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Article

Comparison of Rate of Force Development between Explosive Sustained Contractions and Ballistic Pulse-like Contractions during Isometric Ankle and Knee Extension Tasks

by
Žiga Kozinc
1,2,*,
Jernej Pleša
1,
Daniel Djurić
1 and
Nejc Šarabon
1,3,4
1
Faculty of Health Sciences, University of Primorska, Polje 42, SI-6310 Izola, Slovenia
2
Andrej Marušič Institute, University of Primorska, Muzejski trg 2, SI-6000 Koper, Slovenia
3
Human Health Department, InnoRenew CoE, Livade 6, SI-6310 Izola, Slovenia
4
Ludwig Boltzmann Institute for Rehabilitation Research, Neugebäudeplatz 1, 3100 St. Pölten, Austria
*
Author to whom correspondence should be addressed.
Appl. Sci. 2022, 12(20), 10255; https://doi.org/10.3390/app122010255
Submission received: 21 September 2022 / Revised: 4 October 2022 / Accepted: 10 October 2022 / Published: 12 October 2022
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Abstract

:
Background: The rate of force development (RFD) is a measure of explosive strength, commonly evaluated within the same repetition as peak force/torque, by instructing the participants to produce “fast” and “hard” contraction. Previous studies have indicated that attempting to achieve the maximal RFD and maximal force/torque within the same contraction may not be optimal. Methods: This study assessed the differences between explosive sustained (“fast-and-hard”) and ballistic (pulse-like) contractions on the RFD during isometric ankle and knee extensions in young, recreationally active participants (n = 33; age: 23.4 ± 5.6 years). The participants performed both contraction types for isometric ankle and knee extension tasks. The RFD was calculated for time intervals 50, 100, 150 and 200 ms from the contraction onset and also in between these time points (50–100, 100–150 and 150–200 ms). Findings: The results of this study showed a higher RFD in the ballistic contractions in later time intervals (100–150 and 150–200 ms) compared to that of the explosive sustained contractions (effect size (Hedge’s g) = 0.51–0.73). The differences between explosive sustained and ballistics contractions were larger and occurred for more outcome variables in females than males. Peak RFD was also larger in ballistic contractions, both for females (p = 0.010; Hedge’s g = 0.54) and males (p = 0.003; Hedge’s g = 0.78). The intra-session reliability of the RFD was similar for both contraction types, increasing from moderate to excellent with longer time intervals. Conclusion: Our results suggest that ballistic contractions should be used when the assessment of the RFD is the primary goal. When the aim is to assess both the RFD and peak force, it is suggested to use the combination of separate explosive sustained contractions and ballistic contractions in the protocol.

