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Is the Rate of Force Development Produced in the Squat Jump a Reliable Metric and Is It Related to Jump Height?

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Title: Is the Rate of Force Development Produced in the Squat Jump a Reliable Metric and Is It Related to Jump Height?
Language: English
Authors: Juliano Dal Pupo (ORCID 0000-0003-4084-9474), Rafael Lima Kons (ORCID 0000-0003-1615-5464), Rodrigo Ghedini Gheller (ORCID 0000-0002-2259-8096), Carlos W. P. Gonçalves (ORCID 0000-0002-6481-0446), Amilton Vieira (ORCID 0000-0002-6027-4367)
Source: Measurement in Physical Education and Exercise Science. 2025 29(1):113-119.
Availability: Routledge. Available from: Taylor & Francis, Ltd. 530 Walnut Street Suite 850, Philadelphia, PA 19106. Tel: 800-354-1420; Tel: 215-625-8900; Fax: 215-207-0050; Web site: http://www.tandf.co.uk/journals
Peer Reviewed: Y
Page Count: 7
Publication Date: 2025
Document Type: Journal Articles
Reports - Research
Descriptors: Physical Education, Athletics, Physical Activities, Males, Athletes, Motion, Muscular Strength, Performance, Team Sports
DOI: 10.1080/1091367X.2024.2417100
ISSN: 1091-367X
1532-7841
Abstract: This study aimed to analyze the intrasession reliability of the rate of force development (RFD) in different time windows in squat jumps (SJ) performed at different knee starting angles, and identify the relationship of RFD with vertical jump height (VJH). Twenty male volleyball and basketball players participated. Athletes randomly performed SJ from four different knee-flexion angles. The RFD was analyzed from six different windows, from force onset to instant peak force, in each position using the ground reaction forces. Reliability analysis was performed based on the intraclass correlation coefficient (ICC) and coefficient of variation (CV%); Spearman's rho was used to test the relationship between RFD and VJH. The results demonstrated that RFD[subscript 0-PF] was the only variable assumed to be reliable, demonstrating an ICC > 0.84 and CV < 8.5% in all positions, and significant correlations between RFD metrics and VJH were observed at SJ[subscript 70] and SJ[subscript PREF]. Some variables were underpowered due to the small sample size, as a limitation of the study.
Abstractor: As Provided
Entry Date: 2025
Accession Number: EJ1459069
Database: ERIC
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  Value: &lt;anid&gt;AN0182438458;7mm01jan.25;2025Jan28.02:47;v2.2.500&lt;/anid&gt; &lt;title id=&quot;AN0182438458-1&quot;&gt;Is the Rate of Force Development Produced in the Squat Jump a Reliable Metric and is it Related to Jump Height?&#160;&lt;/title&gt; &lt;p&gt;This study aimed to analyze the intrasession reliability of the rate of force development (RFD) in different time windows in squat jumps (SJ) performed at different knee starting angles, and identify the relationship of RFD with vertical jump height (VJH). Twenty male volleyball and basketball players participated. Athletes randomly performed SJ from four different knee-flexion angles. The RFD was analyzed from six different windows, from force onset to instant peak force, in each position using the ground reaction forces. Reliability analysis was performed based on the intraclass correlation coefficient (ICC) and coefficient of variation (CV%); Spearman&#39;s rho was used to test the relationship between RFD and VJH. The results demonstrated that RFD&amp;lt;sub&amp;gt;0-PF&amp;lt;/sub&amp;gt; was the only variable assumed to be reliable, demonstrating an ICC &amp;gt; 0.84 and CV &amp;lt; 8.5% in all positions, and significant correlations between RFD metrics and VJH were observed at SJ&amp;lt;sub&amp;gt;70&amp;lt;/sub&amp;gt; and SJ&amp;lt;sub&amp;gt;PREF&amp;lt;/sub&amp;gt;. Some variables were underpowered due to the small sample size, as a limitation of the study.&lt;/p&gt; &lt;p&gt;Keywords: Jump performance; repeated measures; athletic performance; muscle power of lower limbs&lt;/p&gt; &lt;hd id=&quot;AN0182438458-2&quot;&gt;Introduction&lt;/hd&gt; &lt;p&gt;Jumping ability, or lower limb ballistic performance, has been considered a determinant factor in many sports (Dal Pupo et al., [&lt;reflink idref=&quot;bib3&quot; id=&quot;ref1&quot;&gt;3&lt;/reflink&gt;]; Loturco et al., [&lt;reflink idref=&quot;bib13&quot; id=&quot;ref2&quot;&gt;13&lt;/reflink&gt;]). In this sense, vertical jump tests are commonly used among professionals in sports training, usually aimed at monitoring the training effects and muscle recovery, or to predict performance (Claudino et al., [&lt;reflink idref=&quot;bib2&quot; id=&quot;ref3&quot;&gt;2&lt;/reflink&gt;]; Twist &amp;amp; Highton, [&lt;reflink idref=&quot;bib21&quot; id=&quot;ref4&quot;&gt;21&lt;/reflink&gt;]).