Brief Summary
Nicola Maffiuletti, Per Aagaard, Anthony Blazevich, Jonathan Folland, Neale Tillin, Jacques Duchateau
https://link.springer.com/content/pdf/10.1007/s00421-016-3346-6.pdf
In this month's research summary, we continue with Maffiuletti et al (2016) as we review the methodological considerations when testing rate of force development (RFD). The following summarises some of the key areas identified by the authors in this paper. RFD variables are typically measured in isometric conditions to avoid the confounding effects of joint angle and velocity changes, so in this summary, the authors specifically focus on isometric RFD unless noted otherwise. RFD measures, especially in the early contraction phase, can be significantly less reliable than MVC force; therefore, a strict methodological approach is needed to ensure reliable and meaningful data collection.
Dynamometre
The purpose of RFD measurements, whether practical (e.g., performance monitoring, injury screening, rehabilitation progression) or experimental, largely dictates the choice of task. For practical purposes, the authors suggest the task should be specific to the activity of interest, as RFD is influenced by the muscle group and joint angle used. RFD is most commonly measured in isolated single-joint tasks (e.g., elbow, knee, or ankle movements) using commercial isokinetic or custom-built dynamometers. Measuring the external lever arm allows conversion between force and torque, aiding data comparison. In some cases, RFD is measured during multi-joint tasks like squats or mid-thigh pulls using force plates. Single-joint tasks provide better control for studying physiological factors, while multi-joint tasks may be more relevant for practical applications. Any compliance or flexibility in the dynamometer system can lead to uncontrolled changes in joint angle and velocity as well as force dissipation, which compromises RFD measurements. While some biological compliance, such as soft-tissue compression, is unavoidable and causes small joint angle changes, dynamometer compliance depends on its rigidity and ability to limit joint movement. Commercial isokinetic dynamometers, often designed for patient comfort, may allow significant joint angle changes (>15° during isometric knee extension), compared to more rigid custom-built devices (4°). This is due to non-rigid components, loose strapping, and excessive padding.
Acquisition and Filtering
The authors suggest that the dynamometer used for measuring RFD should have low baseline noise to enhance measurement accuracy, particularly in identifying contraction onset. Commercial isokinetic dynamometers tend to be noisier than strain gauges. For instance, in knee extensor studies, custom-built dynamometers with strain gauges have shown baseline noise of <0.1% MVC force, compared to >1% in commercial isokinetic dynamometers. Early-phase RFD (within the first 25–50 ms), which can be as low as 2–12% of MVC force, is critical for performance and injury prevention. Thus, low-noise, high-resolution recording equipment is essential for detecting early-phase RFD differences between individuals or sessions. Force signals should be sampled at a high frequency (≥1 kHz). The authors define 5 reasons for this: (1) capture the high RFD human muscle produces (over 10 times maximum isometric force per second); (2) accurately detect contraction onset; (3) synchronise with EMG, which requires a Nyquist limit of ≥1 kHz; (4) measure motor response times such as electromechanical delay (<7 ms for involuntary, <13 ms for voluntary); and (5) use the high-frequency baseline noise pattern to manually identify contraction onset. Post-acquisition, minimal filtering or smoothing is recommended to retain the noise pattern and avoid time shifts in the force signal, which can affect relationships with biological responses like EMG. If filtering is necessary, use a zero-lag, low-pass digital filter at the highest possible cut-off frequency to minimise time shifts.
Protocol
RFD measurements are highly sensitive to the instructions given before the contraction. In a study by Bemben et al. (1990), isometric handgrip contractions were compared under two instructions: (1) reach peak force as quickly as possible by squeezing “hard and fast” and (2) focus only on squeezing as “fast” as possible without concern for peak force. While the “hard and fast” condition produced the highest peak force, the greatest RFD was achieved in the “fast” condition. Similar findings were reported by Sahaly et al. (2001) in isometric elbow flexion and leg press exercises, showing a 20–46% increase in peak RFD when participants were told to push “fast” rather than “hard and fast.” These results suggest that if the goal is to maximise RFD, instructions should emphasise contracting as “fast” as possible, and attempting to achieve both maximal force and RFD in the same contraction may lead to suboptimal outcomes. Therefore, MVC force and RFD measurements should be taken separately, with instructions tailored to each—“hard” for MVC force and “fast” for RFD. However, since RFD is positively correlated with peak force, participants should still aim for high peak forces, even when the focus is on speed. The authors recommend instructing participants to contract “fast and hard,” with an emphasis on speed, and discarding any contractions with low peak force (e.g., below 70–80% of MVC). For separate RFD measurements, explosive contractions should be kept short (0.5–1.5 seconds) to reduce fatigue or discomfort, especially in clinical populations. This allows for a higher number of trials (at least 10) with short rest intervals (15–20 seconds), improving the reliability and representativeness of RFD measurements.