1. Introduction

The rate of force development (RFD) is a commonly used measure of explosive strength in humans [1]. RFD is being increasingly used for assessments in sport science [2], physical therapy and rehabilitation [3], as well as in in studies investigating the basic biomechanics and physiology of human movement [1,4]. A thorough review of the RFD literature was published by Maffiuletti et al. [1], summarizing the multiple underlying mechanisms as well as several methodological considerations pertaining to RFD assessments. It stressed that attempting to achieve the maximal force and RFD within the same contraction may not be optimal [1]. This recommendation was based on studies (summarized further in the following paragraphs) comparing the instructions given to participants to contract as “hard” as possible (no regard to the RFD), as “hard and as fast” as possible (emphasis on peak force, but with a fast start), and as “fast and as hard” as possible (emphasizing the quick RFD, but also sustaining the effort to achieve peak force). The instruction should emphasize the “fast” aspect, in order to obtain the maximal RFD.
The first study to compare the RFD between “hard-and-fast” and “fast-and-hard” instructions reported that the latter elicited a slightly higher peak RFD (~8–12%) in handgrip muscles [5]. A subsequent study reported a similar finding for the ankle, elbow and index-finger muscles, with the differences in the RFD between the instructions ranging from 23% to 53%, depending on the muscle group [6]. The larger difference in comparison to the former study could be attributed to slightly different instructions, as Christ et al. [6] instructed the participants only to attain peak force and allowed for individual strategies in exerting maximal force (for the “hard-and-fast” instruction). In contrast, Bemben et al. [5] instructed the participants to contract as hard and as fast as possible. Differences in the peak RFD (~21%) between “hard-and-fast” and “fast-and-hard” instructions have been recently shown for the knee extension task [7]. Note that these studies only analyzed the peak RFD. A study using an ankle dorsiflexion task reported a ~20–30% higher RFD for the “fast-and-hard” contraction, although only for the peak RFD and the RFD in the 50 and 100 ms time windows, whereas the differences were reversed in the 200–400 ms time windows [8]. A recent study reported that the RFD was higher throughout the first 250 ms for the “fast-and-hard” compared to “hard-and-fast” instructions, for the isometric squat [9].
Another study compared “hard-and-fast” to “fast-and-hard” instructions in the leg press and elbow flexion task and also reported a substantially higher (~27–32%) peak RFD in both tasks for the “fast-and-hard” instruction [10]. This study also reported moderate to high correlations (r = 0.55–0.78) between the peak RFD values from each instruction [10]. A later electromyography study by the same research group demonstrated that the “fast-and-hard” instruction elicits a larger agonist activation at the beginning of the contraction (compared to the “hard-and-fast” instruction), while the antagonist co-activation was unaffected by the instruction [11]. Higher EMG activity could be related to the increased motor unit firing rate [12] or speed of recruitment of motor units [13].
While the emphasis on the “fast” execution of the contraction is needed to obtain the highest RFD, all of the aforementioned studies instructed the participants to focus on the fast start, but to sustain high force contraction in the subsequent 1–4 s as well. Moreover, some studies instructed the participants that the peak force is of no concern during the fast instructions [5] and termed the instruction as “fast” instead of “fast-and-hard”. This is probably not optimal, considering the well-known linear relationship between peak force and RFD [14]. It could also be that using ballistic (pulse-like) contractions (without sustaining the contraction after the peak force is reached), with an emphasis on fast force rise, would elicit an even higher RFD than “fast-and-hard” contractions. Recently, Smajla et al. [15] introduced the theoretical peak RFD, extrapolated from fast ballistic contractions of varying intensities (20–80% of the maximal voluntary force). Interestingly, this extrapolated value was higher than the actual peak RFD obtained during “fast-and-hard” contractions in females, while the opposite was the case in males [15]. Therefore, it could be that ballistics contractions would elicit an even higher RFD than “fast-and-hard” contractions, at least in females. The only study to analyze the difference in the RFD between “hard-and-fast” and “fast-and-hard” instructions, which considered the effect of sex, reported that the differences between instructions tended to be higher in females (~12%) than males (~8%). This indicated that females could be more sensitive to the instructions. Duchateau and Baudry (2014) noted in their review paper that ballistic contractions in their laboratory elicited an ~16% greater RFD compared to sustained contractions, but it was not clear whether the latter were “hard-and-fast” or “fast-and-hard” contractions.
The purpose of this study was to compare the difference between “fast-and-hard” explosive sustained contractions (“fast-and-hard” instruction; referred to as explosive sustained contractions hereafter) and ballistic pulse-like contractions (referred to as ballistic contractions hereafter) on the RFD during isometric ankle and knee extension tasks in male and female participants. Based on the reported theoretical peak RFD extrapolated from submaximal ballistic contractions [15], we hypothesized that the ballistic contractions in our study will elicit a higher RFD than the sustained contractions. For the secondary aims, we checked the intrasession reliability of the RFD across different time intervals and the correlations between contraction types in the corresponding RFD variables. We hypothesized that the early RFD (50 ms) will show moderate reliability in both contraction types, whereas the late RFD (100–200 ms) will show good-to-excellent reliability [1]. Based on the studies comparing “hard-and-fast” to “fast-and-hard” contractions [10], we hypothesized that the correlations between contraction types will be moderate to strong (r = 0.4–0.9).

2. Materials and Methods

2.1. Participants

A convenience sample of 33 healthy young participants (17 males, 16 females) was recruited for the study (age: 23.4 ± 5.6 years; body mass: 78.2 ± 3.4 kg; body height: 178.1 ± 19.2 cm). The participants were recreationally active in various sport activities and reported performing resistance exercise 2–4 times a week. Participants with any lower leg injuries in the past 6 months, known neurological diseases or any current musculoskeletal pain were excluded from the study. Participants were thoroughly informed about the testing procedures, and written informed consent was required prior to commencing the study. The experiment was approved by National Medical Ethics Committee and was conducted in accordance with the latest revision of the Declaration of Helsinki.