&lt;/p&gt; &lt;p&gt;Jumping performance is determined by the capability to accelerate the body mass as much as possible to reach the highest velocity at the end of a push-off and, consequently, the jump height. From Newton&#39;s second law of motion, the velocity reached by the body center of mass at the end of a push-off (take-off velocity) directly depends on the mechanical impulse developed in the movement direction (Kirby et al., [&lt;reflink idref=&quot;bib8&quot; id=&quot;ref5&quot;&gt;8&lt;/reflink&gt;]; Knudson, [&lt;reflink idref=&quot;bib9&quot; id=&quot;ref6&quot;&gt;9&lt;/reflink&gt;]; McBride et al., [&lt;reflink idref=&quot;bib14&quot; id=&quot;ref7&quot;&gt;14&lt;/reflink&gt;]; Winter, [&lt;reflink idref=&quot;bib24&quot; id=&quot;ref8&quot;&gt;24&lt;/reflink&gt;]). In practical terms and thinking of physical capacities, developing high impulse during lower limb push-off, and in turn accelerating a mass as much as possible, has often been assumed to depend on the capacity to apply force rapidly and to produce high amounts of mechanical work over a short duration (i.e., power output) (Linthorne, [&lt;reflink idref=&quot;bib12&quot; id=&quot;ref9&quot;&gt;12&lt;/reflink&gt;]). Several studies have shown a positive and moderate-strong correlation of mechanical power output with impulse or vertical jump height (VJH) (Kons et al., [&lt;reflink idref=&quot;bib10&quot; id=&quot;ref10&quot;&gt;10&lt;/reflink&gt;]). However, the role of the capacity to produce rapid force and the rate of force produced is not well elucidated in the literature.&lt;/p&gt; &lt;p&gt;Some studies have investigated the relationship between RFD and vertical jump performance but have reported inconclusive results. For example, two studies found a non-significant and trivial correlation of RFD&lt;subs&gt;0-30 ms&lt;/subs&gt; (Dal Pupo et al., [&lt;reflink idref=&quot;bib4&quot; id=&quot;ref11&quot;&gt;4&lt;/reflink&gt;]) and average RFD (Ebben et al., [&lt;reflink idref=&quot;bib5&quot; id=&quot;ref12&quot;&gt;5&lt;/reflink&gt;]) in the countermovement jump (CMJ) and squat jump (SJ) with VJH; on the other hand, other studies (McLellan et al., [&lt;reflink idref=&quot;bib15&quot; id=&quot;ref13&quot;&gt;15&lt;/reflink&gt;]; Miller et al., [&lt;reflink idref=&quot;bib17&quot; id=&quot;ref14&quot;&gt;17&lt;/reflink&gt;]) verified significant and moderate correlations of peak RFD with VJH. Despite the biological basis of rapid force on vertical jump performance, the lack of a significant relationship found between RFD and VJH in some studies may result from methodological characteristics associated with the measurement of RFD. One important concern may be the different time windows or jump durations used in the analyses of RFD. As Ebben et al. ([&lt;reflink idref=&quot;bib5&quot; id=&quot;ref15&quot;&gt;5&lt;/reflink&gt;]) described, RFD may manifest in short and long components of the stretch-shortening cycle (SSC). The short component seems to occur within 100–250 ms of the muscle activation and, when applied to the lower body, is characterized by small angular displacements of the ankle, knee, and hip joints, such as during quick jumping; on the other hand, the long component is typified by muscle activation of more than 250 ms and is demonstrated during movements that involve larger angular displacements of the ankle, knee, and hip joints, such as during maximal vertical jumps. When assessing RFD in dynamic movements with large angular displacement (e.g., vertical jumps), short- or long-time windows may be analyzed, but it is unclear if they present similar reliability.&lt;/p&gt; &lt;p&gt;Another issue potentially influencing the relationship between RFD and jump performance is the observed low reliability of RFD measures (McLellan et al., [&lt;reflink idref=&quot;bib15&quot; id=&quot;ref16&quot;&gt;15&lt;/reflink&gt;]; Moir et al., [&lt;reflink idref=&quot;bib18&quot; id=&quot;ref17&quot;&gt;18&lt;/reflink&gt;]), possibly due to variations in squat depth between jump trials by athletes (Miller et al., [&lt;reflink idref=&quot;bib17&quot; id=&quot;ref18&quot;&gt;17&lt;/reflink&gt;]). It is known that changing the squat depth influences jump impulse and RFD (Gheller et al., [&lt;reflink idref=&quot;bib6&quot; id=&quot;ref19&quot;&gt;6&lt;/reflink&gt;]). Furthermore, when jumping to maximize jumping height from a short-amplitude condition, individuals need to optimize force production, which likely leads to higher RFD. Thus, a study controlling the squat depth is necessary to investigate the magnitude and reliability of the RFD, as well as to investigate the influence of the RFD on vertical jump performance.&lt;/p&gt; &lt;p&gt;Therefore, the current study investigated the intrasession reliability of RFD across different time windows during squat jumps with varied knee starting angles, as well as the relationship between RFD and jump height.