Contraction Onset
Various methods for detecting onset have been proposed, including threshold-based techniques and systematic manual/visual approaches. In research targeting these methods, contraction onset is typically defined as the point at which the force surpasses a predetermined threshold. This approach is popular because it is easily automated, making it both reliable and time-efficient. The threshold can be set in absolute terms or relative to the individual's MVC, such as 2% or 2.5% MVC. The authors suggest that while absolute thresholds offer simplicity, they may not be ideal for comparing individuals, groups, or muscle sets with varying functional capacities. Relative thresholds are generally preferred, assuming they are based on a robust reference measure. As noted above, studies using automated threshold methods (whether absolute or relative) often employ relatively high thresholds. This is likely due to the use of commercially available dynamometers, which tend to produce significant inherent noise (around ~5 Nm or >1% MVC) compared to custom-built dynamometers (less than 0.1 Nm). These higher thresholds may inaccurately pinpoint the actual force onset. Recent studies using low-noise dynamometers and systematic manual detection have found that knee extensor torques exceeding 5 Nm or 2.5% MVC occur only after more than 25 ms from the onset of contraction. Inaccuracies in high-threshold automated methods can compromise the measurement of RFD, especially in the early phase of contraction (e.g., the first 50 ms). An alternative approach is to define the threshold relative to the baseline noise in the force recording, which has produced much lower thresholds when used with custom-built dynamometers (~1 N or 0.5 Nm). Another distinct method is systematic manual/visual detection, which identifies the precise moment of force onset. While this method shows good validity, subjectivity could impact its reliability, and it is considerably less time-efficient than automated techniques. To enhance the reliability of manual onset detection, systematic approaches involve using specific criteria, such as “the last trough before the force rises above baseline noise” or “the last time the first derivative of the filtered torque signal crosses zero before the torque increase.” Consistently viewing force recordings on a fixed scale (e.g., 300 ms vs. 1 N) also helps. When paired with a low-noise dynamometer and minimal filtering, this method has been shown to produce reliable onset timings, with inconsistencies being small compared to the >25 ms delay seen in some high-threshold automated methods. However, manual methods may be far less reliable when using a high-noise dynamometer or excessive filtering.
Preload/Pretension
Pre-tension in the muscle before an explosive contraction changes the force–time curve, increasing the initial (40 ms) torque-time integral and reducing peak RFD, partly due to alterations in motor unit discharge patterns. Similarly, a countermovement (negative/antagonist force) just before the contraction affects RFD based on its amplitude and duration. Therefore, pre-contraction conditions should be standardised across contractions, participants, and sessions for reliable RFD measurements. Contractions with uncontrolled pre-tension or countermovement should be discarded. Practically, this can be achieved by displaying baseline force in real time and rejecting trials with unstable baselines (e.g., shifts >0.5 N in the previous 200 ms). During analysis, strict criteria for baseline stability should be applied. Lower baseline noise allows for more accurate detection of countermovement or pre-tension.
Concluding remarks and recommendations
To briefly summarise the observations presented in this methodological section, the authors recommend the application of rigorous procedures for the assessment of RFD, which include the following precautions:
At KangaTech, we have been taking such methodological considerations for RFD testing into account when developing our latest product. The KT360 PRO brings innovation to RFD testing by providing an advanced platform for measuring isometric force development with precision and efficiency. The system’s high sampling frequency offers accuracy when assessing an individual’s capacity to rapidly produce force. With the ability to deliver real-time feedback and data analysis, coupled with the robust nature of the system, the KT360 PRO sets a new standard for measuring isometric RFD, offering valuable insights into muscular strength and explosive power. For any further information relating to the KT360 PRO, please contact support@kangatech.com.