2.2. Study Design

The participants were asked to refrain from any vigorous physical activity for 48 h before the measurement session. The experiment was conducted within a single session, lasting approximately 50 min. The participants performed a standardized warm up, consisting of 5 min of jogging at a moderate pace on a treadmill, 5 min of dynamic stretching and a set of 5 bodyweight resistance exercises (10 squats, 5 push-ups, 5 lunges per leg, 5 submaximal countermovement jumps, 10 calf raises). Then, the participants went on to perform the RFD tasks on isometric dynamometers. The order of the assessments for different joints (ankle, knee) and contraction types (sustained, pulse) was randomized. The breaks between the assessments for different joints was set to 5 min, to ensure there was enough time to prepare for each participant (setting up on the different dynamometers), while the breaks between the contraction types were set to 2 min.

2.3. Apparatus

All assessments were conducted using isometric dynamometers (S2P, Science to Practice, Ljubljana, Slovenia) with embedded force sensors (model 1-Z6FC3/200 kg). These dynamometers are constructed of rigid metal elements with minimal padding and encompass several straps used for fixation, thereby allowing for a reliable assessment of RFD. For the assessment of the ankle extension RFD (Figure 1, top left), the participant’s shins were tightly secured within the dynamometer frame, and the feet were placed on a rigid metal plate mounted on the force sensor. The axis of the dynamometer was carefully aligned with the medial malleolus, and the ankle was in the neutral position (90°). The foot was tightly fixated against the plate with a strap. The knee and hip angles were set at 90° as well, which was achieved by adjusting the height and the depth of the dynamometer seat. For the assessment of the knee extension RFD (Figure 1, top right), the participant was seated in the dynamometer, which was tightly fixated across the pelvis and across the lower portion of the thighs, just above the knees. The axis of the dynamometer was aligned with the lateral femoral condyle, and the knee angle was set to 60° (with 0° representing knee extension). Both dynamometers have been previously reported to produce highly reliable assessment of maximal force/torque and acceptably reliable assessments of RFD [16,17].

2.4. Procedures

After being fixated within the dynamometer, the participants performed warm-up trials for familiarization and to verify that they understood the tasks. For the sustained contractions, three warm-up trials were performed with the instruction to use 50%, 75% and 90% of maximal effort (30 s breaks between repetitions). For the ballistic contractions, six familiarization repetitions (15 s breaks between repetitions) were performed, with the instruction to gradually increase the effort from 50% to 90% of the maximum. For the sustained contraction, the instruction was to produce the force as “fast and as hard as possible” and to maintain maximal exertion for ~3–4 s. The participants were instructed and constantly reminded to focus on emphasizing the explosive start (the fast part of the contraction). For the ballistic contractions, the participants were instructed to “produce a brief pulse-like contraction as fast as possible”, aiming to achieve maximal possible peak force, but without maintaining the contraction after the peak force is reached. In both contraction types, no countermovement was allowed. The force-time signals were inspected during the protocol, and the trial was repeated in case of visible countermovement. Three repetitions of sustained contraction and six repetition of pulse contractions were performed. The breaks between the repetitions were 2 min for the sustained contractions and 30 s for the pulse contractions. Loud verbal encouragement was provided at all times, and the participants received real-time feedback on force trace on a computer screen placed ~2 m away, slightly below eye level.

2.5. Data Analysis

The force signals were sampled at 1000 Hz and were further automatically processed in the manufacturer’s software (Analysis and Reporting Software, S2P, Ljubljana, Slovenia). A moving average filter (window: 5 ms) was applied, after which the RFD was calculated as Δ force/Δ time for 0–50, 0–100, 0–150 and 0–200, as well as for 50–100, 100–150 and 150–200 ms time intervals, after the onset of the contraction. To ensure an accurate determination of force-rise onset, manual marker positioning was done within the software, as recommended in the literature [1]. Peak torque was determined from the sustained contraction as the mean value of the 1 s interval. For all of the outcome variables, the best repetition (i.e., highest value) was considered for the main statistical analyses (differences and correlations). All values for the respective task (3 for sustained and 6 for ballistic contractions) were considered for the reliability analysis. The outcomes were not normalized to peak force or body mass, as the main purpose was to investigate the effect of contraction type (within-subject factor).