&lt;/p&gt; &lt;hd id=&quot;AN0182438458-3&quot;&gt;Material and methods&lt;/hd&gt; &lt;p&gt;&lt;/p&gt; &lt;hd id=&quot;AN0182438458-4&quot;&gt;Participants&lt;/hd&gt; &lt;p&gt;Twenty male amateur volleyball and basketball players (24 &#177; 4 years; 82 &#177; 10 kg; 185 &#177; 6 cm; 14 &#177; 3% body fat) volunteered to participate in this study. The statistical power was calculated (post-hoc analysis) based on the current sample size and an average value of 0.73 (ranging from 0.39 to 1.0) was obtained for ICC analysis and 0.50 (ranging from 0.05 to 0.99) for correlation analysis, depending on the variable and knee angle tested. The participants had trained (specific training) three times per week for at least the 4 years that preceded the study, and were all currently competing at the college level. None of the participants reported injuries or other conditions that could prevent them from training or otherwise influence their maximal physical performance. All participants were informed about the procedures and signed an informed consent form.&lt;/p&gt; &lt;hd id=&quot;AN0182438458-5&quot;&gt;Design and procedures&lt;/hd&gt; &lt;p&gt;This is a cross-sectional study in which the participants visited the laboratory twice, 2 days apart. The first day was dedicated to familiarization with the jump protocol. On the second day, participants randomly performed SJ from different knee-flexion angles (i.e., different levels of squat depth): preferred position (SJ&lt;subs&gt;PREF&lt;/subs&gt;), and at maximum knee-flexion angles of 70&#176; (SJ&lt;subs&gt;70&lt;/subs&gt;), 90&#176; (SJ&lt;subs&gt;90&lt;/subs&gt;), and 110&#176; (SJ&lt;subs&gt;110&lt;/subs&gt;).&lt;/p&gt; &lt;p&gt;Each knee-angle position was determined in the familiarization session, when participants were instructed to squat in a slow and controlled manner until they reached the required knee angle. The squat depth position was maintained for approximately 5 s, allowing the knee-angle measurement using a goniometer. A rigid bar was then positioned under the participants&#39; hips (posterior thigh) and attached to a vertical support. This mechanism allowed the adjustment of the bar height according to the specific knee-flexion angle (see Figure 1). During the jumps, participants squatted until they could feel the bar on their posterior thigh, then remained static for a few seconds before performing the concentric action as fast as possible. In the preferred SJ position, the participants were free to squat until an individual and satisfactory maximum knee-flexion angle. Individuals were required to keep their hands on their hips (akimbo) during the jumps. Participants performed three SJs in each condition, with the order of the conditions randomized. Participants performed the SJs on a force platform (Quattro Jump, 9290 AD, Kistler, Winterthur, Switzerland) sampling at 500 hz, with their hands on their hips.&lt;/p&gt; &lt;p&gt;Graph: Figure 1. Initial position in four different conditions of knee starting angle during the squat jump on a force platform (FP).&lt;/p&gt; &lt;hd id=&quot;AN0182438458-6&quot;&gt;Data processing&lt;/hd&gt; &lt;p&gt;The RFD and VJH were calculated from vertical ground reaction forces (GRF) using custom-made Python code (libraries: pandas, matplotlib, numpy, and scipy). The offset was adjusted by subtracting the mean residual force measured during the flight phase of the jump, guaranteeing a zeroed measure. Body weight and mass were measured during the 1-s period of the weighing phase with the lowest standard deviation (SD). The start of the jump was identified as the instant when the force signal reached the threshold of 5 &#215; SD of the body weight (BW) minus 30 ms (McMahon et al., [&lt;reflink idref=&quot;bib16&quot; id=&quot;ref20&quot;&gt;16&lt;/reflink&gt;]). The RFD was calculated by dividing the net force by the time-lapse from movement onset to peak force (Figure 2). Jump height was estimated using the local gravity acceleration equation &lt;emph&gt;v&lt;/emph&gt;&lt;sups&gt;2&lt;/sups&gt; ∙ 19.58&lt;sups&gt;−1&lt;/sups&gt;. The center of mass velocity was calculated by numerically integrating acceleration time using the trapezoidal rule.&lt;/p&gt; &lt;p&gt;Graph: Figure 2. Rate of force development (RFD) measurements starting from force onset to specific time point (RFD0-50, RFD0-100, RFD0-200, RFD50-100, RFD100-200), and from force onset to instant of peak force (PF), RFD0-PF (e.g., 375 ms). Data from a representative participant.&lt;/p&gt; &lt;p&gt;Data were further analyzed objectively in order to remove any jump that presented a countermovement, since visual inspection of the force–time curve is insufficient to detect this movement (Shepard and Doyle, Sheppard &amp;amp; Doyle, [&lt;reflink idref=&quot;bib20&quot; id=&quot;ref21&quot;&gt;20&lt;/reflink&gt;]). We discarded data when a 10% BW threshold was exceeded in the downward direction. Of the 260 jumps performed, 27 (10.4%) exceeded the 10% BW threshold and were removed from the final results.