may rise even to 10,000 N·s⁻¹ for ECC ²⁴.

The implications discussed and the entire confusion of finding oneself in the “beautiful mess” of such sensitive variables as RFD incline insightful performance practitioners to use RFD₅₀₋₂₀₀ ₘₛ as a more precise indicator of “rapid dynamics” of athletic movement, which sounds reasonable, but is not a golden standard, let alone one confirmed by science ²⁶. To top it off—it suffices to recall the natural differences in RFD profiles between limbs, observed for instance during unilateral jumps. These—as well as other nuances, of which I myself am surely unaware, argue for the need to create individual scales containing ranges interpreted as “healthy” or “abnormal.”

Case study: ACLR.

Let us discuss the RFD results in the course of model rehab after anterior cruciate ligament (ACL) reconstruction in an elite–level soccer player. A graphical visualisation of the data, set along the timeline following ACLR and enriched with the context of medical events and RTS decisions, will tell us much more than barren theorising:

Diagram 2. Trends evolution of RFD₀₋₂₀₀ ₘₛ in the CON phase (solid line—lower results) and ECC phase (dashed line—higher results) in the CMJ test for the operated limb (op–limb) and the healthy limb. The vertical distance between the lines represents the scale of asymmetry between the limbs, while the second Y–axis shows the thresholds of expected reference norms for the given athletic population.

Important: the above visualization is based solely on approximation—on daily measurements, their analysis and the search for usual scenarios. It should be treated primarily as an didactical tool: an example of a visual report presenting RFD values in relation to time–post–surgery and as a “theoretical” foundation for storytelling (clinical interpretation):

Regular testing was initiated in the 3rd month after ACLR and repeated every month. The period of 3–6 months reflected the mid–phase of rehabilitation, dominated by strength interventions, although the athlete was already performing progressive field sessions within the return to running (RTR) program. Months 3–4 were additionally marked by complications in scar healing, yet the absence of further issues and positive PF results opened the way to partial participation in restricted soccer training around the 6th month post–surgery. At this time, a significant correction of RFD asymmetry was noted (from ~20% to ~10%), along with an increase in movement velocities in the ECC and CON phases, as well as greater efficiency and symmetry in tolerating RFD breaking decelerations. Despite typical result fluctuations—sensitive to fatigue—a clear convergence of outcomes between both limbs is observable, indicating a reduction of asymmetry between them.

Summary.

Despite the growing interest in the subject, prospective studies are still lacking in which “burst-like” RFD would be used as an independent criterion for return-to-sport (RTS) decision-making, with a clearly defined threshold value.

Even Aspetar Hospital in Doha—one of the most influential orthopedic–sports clinics—does not formulate specific norms for RFD in the latest guidelines on rehabilitation after ACLR (available for download here). RFD is treated rather as an element of general explosiveness and reactive strength, developed indirectly through such indicators as the reactive strength index (RSI) or ground contact time in single–leg drop jump (SLDJ) tests. In Aspetar’s protocols, it is considered that any strength training may support the development of explosiveness, provided it is performed with a clear intention of acting as fast as possible. In practice, RFD is thus a “proxy” for RSI and is not assessed at the moment as a separate, key indicator of successful RTS, but rather as an integral part of the continuum of exercises, jump progressions, sport-specific tasks and biomechanical tests (as in the presentation attached above).

This does not mean, however, that we should abandon the curiosity of “peeking behind the curtain” of the first 0–200 ms, when the muscle executes a sudden "peaking", although for unambiguous conclusions—or perhaps the refutation of many theses of this text—we will still need to wait a little longer ...

Enrich the discussion.

Respond to the ideas included in the text by writing to: me@pawelkrotki.com.