2.6. Statistical Analysis

Statistical analyses were done with SPSS (version 26.0, SPSS Inc., Chicago, IL, USA). Descriptive statistics are reported as mean ± standard deviation. The normality of the data distribution was verified with Shapiro–Wilk tests. The intravisit reliability was assessed with two-way random intraclass correlation coefficient (ICC) for single measures (absolute agreement) and the typical error, which was expressed as percentage of the mean value. The reliability according to ICC was interpreted as poor (<0.5), moderate (0.5–0.75), good (0.75–0.9) and excellent (>0.9) [18]. Two-way mixed-model ANOVA was used to analyze the effect of the contraction type as within-subject factor (sustained vs. ballistic) and sex as between-subject factor (male vs. female), as well as the interaction between the two main effects. Effect sizes were expressed as partial eta-squared (η2) and interpreted as trivial (<0.01), small (0.01–0.06), medium (0.06–0.14) and large (>0.14) [19]. Further separate comparisons between contractions types for each sex were done with pair-wise t-tests, with effects sizes calculated as Hedge’s g and interpreted as trivial (<0.2), small (>0.2–0.6), moderate (>0.6–1.2), large (>1.2–2.0) and very large (>2.0–4.0) [20]. Since multiple outcome variables were separately analyzed, a Bonferroni–Holm correction was applied to control for the family-wise error rate [21]. Correlations between outcomes of the two contraction types were assessed with Pearson’s correlation coefficients and interpreted as negligible (<0.1), weak (0.1–0.39), moderate (0.4–0.69), strong (0.7–0.89) and very strong (>0.9) [22]. The threshold for statistical significance was set at α < 0.05.

3. Results

3.1. Descriptive Statistics and Reliability

All participants completed the full set of measurements, and no data were discarded. The descriptive statistics with reliability analyses are available in Table 1 and Table 2 for the ankle and knee extension tasks, respectively. Males produced higher peak forces than females in the ankle extension task (375.0 ± 57.2 N vs. 267.1 ± 51.9 N; p < 0.001) and knee extension task (721.8 ± 158.1 N vs. 434.6 ± 114.2 N; p < 0.001). Regarding the ankle extension, the reliability was good for most of the RFD variables. The peak RFD showed good (ICC = 0.81) and moderate (ICC = 0.69) relative reliability for the ankle task in sustained and ballistics contractions, respectively. The early RFD (0–50 ms) also had only moderate reliability for the ballistic contractions (ICC = 0.61). For the sustained contractions, the RFD in later and longer intervals (100–150, 150–200 ms) and also in 0–150 and 0–200 ms reached excellent reliability scores (ICC = 0.92–0.95). Typical errors ranged between 7.3% and 22.9% for the sustained contractions and between 8.6 and 33.3% for the ballistic contractions. For both contraction types, the typical errors notably decreased as the time interval increased.
Regarding the knee extension task, the reliability was also good for most of the outcome variables. The peak RFD showed only moderate reliability for the knee task, for both contraction types (ICC = 0.59–0.61). Moreover, only moderate reliability was found for variables for the 0–50 ms time interval for both sustained (ICC = 0.61) and ballistic contractions (ICC = 0.65). Excellent reliability was found only for the RFD at 0–200 ms for the sustained task (ICC = 0.94). Typical errors ranged between 8.6% and 40.1% for the sustained contractions and between 11.0% and 39.6% for the ballistic contractions. As in the ankle extension task, the typical errors for the knee task decreased as the time interval increased.

3.2. Effect of Task on Peak RFD

Regarding the peak RFD for the ankle task, there was no sex × contraction-type interaction (p = 0.179). The main effect of sex (p = 0.010; η2 = 0.19) and contraction type (p = 0.026; η2 = 0.15) were both statistically significant. Paired t-test showed a statistically significant difference between contraction types (larger RFD in ballistic contractions) in females (p = 0.006; Hedge’s g = 0.68), but not in males (p = 0.071).
Regarding the peak RFD for the knee task, there was a statistically significant sex × contraction type interaction (p = 0.015; η2 = 0.15). The main effect of sex (p < 0.001; η2 = 0.42) and contraction type (p < 0.001; η2 = 0.35) were both statistically significant. A paired t-test showed statistically significant differences between contraction types in females (p = 0.010; Hedge’s g = 0.54) and males (p = 0.003; Hedge’s g = 0.78).