&lt;/p&gt; &lt;hd id=&quot;AN0182438458-7&quot;&gt;Statistical analyses&lt;/hd&gt; &lt;p&gt;For reliability analysis, a two-way mixed-effects model intraclass correlation coefficient (ICC) for absolute agreement (Koo &amp;amp; Li, [&lt;reflink idref=&quot;bib11&quot; id=&quot;ref22&quot;&gt;11&lt;/reflink&gt;]) and within-participant coefficient of variation (CV% = standard deviation divided by mean times 100) were applied among the three trials for each participant. The CV% was interpreted as good (≤5%), moderate (5.01–10%), and poor (&amp;gt;10.01%) (Ugliara et al., [&lt;reflink idref=&quot;bib22&quot; id=&quot;ref23&quot;&gt;22&lt;/reflink&gt;]; Vieira et al., [&lt;reflink idref=&quot;bib23&quot; id=&quot;ref24&quot;&gt;23&lt;/reflink&gt;]), while the ICC was interpreted as poor (&amp;lt;0.500), moderate (0.501–0.750), good (0.751–0.900), and excellent (&amp;gt;0.900) (Koo &amp;amp; Li, [&lt;reflink idref=&quot;bib11&quot; id=&quot;ref25&quot;&gt;11&lt;/reflink&gt;]). Variables were assumed as &quot;reliable&quot;&#39; when the mean CV% was ≤10% (Atkinson &amp;amp; Nevill, [&lt;reflink idref=&quot;bib1&quot; id=&quot;ref26&quot;&gt;1&lt;/reflink&gt;]) and the mean ICC was ≥0.75 (Koo &amp;amp; Li, [&lt;reflink idref=&quot;bib11&quot; id=&quot;ref27&quot;&gt;11&lt;/reflink&gt;]). Data distribution was verified by running the Shapiro–Wilk test, visually checking QQ plots, and calculating skewness/kurtosis error ratios. Although ratios and QQ plots indicated a normal distribution, some data did not pass the Shapiro–Wilk test. Therefore, we adopt a conservative approach running 2-tailed Spearman&#39;s rho coefficients (in which normal distribution is not an assumption) to investigate the relationships between jump height and RFD. The magnitude of each relationship was interpreted as trivial (0.00–0.09), small (0.10–0.29), moderate (0.30–0.49), large (0.50–0.69), very large (0.70–0.89), and nearly perfect (0.90–1.00) (Hopkins et al., [&lt;reflink idref=&quot;bib7&quot; id=&quot;ref28&quot;&gt;7&lt;/reflink&gt;]). Data were analyzed using IBM SPSS Statistics software (version 26; IBM Co., Chicago, IL, USA).&lt;/p&gt; &lt;hd id=&quot;AN0182438458-8&quot;&gt;Results&lt;/hd&gt; &lt;p&gt;Table 1 presents the descriptive values (mean and standard deviation) of RFD obtained in each SJ knee starting angle.&lt;/p&gt; &lt;p&gt;Table 1. Vertical jump height (m) and rate of force development (N.s&lt;sups&gt;−1&lt;/sups&gt;) obtained in the SJ performed at different knee flexion starting angles.&lt;/p&gt; &lt;p&gt; &lt;ephtml&gt; &amp;lt;table&amp;gt;&amp;lt;thead&amp;gt;&amp;lt;tr&amp;gt;&amp;lt;td /&amp;gt;&amp;lt;td&amp;gt;SJ&amp;lt;sub&amp;gt;PREF&amp;lt;/sub&amp;gt;&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;SJ&amp;lt;sub&amp;gt;70&amp;lt;/sub&amp;gt;&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;SJ&amp;lt;sub&amp;gt;90&amp;lt;/sub&amp;gt;&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;SJ&amp;lt;sub&amp;gt;110&amp;lt;/sub&amp;gt;&amp;lt;/td&amp;gt;&amp;lt;/tr&amp;gt;&amp;lt;/thead&amp;gt;&amp;lt;tbody&amp;gt;&amp;lt;tr&amp;gt;&amp;lt;td&amp;gt;VJH&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;0.334 &amp;amp;#177; 0.054&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;0.322 &amp;amp;#177; 0.060&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;0.319 &amp;amp;#177; 0.051&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;0.310 &amp;amp;#177; 0.043&amp;lt;/td&amp;gt;&amp;lt;/tr&amp;gt;&amp;lt;tr&amp;gt;&amp;lt;td&amp;gt;RFD&amp;lt;sub&amp;gt;0-50 ms&amp;lt;/sub&amp;gt;&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;2.773 &amp;amp;#177; 1.018&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;1.904 &amp;amp;#177; 724&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;1.930 &amp;amp;#177; 871&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;2.718 &amp;amp;#177; 1.161&amp;lt;/td&amp;gt;&amp;lt;/tr&amp;gt;&amp;lt;tr&amp;gt;&amp;lt;td&amp;gt;RFD&amp;lt;sub&amp;gt;0-100 ms&amp;lt;/sub&amp;gt;&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;4.142 &amp;amp;#177; 1.627&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;2.834 &amp;amp;#177; 1.176&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;3.040 &amp;amp;#177; 1.556&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;4.358 &amp;amp;#177; 2.580&amp;lt;/td&amp;gt;&amp;lt;/tr&amp;gt;&amp;lt;tr&amp;gt;&amp;lt;td&amp;gt;RFD&amp;lt;sub&amp;gt;50-100 ms&amp;lt;/sub&amp;gt;&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;4.301 &amp;amp;#177; 1.654&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;2.394 &amp;amp;#177; 714&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;3.102 &amp;amp;#177; 1.363&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;5.238 &amp;amp;#177; 1.726&amp;lt;/td&amp;gt;&amp;lt;/tr&amp;gt;&amp;lt;tr&amp;gt;&amp;lt;td&amp;gt;RFD&amp;lt;sub&amp;gt;100-200 ms&amp;lt;/sub&amp;gt;&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;5.511 &amp;amp;#177; 2.492&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;3.764 &amp;amp;#177; 1.779&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;4.151 &amp;amp;#177; 2.393&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;5.997 &amp;amp;#177; 4.208&amp;lt;/td&amp;gt;&amp;lt;/tr&amp;gt;&amp;lt;tr&amp;gt;&amp;lt;td&amp;gt;RFD&amp;lt;sub&amp;gt;0-200 ms&amp;lt;/sub&amp;gt;&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;4.460 &amp;amp;#177; 2.286&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;1.953 &amp;amp;#177; 890&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;3.164 &amp;amp;#177; 1.731&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;6.118 &amp;amp;#177; 2.938&amp;lt;/td&amp;gt;&amp;lt;/tr&amp;gt;&amp;lt;tr&amp;gt;&amp;lt;td&amp;gt;RFD&amp;lt;sub&amp;gt;0-PF&amp;lt;/sub&amp;gt;&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;4.130 &amp;amp;#177; 1.414&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;2.235 &amp;amp;#177; 518&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;3.326 &amp;amp;#177; 1.181&amp;lt;/td&amp;gt;&amp;lt;td&amp;gt;5.495 &amp;amp;#177; 2.062&amp;lt;/td&amp;gt;&amp;lt;/tr&amp;gt;&amp;lt;tr&amp;gt;&amp;lt;td&amp;gt;VJH: Vertical jump height, RFD: Rate of Force Development; PREF: preferential; SJ: Squat Jump&amp;lt;/td&amp;gt;&amp;lt;/tr&amp;gt;&amp;lt;/tbody&amp;gt;&amp;lt;/table&amp;gt; &lt;/ephtml&gt; &lt;/p&gt; &lt;p&gt;The RFD&lt;subs&gt;0-PF&lt;/subs&gt; was the only variable that demonstrated acceptable ICC and CV% scores and, therefore, was assumed to be &quot;reliable&quot; - this was true for all SJ positions (Figure 3). The RFD&lt;subs&gt;0-PF&lt;/subs&gt; in the SJ&lt;subs&gt;70&lt;/subs&gt; attained the best reliability scores, with the lowest CV% of 4.2 (95% CI: from 2.9 to 5.5) and the highest ICC of 0.95 (95% CI: from 0.90 to 0.98). This was followed by the SJ&lt;subs&gt;90&lt;/subs&gt; with a CV% of 4.9 (95% CI: 2.8–7.0) and ICC of 0.93 (95% CI: 0.84–0.97), SJ&lt;subs&gt;PREF&lt;/subs&gt; with a CV% of 7.5 (95% CI: 5.7–9.2) and ICC of 0.93 (95% CI: 0.83–0.97), and lastly SJ&lt;subs&gt;110&lt;/subs&gt; with a CV% of 8.5 (95% CI: 5.8–11.2) and ICC of 0.84 (95% CI: 0.70–0.93).&lt;/p&gt; &lt;p&gt;Graph: Figure 3. Intrasession reliability of rate of force development obtained during the squat jump (SJ) tests performed at different knee-angle start positions – preferred position (SJPREF), 70&#176; (SJ70), 90&#176; (SJ90), and 110&#176; (SJ110). Data are presented as mean with the 95% confidence interval (CI, vertical lines) of the coefficient of variations (upper panel).&lt;/p&gt; &lt;p&gt;In the SJ&lt;subs&gt;PREF&lt;/subs&gt;, we found small to moderate (coefficients ranging from 0.28 to 0.47) correlations between all RFD metrics and VJH. In the SJ&lt;subs&gt;70&lt;/subs&gt;, we found moderate to large (0.36–0.62) correlation coefficients. Finally, in the SJ&lt;subs&gt;90&lt;/subs&gt;, only the RFD0-PF demonstrated a moderate correlation with jump height, while the SJ&lt;subs&gt;110&lt;/subs&gt; showed no correlations (Figure 4—left panel).&lt;/p&gt; &lt;p&gt;Graph: Figure 4. Left panel: correlation matrix between rate of force development (RFD0-50, RFD0-100, RFD0-200, RFD50-100, RFD100-200, and RFD0-PF) and squat jump height at different knee starting angles. Right panel: scatter plot depicting the &quot;large&quot; relationship between RFD0-PF (Spearman coefficient of 0.62, p =.01, 2-tailed) and VJH at SJ70.&lt;/p&gt; &lt;hd id=&quot;AN0182438458-9&quot;&gt;Discussion&lt;/hd&gt; &lt;p&gt;Our main results showed that RFD&lt;subs&gt;0-PF&lt;/subs&gt; was the only variable with acceptable reliability (i.e., CV &amp;lt; 10% and ICC &amp;gt; 0.75) for all SJ positions analyzed.&lt;/p&gt; &lt;p&gt;Quantifying the level of reliability of a measurement is crucial in the context of sports training, as it allows the determination of other variables, e.g., when a change in the training status of an athlete is true and greater than the error of the measurement. When considering vertical jump evaluations, although metrics such as jump height and power output are already commonly accepted as reliable (Koo &amp;amp; Li, [&lt;reflink idref=&quot;bib11&quot; id=&quot;ref29&quot;&gt;11&lt;/reflink&gt;]; Atkinson &amp;amp; Nevill, [&lt;reflink idref=&quot;bib1&quot; id=&quot;ref30&quot;&gt;1&lt;/reflink&gt;]), the reliability of RFD is not widely established. Low test–retest reliability (CV from 14.8% to 17.9%) was reported by McLellan et al. ([&lt;reflink idref=&quot;bib15&quot; id=&quot;ref31&quot;&gt;15&lt;/reflink&gt;]) for peak and average RFD in the SJ and CMJ, while the study of Moir et al. ([&lt;reflink idref=&quot;bib18&quot; id=&quot;ref32&quot;&gt;18&lt;/reflink&gt;]) reported CV values from 17.2 to 31.5 for the same variables in CMJ.