3.3. Effect of Task on Interval-Specific RFD

The RFD outcomes are summarized in Figure 2 and Figure 3 for the ankle and knee extension tasks, respectively. As expected, there was a main effect of sex for all RFD variables. The effect sizes pertaining to sex differences were moderate or large for the ankle extension task (η2 = 0.12–0.21; p = 0.007–0.049) and large for the knee extension task (η2 = 0.38–0.55; p < 0.001). There were no sex × contraction-type interactions (p = 0.149–0.608).
The main effect of contraction type was present in the ankle extension task for the two latest intervals, 100–150 ms and 150–200 ms (p = 0.004–0.005; η2 = 0.22–0.23), as well as the RFD in the 0–150 ms interval (p = 0.044; η2 = 0.13). Pairwise comparisons in males showed that ballistic contractions produced a higher RFD than sustained contractions at the 100–150 and 150–200 ms intervals (mean difference: 386.3 N/s and 290.5 N/s; p = 0.035–0.46; Hedge’s g = 0.51–0.55). Similarly, pairwise tests in females indicated a higher RFD at the 100–150 and 150–200 ms intervals in ballistic contractions compared to sustained contractions (mean difference: 172.5 N/s and 149.6 N/s; p = 0.003–0.007; Hedge’s g = 0.58–0.61) (Figure 2).
In the knee extension task, the main effect of contraction on the RFD was statistically significant at the 100–150 and 150–200 ms intervals (p = 0.004–0.024; η2 = 0.16–0.24), as well as the 0–200 ms interval (p = 0.002; η2 = 0.26). In males, there were no differences between sustained and ballistic contractions (p = 0.132–0.813). In females, ballistic contractions produced a statistically significantly higher RFD at the 100–150 and 150–200 ms intervals (mean difference: 454.3 N/s and 486.6 N/s; p = 0.001–0.003; Hedge’s g = 0.58–0.73), and a statistically significantly higher RFD at the 0–100, 0–150 and 0–200 ms intervals (mean difference: 302.3 N/s, 250.1 N/s and 410.2 N/s; p = 0.003–0.016; Hedge’s g = 0.42–0.71) (Figure 3).

3.4. Correlations between the Tasks

In females, the correlations between the contraction types in the corresponding variables were statistically significant (all p < 0.006) and were moderate to strong (Pearson’s r: 0.64–0.78 for the ankle RFD; 0.66–0.81 for the knee RFD;). The only exception was the RFD for the ankle at the 0–50 ms time interval (r = 0.46–0.48; p = 0.056–0.075). In males, the correlations for all ankle variables were moderate and statistically significant (r = 0.51–0.55; p = 0.020–0.033), except for at the 0–50 ms time interval (r = 0.46; p = 0.062). A very similar pattern was present for the knee variables in males, with moderate statistically significant correlations (r = 0.47–0.57; p = 0.017–0.048), except for the early RFD (0–50 ms) (r = 0.27–0.33; p = 0.192–0.279).