&lt;/p&gt; &lt;p&gt;In the current study, we analyzed the RFD in different time windows during SJ and verified that the RFD from zero to peak force (0-PF) was the most reliable (ICC ≥ 0.84 and CV ≤ 8.5%), demonstrating better reliability metrics than the studies cited above. Although, in general, larger time windows (e.g., at least 200 ms) appear to demonstrate better reliability, our findings show that this is not consistent across all conditions. For example, even in the 0–200 ms window, certain squat positions (such as SJ&lt;subs&gt;PREF&lt;/subs&gt; and SJ&lt;subs&gt;70&lt;/subs&gt;) did not meet acceptable reliability thresholds (CV &amp;gt; 10%, ICC &amp;lt; 0.75). The poor reliability may be attributed to the instability of the GRF mainly seen at the beginning of the vertical jump because of the adjustments made by individuals on the lower limbs, in an attempt to gather the inertia to accelerate the body as fast as possible. It was reported that some compensatory strategies may occur within the motor system, whereby reciprocal alterations are produced so that the outcome of the movement (jump height) is preserved between trials (Moir et al., [&lt;reflink idref=&quot;bib18&quot; id=&quot;ref33&quot;&gt;18&lt;/reflink&gt;]).&lt;/p&gt; &lt;p&gt;One factor that can influence the reliability of RFD, as pointed out by Miller et al. ([&lt;reflink idref=&quot;bib17&quot; id=&quot;ref34&quot;&gt;17&lt;/reflink&gt;]), is the different countermovement depths adopted by individuals in the trials. It is known that modulation of squat depth may affect some variables related to vertical jump performance, such as impulse and RFD (Gheller et al., [&lt;reflink idref=&quot;bib6&quot; id=&quot;ref35&quot;&gt;6&lt;/reflink&gt;]). Considering this, we used a &quot;rigid&quot; bar positioned under the participants&#39; hips and attached to a vertical support, aiming to &quot;stabilize&quot; the individuals at the beginning of the jump in each position. With this control, we were able to test the reliability of the RFD in the SJ performed at different squat depths. We found similar results in all angles tested, i.e., the only variable with acceptable reliability (RFD&lt;subs&gt;0-PF&lt;/subs&gt;) was observed in all SJ positions.&lt;/p&gt; &lt;p&gt;To meet the second objective of the present study, we analyzed the correlations of RFD with VJH. Significant correlations were observed at SJ&lt;subs&gt;70&lt;/subs&gt; (moderate to large effects) and at SJ&lt;subs&gt;PREF&lt;/subs&gt; (small to moderate effects); and the variable RFD&lt;subs&gt;0-PF&lt;/subs&gt; at SJ&lt;subs&gt;70&lt;/subs&gt; presented the largest correlation (Figure 4). Our results corroborate the study of McLellan et al. ([&lt;reflink idref=&quot;bib15&quot; id=&quot;ref36&quot;&gt;15&lt;/reflink&gt;]) who also observed significant correlations of the peak and average RFD with VJH during the CMJ, although the jump was performed without control of knee flexion angle. A biological basis that can explain the importance of RFD on vertical jump performance is that developing high impulse during a lower limb push-off and, in turn, accelerating a mass as much as possible, depends on the capacity to apply force rapidly during the propulsive phases and to produce high amounts of mechanical work over a short duration (Linthorne, [&lt;reflink idref=&quot;bib12&quot; id=&quot;ref37&quot;&gt;12&lt;/reflink&gt;]; Morin et al., [&lt;reflink idref=&quot;bib19&quot; id=&quot;ref38&quot;&gt;19&lt;/reflink&gt;]). Despite the evident biological basis, some studies (Dal Pupo et al., [&lt;reflink idref=&quot;bib4&quot; id=&quot;ref39&quot;&gt;4&lt;/reflink&gt;]; Miller et al., [&lt;reflink idref=&quot;bib17&quot; id=&quot;ref40&quot;&gt;17&lt;/reflink&gt;]; Ebben et al., [&lt;reflink idref=&quot;bib5&quot; id=&quot;ref41&quot;&gt;5&lt;/reflink&gt;]) reported a poor relationship between RFD and VJH. It is possible the methodological issues associated with the measurement of RFD and the high variability may explain the previously reported poor correlation (Dal Pupo et al., [&lt;reflink idref=&quot;bib4&quot; id=&quot;ref42&quot;&gt;4&lt;/reflink&gt;]; McLellan et al., [&lt;reflink idref=&quot;bib15&quot; id=&quot;ref43&quot;&gt;15&lt;/reflink&gt;]).&lt;/p&gt; &lt;p&gt;It has been reported that depending on the level of squat depth used in vertical jumps, different rates of force are produced. Usually, athletes in a short-amplitude condition try to optimize force production with greater acceleration, producing high RFD, aiming to maximize jumping height. On the other hand, in jumps performed from a deeper squat depth, there is more time for impulse transmission, so that lower values of RFD are found, and higher impulses may be produced (Gheller et al., [&lt;reflink idref=&quot;bib6&quot; id=&quot;ref44&quot;&gt;6&lt;/reflink&gt;]). Thus, it could be expected that the largest correlations between RFD and VJH would be verified in the SJ&lt;subs&gt;110&lt;/subs&gt;. It is possible to speculate that the positions close to a knee angle of 70&#176; or a self-selected angle allow a better body adjustment to optimize the rapid force production and the generated impulse, reflecting positively on VJH. From this, as a practical application, our results suggest that those aiming to optimize jump height should emphasize lower-limb exercises from deeper squat positions (i.e., smaller knee angles), where the relationship between rapid force production and performance appears to be optimized.&lt;/p&gt; &lt;p&gt;The results of the present study showed the importance of RFD, an indicator of rapid or &quot;explosive&quot; strength, on vertical jump performance. Therefore, training methods emphasizing explosive techniques designed to improve RFD should lead to improvements in VJH and, ultimately, improved dynamic sports performance. In addition, attention must be given to the reliability metrics since, to identify a real change that occurs due to training, it is necessary to first identify the measurement error. In a vertical jump assessment, we suggest measuring RFD from the beginning of the propulsive phase until the peak force, and larger time windows are preferred over shorter ones.&lt;/p&gt; &lt;p&gt;Our results demonstrate that the reliability of RFD primarily depends on the size of the time window, with a lesser degree of influence from other factors. However, some variables, particularly in the reliability analysis, were underpowered. We did not conduct an a priori sample size calculation in this study because the participants were selected based on accessibility and logistical reasons, which we acknowledge as a limitation. It is well-established that limitations in sample size can reduce the statistical power of a study, thereby increasing the likelihood of Type II errors, where true effects (e.g., significant relationships between variables) may go undetected. This could lead to incorrect conclusions about the absence of a significant effect. Therefore, we recommend that future studies increase the sample size to enhance the robustness of the findings and reduce the potential for statistical error, particularly when the focus is on short-time windows.&lt;/p&gt; &lt;hd id=&quot;AN0182438458-10&quot;&gt;Conclusion&lt;/hd&gt; &lt;p&gt;The reliability of RFD is dependent on the time windows evaluated. Based on our results, we conclude that only the RFD measured from zero to peak force showed acceptable reliability (i.e., CV &amp;lt; 10% and ICC &amp;gt; 0.75) for all SJ positions analyzed. In general, higher ICC and CV values were observed in the larger time windows of RFD (100–200, 0–200, and 0-PF) compared to the shorter ones. In addition, the RFD&lt;subs&gt;0-PF&lt;/subs&gt; presented the largest correlation with VJH, demonstrating the importance of rapid or &quot;explosive&quot; force on lower limb ballistic performance.&lt;/p&gt; &lt;hd id=&quot;AN0182438458-11&quot;&gt;Acknowledgments&lt;/hd&gt; &lt;p&gt;To the athletes who participated in this study&lt;/p&gt; &lt;hd id=&quot;AN0182438458-12&quot;&gt;Disclosure statement&lt;/hd&gt; &lt;p&gt;No potential conflict of interest was reported by the author(s).&lt;/p&gt; &lt;ref id=&quot;AN0182438458-13&quot;&gt; &lt;title&gt; References &lt;/title&gt; &lt;blist&gt; &lt;bibl id=&quot;bib1&quot; idref=&quot;ref26&quot; type=&quot;bt&quot;&gt;1&lt;/bibl&gt; &lt;bibtext&gt; Atkinson, G., &amp;amp; Nevill, A. M. (1998). 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Gon&#231;alves and Amilton Vieira&lt;/p&gt; &lt;p&gt;Reported by Author; Author; Author; Author; Author&lt;/p&gt; &lt;/aug&gt; &lt;nolink nlid=&quot;nl1&quot; bibid=&quot;bib13&quot; firstref=&quot;ref2&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl2&quot; bibid=&quot;bib21&quot; firstref=&quot;ref4&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl3&quot; bibid=&quot;bib14&quot; firstref=&quot;ref7&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl4&quot; bibid=&quot;bib24&quot; firstref=&quot;ref8&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl5&quot; bibid=&quot;bib12&quot; firstref=&quot;ref9&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl6&quot; bibid=&quot;bib10&quot; firstref=&quot;ref10&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl7&quot; bibid=&quot;bib15&quot; firstref=&quot;ref13&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl8&quot; bibid=&quot;bib17&quot; firstref=&quot;ref14&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl9&quot; bibid=&quot;bib18&quot; firstref=&quot;ref17&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl10&quot; bibid=&quot;bib16&quot; firstref=&quot;ref20&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl11&quot; bibid=&quot;bib20&quot; firstref=&quot;ref21&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl12&quot; bibid=&quot;bib11&quot; firstref=&quot;ref22&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl13&quot; bibid=&quot;bib22&quot; firstref=&quot;ref23&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl14&quot; bibid=&quot;bib23&quot; firstref=&quot;ref24&quot;&gt;&lt;/nolink&gt; &lt;nolink nlid=&quot;nl15&quot; bibid=&quot;bib19&quot; firstref=&quot;ref38&quot;&gt;&lt;/nolink&gt;
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  Data: Is the Rate of Force Development Produced in the Squat Jump a Reliable Metric and Is It Related to Jump Height?