4. Discussion

The purpose of this study was to test the differences between explosive sustained (“fast-and-hard” instruction) contractions and ballistic (pulse-like) contractions on the RFD during isometric ankle and knee extension tasks. We hypothesized that the ballistic contractions in our study would elicit a higher RFD than the sustained contractions. Our hypothesis was partially confirmed, as the RFD was indeed higher in the ballistic contractions, but only during late time intervals. Thus, ballistic contractions seem to enable the participants to achieve a higher RFD compared to sustained contractions (even if an emphasis on the fastest possible start is given for the sustained contraction). The differences between contraction types were more evident in females, which is also in line with our hypothesis. The exception was the peak RFD, where males tended to show a higher difference between the contractions; however, this outcome had low reliability and a very high between-participant variability. The intrasession reliability of the RFD was moderate to excellent, with very similar results for both contraction types. We also found moderate-to-strong correlations between the RFD in each of the contraction types in females, but only moderate correlations in males.
Altogether, our results indicated that ballistic contractions should be preferred over sustained contractions, when RFD assessment is the primary aim, and that both contraction types produce reliable RFD outcomes, with the exception of the peak RFD and early (0–50 ms) RFD. When the aim is to assess both the RFD and peak force, it is suggested to use a combination of separate “hard-and-fast” sustained contractions and ballistic contractions in the protocol. In case of time constraints, “fast-and-hard” instruction can be a reasonably valid solution, considering that the RFD values between sustained and ballistic contractions are moderately to highly correlated. Similarly high correlations have also been reported for the RFD obtained with contractions between “hard-and-fast” and “fast-and-hard” [10]. The lower reliability of the early RFD is consistent with previous studies [23] and has been attributed to neural factors, as early evoked force is much less variable. The reliability of the RFD in ballistic contractions in later time intervals is comparable or even higher compared to the previously reported reliability for “hard-and-fast” contractions [16,24,25]. In sum, ballistic contractions exhibited acceptable intrasession reliability and elicited a higher RFD than sustained contractions.
Several underlying mechanisms could be responsible for the difference in the RFD between sustained and ballistic contractions. Given that muscle/motor unit activity was not measured in this study, the discussion can only be speculative at the moment. The primary candidates for the underlying mechanisms include increased motor unit firing rate [12], speed of motor unit recruitment [13,26] and increased presence of doublet discharges [27]. Indeed, Duchateau and Baudry (2014) [12] have made a strong case that motor unit recruitment at the onset of contraction is the primary determinant of the RFD during ballistic contractions. While motor unit firing rate is considered as a primary determinant for the RFD in general [1], recent evidence also suggests a very important role for the speed of motor unit recruitment (i.e., the number of motor units recruited per second) [13]. Another possible determinant of the RFD is the co-activation of the antagonist muscles [28]. In submaximal ballistic contractions, the antagonist muscles are responsible for decelerating the force rise prior to peak force [29]. While coordinated antagonist activation is likely crucial for accurate peak-force control in such contractions [14], it remains to be determined if antagonist activity contributed to the differences in the RFD between ballistic and sustained contractions. Further studies comparing pulse and sustained contraction, preferably using high-density electromyography, are needed to reveal the exact underlying mechanisms of the differences observed in this study.
During submaximal ballistic contractions, there is a linear relationship between the peak force and RFD [14,30]. A previous study [15] introduced a theoretical peak RFD, which was extrapolated from the submaximal ballistic pulses of varying intensities. This theoretical peak RFD was higher than the actual RFD in explosive sustained contractions (“fast-and-hard” instruction) in females, but the opposite was true in males. This is partially in line with our findings, as the differences between ballistic and sustained contractions in our study were more pronounced in females compared to males. The reason for this difference is difficult to decipher. Differences in the RFD between sexes are minimal when normalized to maximum strength and/or body size [31,32,33]. This was also the case in our study, as the sexes had an equal RFD normalized to peak force (all p > 0.129). Males and females also show very similar scaling of the RFD with peak force in ballistic contractions [30,34]. It could be speculated that females have more difficulties in attaining the highest possible RFD in sustained contractions. Females have been reported to exhibit longer electromechanical delay [35], which could be a modulating factor in the RFD, but would likely primarily affect the early phase of force rise. On the other hand, sex differences in neural mechanisms, such as cortical and intracortical inhibition, could also play a role. The RFD in females was also more sensitive to whole-body vibration intervention, compared to the RFD in males [36], which points to possible sensory and central mechanism. However, the exact mechanism underlying the sex differences in RFD changes are still to be determined.
This study does not come without limitations. Our findings should be further verified using different joints and movements. Moreover, our sample consisted of individuals who were moderately trained with resistance exercise and moderately physically active. Further studies are needed to reveal if the results are applicable to other populations (e.g., elite resistance-trained athletes, older adults and patients with neuromuscular diseases). Moreover, intersession reliability of RFD in pulse contractions should also be verified. Finally, future experimental protocols should include in-depth investigation by recording motor unit behavior and possibly even spinal and corticospinal excitability, to provide an understating of the underlying mechanisms for the differences observed in this study.

Practical Application

This study has several important implications for the sport and health sciences. The RFD has been shown to be a determinant of athletic performance [2] and a useful marker in injury rehabilitation [37] and functional status in the elderly [38], and it also shows potential for the diagnostics of neurological diseases [39]. Accurate determination of the RFD is needed to increase confidence in the outcomes in these areas. This study shows that ballistic pulse-like contractions may elicit a higher RFD than explosive and sustained contractions. This indicates that ballistics contractions are to be preferred to determine the maximal possible RFD. Although the difference between contraction types was not large, they may affect the results and their interpretation. Furthermore, it could be that larger differences would arise in different populations and/or muscle groups, which is an interesting avenue to explore in future studies. Accurate determination of the RFD will help in decision-making during training design for athletes (e.g., identifying target muscle groups with low explosive strength), as well as contribute to diagnostics in clinical practice, particularly for neurological diseases such as multiple sclerosis [39].

5. Conclusions

In conclusion, this study demonstrates that ballistic (pulse-like) contractions elicit a higher RFD in isometric ankle and knee extension tasks in comparison to explosive sustained contractions, even in the case of a fast start being emphasized in the latter. Considering that both contraction types showed similar reliability, we recommend that ballistic contractions are used when RFD assessment is the primary goal.

Author Contributions

Conceptualization, Ž.K.; methodology, Ž.K.; software, N.Š.; validation, N.Š.; formal analysis, Ž.K. and J.P.; investigation, J.P. and D.D.; resources, Ž.K., J.P., D.D. and N.Š.; data curation, J.P. and D.D.; writing—original draft preparation, Ž.K.; writing—review and editing, J.P., D.D. and N.Š.; visualization, Ž.K. and N.Š.; supervision, Ž.K.; project administration, Ž.K. and N.Š. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

The experiment was approved by the National Medical Ethics Committee (approval no. 0120–99/2018/5) and was conducted in accordance with the latest revision of the Declaration of Helsinki.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 12 10255 g001 550
Figure 1. Dynamometers used for ankle (top left) and knee (top right) extension assessments. Examples of unfiltered signal traces for explosive sustained (left) and ballistic (right) contractions are shown on the bottom half of the figure. The webbing straps on the ankle dynamometer are routinely used because the rigid elements of the dynamometer can give out, in rare case of assessing very strong individuals (~600 N and above), which was not an issue in the current investigation.
Figure 1. Dynamometers used for ankle (top left) and knee (top right) extension assessments. Examples of unfiltered signal traces for explosive sustained (left) and ballistic (right) contractions are shown on the bottom half of the figure. The webbing straps on the ankle dynamometer are routinely used because the rigid elements of the dynamometer can give out, in rare case of assessing very strong individuals (~600 N and above), which was not an issue in the current investigation.
Applsci 12 10255 g001
Applsci 12 10255 g002 550
Figure 2. Comparison of RFD outcomes between explosive sustained and pulse contractions for the ankle extension task across time intervals for females (top graph) and males (bottom graph). * indicates statistically significant pair-wise comparison.
Figure 2. Comparison of RFD outcomes between explosive sustained and pulse contractions for the ankle extension task across time intervals for females (top graph) and males (bottom graph). * indicates statistically significant pair-wise comparison.
Applsci 12 10255 g002
Applsci 12 10255 g003 550
Figure 3. Comparison of RFD outcomes between explosive sustained and pulse contractions for the knee extension task across time intervals for females (top graph) and males (bottom graph). * indicates statistically significant pair-wise comparison.
Figure 3. Comparison of RFD outcomes between explosive sustained and pulse contractions for the knee extension task across time intervals for females (top graph) and males (bottom graph). * indicates statistically significant pair-wise comparison.
Applsci 12 10255 g003
Table
Table 1. Descriptive statistics and reliability analysis for ankle extension task.
Table 1. Descriptive statistics and reliability analysis for ankle extension task.
Outcome VariableMeanSDICC (95% CI)TE (%)
Sustained Contractions
Peak force (N)322.776.80.98 (0.97–0.99)3.8 (3.3–4.6)
Peak RFD (N/s)2338.4613.10.81 (0.65–0.92)13.12 (7.8–19.2)
RFD 0–50 (N/s)1146.0434.90.78 (0.66–0.87)22.2 (19.1–26.8)