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  Data: &lt;searchLink fieldCode=&quot;AR&quot; term=&quot;%22Juliano+Dal+Pupo%22&quot;&gt;Juliano Dal Pupo&lt;/searchLink&gt; (ORCID &lt;externalLink term=&quot;https://orcid.org/0000-0003-4084-9474&quot;&gt;0000-0003-4084-9474&lt;/externalLink&gt;)&lt;br /&gt;&lt;searchLink fieldCode=&quot;AR&quot; term=&quot;%22Rafael+Lima+Kons%22&quot;&gt;Rafael Lima Kons&lt;/searchLink&gt; (ORCID &lt;externalLink term=&quot;https://orcid.org/0000-0003-1615-5464&quot;&gt;0000-0003-1615-5464&lt;/externalLink&gt;)&lt;br /&gt;&lt;searchLink fieldCode=&quot;AR&quot; term=&quot;%22Rodrigo+Ghedini+Gheller%22&quot;&gt;Rodrigo Ghedini Gheller&lt;/searchLink&gt; (ORCID &lt;externalLink term=&quot;https://orcid.org/0000-0002-2259-8096&quot;&gt;0000-0002-2259-8096&lt;/externalLink&gt;)&lt;br /&gt;&lt;searchLink fieldCode=&quot;AR&quot; term=&quot;%22Carlos+W%2E+P%2E+Gon&#231;alves%22&quot;&gt;Carlos W. P. Gon&#231;alves&lt;/searchLink&gt; (ORCID &lt;externalLink term=&quot;https://orcid.org/0000-0002-6481-0446&quot;&gt;0000-0002-6481-0446&lt;/externalLink&gt;)&lt;br /&gt;&lt;searchLink fieldCode=&quot;AR&quot; term=&quot;%22Amilton+Vieira%22&quot;&gt;Amilton Vieira&lt;/searchLink&gt; (ORCID &lt;externalLink term=&quot;https://orcid.org/0000-0002-6027-4367&quot;&gt;0000-0002-6027-4367&lt;/externalLink&gt;)
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  Data: &lt;searchLink fieldCode=&quot;DE&quot; term=&quot;%22Physical+Education%22&quot;&gt;Physical Education&lt;/searchLink&gt;&lt;br /&gt;&lt;searchLink fieldCode=&quot;DE&quot; term=&quot;%22Athletics%22&quot;&gt;Athletics&lt;/searchLink&gt;&lt;br /&gt;&lt;searchLink fieldCode=&quot;DE&quot; term=&quot;%22Physical+Activities%22&quot;&gt;Physical Activities&lt;/searchLink&gt;&lt;br /&gt;&lt;searchLink fieldCode=&quot;DE&quot; term=&quot;%22Males%22&quot;&gt;Males&lt;/searchLink&gt;&lt;br /&gt;&lt;searchLink fieldCode=&quot;DE&quot; term=&quot;%22Athletes%22&quot;&gt;Athletes&lt;/searchLink&gt;&lt;br /&gt;&lt;searchLink fieldCode=&quot;DE&quot; term=&quot;%22Motion%22&quot;&gt;Motion&lt;/searchLink&gt;&lt;br /&gt;&lt;searchLink fieldCode=&quot;DE&quot; term=&quot;%22Muscular+Strength%22&quot;&gt;Muscular Strength&lt;/searchLink&gt;&lt;br /&gt;&lt;searchLink fieldCode=&quot;DE&quot; term=&quot;%22Performance%22&quot;&gt;Performance&lt;/searchLink&gt;&lt;br /&gt;&lt;searchLink fieldCode=&quot;DE&quot; term=&quot;%22Team+Sports%22&quot;&gt;Team Sports&lt;/searchLink&gt;
– Name: DOI
  Label: DOI
  Group: ID
  Data: 10.1080/1091367X.2024.2417100
– Name: ISSN
  Label: ISSN
  Group: ISSN
  Data: 1091-367X&lt;br /&gt;1532-7841
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: This study aimed to analyze the intrasession reliability of the rate of force development (RFD) in different time windows in squat jumps (SJ) performed at different knee starting angles, and identify the relationship of RFD with vertical jump height (VJH). Twenty male volleyball and basketball players participated. Athletes randomly performed SJ from four different knee-flexion angles. The RFD was analyzed from six different windows, from force onset to instant peak force, in each position using the ground reaction forces. Reliability analysis was performed based on the intraclass correlation coefficient (ICC) and coefficient of variation (CV%); Spearman&#39;s rho was used to test the relationship between RFD and VJH. The results demonstrated that RFD[subscript 0-PF] was the only variable assumed to be reliable, demonstrating an ICC &gt; 0.84 and CV &lt; 8.5% in all positions, and significant correlations between RFD metrics and VJH were observed at SJ[subscript 70] and SJ[subscript PREF]. Some variables were underpowered due to the small sample size, as a limitation of the study.
– Name: AbstractInfo
  Label: Abstractor
  Group: Ab
  Data: As Provided
– Name: DateEntry
  Label: Entry Date
  Group: Date
  Data: 2025
– Name: AN
  Label: Accession Number
  Group: ID
  Data: EJ1459069
PLink https://search.ebscohost.com/login.aspx?direct=true&site=eds-live&db=eric&AN=EJ1459069
RecordInfo BibRecord:
  BibEntity:
    Identifiers:
      – Type: doi
        Value: 10.1080/1091367X.2024.2417100
    Languages:
      – Text: English
    PhysicalDescription:
      Pagination:
        PageCount: 7
        StartPage: 113
    Subjects:
      – SubjectFull: Physical Education
        Type: general
      – SubjectFull: Athletics
        Type: general
      – SubjectFull: Physical Activities
        Type: general
      – SubjectFull: Males
        Type: general
      – SubjectFull: Athletes
        Type: general
      – SubjectFull: Motion
        Type: general
      – SubjectFull: Muscular Strength
        Type: general
      – SubjectFull: Performance
        Type: general
      – SubjectFull: Team Sports
        Type: general
    Titles:
      – TitleFull: Is the Rate of Force Development Produced in the Squat Jump a Reliable Metric and Is It Related to Jump Height?
        Type: main
  BibRelationships:
    HasContributorRelationships:
      – PersonEntity:
          Name:
            NameFull: Juliano Dal Pupo
      – PersonEntity:
          Name:
            NameFull: Rafael Lima Kons
      – PersonEntity:
          Name:
            NameFull: Rodrigo Ghedini Gheller
      – PersonEntity:
          Name:
            NameFull: Carlos W. P. Gonçalves
      – PersonEntity:
          Name:
            NameFull: Amilton Vieira
    IsPartOfRelationships:
      – BibEntity:
          Dates:
            – D: 01
              M: 01
              Type: published
              Y: 2025
          Identifiers:
            – Type: issn-print
              Value: 1091-367X
            – Type: issn-electronic
              Value: 1532-7841
          Numbering:
            – Type: volume
              Value: 29
            – Type: issue
              Value: 1
          Titles:
            – TitleFull: Measurement in Physical Education and Exercise Science
              Type: main
ResultId 1