RFD 0–100 (N/s)1427.1467.90.87 (0.78–0.92)14.5 (12.5–17.6)
RFD 0–150 (N/s)1363.1394.00.92 (0.87–0.96)9.4 (8.1–11.4)
RFD 0–200 (N/s)1211.5331.40.95 (0.91–0.97)7.4 (6.4–8.9)
RFD 50–100 (N/s)1589.8566.40.85 (0.76–0.91)16.6 (14.3–20.1)
RFD 100–150 (N/s)1537.7455.40.92 (0.87–0.95)9.8 (8.4–11.8)
RFD 150–200 (N/s)1375.6380.30.95 (0.91–0.97)7.3 (6.3–8.8)
Ballistic Contractions
Peak RFD (N/s)2994.81766.10.69 (0.44–0.82)24.12 (12.1–38.7)
RFD 0–50 (N/s)1109.1711.90.61 (0.46–0.74)33.3 (29.5–38.8)
RFD 0–100(N/s)1562.4720.90.82 (0.73–0.89)15.0 (13.3–17.5)
RFD 0–150 (N/s)1532.7589.20.89 (0.83–0.93)9.3 (8.2–10.8)
RFD 0–200 (N/s)1340.0494.10.88 (0.82–0.93)10.5 (9.3–12.2)
RFD 50–100 (N/s)1765.7915.60.78 (0.68–0.87)18.0 (15.9–21.0)
RFD 100–150 (N/s)1820.4679.00.88 (0.82–0.93)10.2 (9.0–11.9)
RFD 150–200 (N/s)1597.9542.80.90 (0.84–0.94)8.6 (7.6–10.0)
SD—standard deviation; ICC—intraclass correlation coefficient; CI—confidence interval; TE—typical error; RFD—rate of force development.
Table
Table 2. Descriptive statistics and reliability analysis for knee extension task.
Table 2. Descriptive statistics and reliability analysis for knee extension task.
Outcome VariableMeanSDICC (95% CI)TE (%)
Sustained Contractions
Peak force [N]582.6199.70.97 (0.95–0.98)6.5 (5.6–7.9)
Peak RFD [N/s]5546.9271.80.59 (0.39–0.83)23.1 (12.1–34.5)
RFD 0–50 [N/s]3190.41594.80.61 (0.43–0.75)37.2 (32–45)
RFD 0–100 [N/s]3007.41126.70.82 (0.71–0.89)18 (15.4–21.8)
RFD 0–150 [N/s]2556.6919.70.90 (0.84–0.94)12.4 (10.6–15)
RFD 0–200 [N/s]2143.9750.70.94 (0.91–0.97)8.8 (7.6–10.7)
RFD 50–100 [N/s]3517.01367.40.78 (0.66–0.87)20.8 (17.9–25.1)
RFD 100–150 [N/s]2837.31004.40.89 (0.83–0.94)12.5 (10.7–15.1)
RFD 150–200 [N/s]2353.1823.60.90 (0.84–0.94)12.3 (10.5–14.8)
Ballistic Contractions
Peak RFD [N/s]7454.14466.20.61 (0.39–0.85)24.2 (11.1–36.6)
RFD 0–50 [N/s]3207.61330.70.65 (0.51–0.77)32.7 (28.9–38.1)
RFD 0–100 [N/s]3187.91008.50.89 (0.83–0.93)12.1 (10.6–14.1)
RFD 0–150 [N/s]2676.7840.80.89 (0.84–0.94)11.5 (10.2–13.4)
RFD 0–200 [N/s]2078.7731.30.86 (0.78–0.91)17.4 (15.4–20.2)
RFD 50–100 [N/s]3893.91268.20.73 (0.62–0.83)20.1 (17.8–23.4)
RFD 100–150 [N/s]3173.9942.40.89 (0.83–0.93)11.0 (9.7–12.7)
RFD 150–200 [N/s]2526.2801.30.84 (0.76–0.9)14.7 (12.9–17.2)
SD—standard deviation; ICC—intra-class correlation coefficient; CI—confidence interval; TE—typical error; RFD—rate of force development.
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Kozinc, Ž.; Pleša, J.; Djurić, D.; Šarabon, N. Comparison of Rate of Force Development between Explosive Sustained Contractions and Ballistic Pulse-like Contractions during Isometric Ankle and Knee Extension Tasks. Appl. Sci. 2022, 12, 10255. https://doi.org/10.3390/app122010255

AMA Style

Kozinc Ž, Pleša J, Djurić D, Šarabon N. Comparison of Rate of Force Development between Explosive Sustained Contractions and Ballistic Pulse-like Contractions during Isometric Ankle and Knee Extension Tasks. Applied Sciences. 2022; 12(20):10255. https://doi.org/10.3390/app122010255

Chicago/Turabian Style

Kozinc, Žiga, Jernej Pleša, Daniel Djurić, and Nejc Šarabon. 2022. "Comparison of Rate of Force Development between Explosive Sustained Contractions and Ballistic Pulse-like Contractions during Isometric Ankle and Knee Extension Tasks" Applied Sciences 12, no. 20: 10255. https://doi.org/10.3390/app122010255

APA Style

Kozinc, Ž., Pleša, J., Djurić, D., & Šarabon, N. (2022). Comparison of Rate of Force Development between Explosive Sustained Contractions and Ballistic Pulse-like Contractions during Isometric Ankle and Knee Extension Tasks. Applied Sciences, 12(20), 10255. https://doi.org/10.3390/app122010255

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