Can the Countermovement Jump or Squat Jump predict the kinematic performance of the rear Bandal Chagi kick?

. This study aimed to measure the associative and predictive strength of biomechanical variables of the countermovement jump (CMJ) and squat jump (SJ), and performance of the bandal-chagi kick performed with the dominant (D) rear lower limb in elite Taekwondo Athletes. For this, a total of 27 international level athletes (15 males and 12 females) performed the CMJ and SJ on two force platforms. The bandal-chagi was monitored using a motion capture system (VICON). The main results indicated that non-dominant (ND) Stiffness for the CMJ showed predictive power alone (p=0.048) or in combination with eccentric breaking rating of force development (RFD) (p=0.031) for total hip angular speed. Takeoff peak force/BM for the SJ predicted total time (p=0.002). Con-centric RFD/BM predicted total linear foot acceleration, either alone (p=0.001) or with vertical speed at takeoff (p=0.042). The total hip angular acceleration predictor was takeoff peak force D alone (p=0.019) or in combination with peak power/BM (p=0.041). In conclusion, CMJ and SJ tests showed specific associations with TKD kick performance. These analyses in this study indicated that they are good predictors for TKD athletes, producing results consistent with the participants ’ levels.


Introduction
Taekwondo (TKD) is an Olympic combat sport (Santos et al., 2014).To win a bout, athletes strive to score points while avoiding counterattacks from their opponents (da Silva Santos, Loturco, & Franchini, 2018).It is consequently crucial that offensive actions, such as punches and kicks, are swift and accurate (da Silva Santos et al., 2018;Falco et al., 2009).Scoring criteria are according to competitive rules, and vary based on the applied technique and targeted body area (Santos et al., 2014).Athletes often opt for kicks since they generally score higher than punches (Kwok, 2012), with the bandal-chagi (roundhouse kick) being the most frequent choice (Ha, Choi, & Kim, 2009;Sousa, Puerto, Beltrán, Louro, & Godoy, 2024).Moreover, proficiency in executing this kick correlates with competitive success; medalist athletes use this kick more frequently, as noted by Kwok (2012).
Given that the technique is adaptable to an athlete's morphology (Falco et al., 2009), some studies (Gavagan & Sayers, 2017;Ha et al., 2009) have delved into understanding how anthropometric and biomechanical variables can aid coaches in enhancing fighter performance (Ojeda-Aravena, Azócar-Gallardo, Hérnandez-Mosqueira, & Herrera-Valenzuela, 2020).Certain technical-tactical and biomechanical analysis studies (Estevan, Alvarez, Falco, Molina-García, & Castillo, 2011;Falco et al., 2009) have identified kick application speed as a determinant of competitive success in TKD.However, it is essential to understand that a comprehensive analysis of bandal-chagi performance necessitates kinematic monitoring due to the technique's angular nature (Falco et al., 2009).The advent of camera capture systems has ushered in an era of refined precision in assessing sports techniques (Windolf, Götzen, & Morlock, 2008).Accordingly, using such equipment has deepened our understanding of the biomechanical variables integral to this kick (Estevan, Jandacka, & Falco, 2013;Gavagan & Sayers, 2017;Kim, Kim, & Im, 2011).For instance, Kim et al. (2011) documented an angular velocity of 56.6g/s in hip flexion-extension movements, while the knee average was 95.5g/s.Gavagan and Sayers (2017) reported 341g/s and 943g/s for the hip and knee angular velocities, respectively, and 14.66 m/s for linear velocity in a stationary position.Similarly, experienced athletes in a study by Estevan et al. (2013) exhibited an average foot speed of 9.90 m/s.In relation to kicks, da Silva, Misuta, Silvatti, Mercadante, and Barros (2011) observed an average foot acceleration of 127.65 m/s^2 and 421 rad/s^2 for the knee joint.
An important consideration to highlight is that while motion capture systems boast advanced capabilities, they often lack portability and cost-effectiveness (Fiorentino, Uva, Foglia, & Bevilacqua, 2013).Consequently, there is a growing interest in exploring alternative methods that bridge the gap between the technical insights provided by motion capture and the more accessible biomechanical tests.In this regard, jump tests have emerged as a viable option for evaluating TKD fighters (Avci & Celik, 2023;Chiodo et al., 2012;da Silva Santos, Herrera-Valenzuela, Ribeiro da Mota, & Franchini, 2016;Norjali Wazir et al., 2019).Considering the dominance of lower limb attacks in the sport (Ha et al., 2009), both the countermovement jump (CMJ) and squat jump (SJ) tests have been leveraged to forecast kicking prowess in TKD (Chiodo et al., 2012;da Silva Santos et al., 2016;Norjali Wazir et al., 2019).However, existing knowledge regarding these jumps' utility in TKD remains limited, predominantly because evaluators have primarily gauged jump height as the sole performance indicator (Ojeda-Aravena et al., 2023).The prevalent use of contact mats instead of force platforms partly explains this restricted perspective (Chiodo et al., 2012;da Silva Santos et al., 2016;Norjali Wazir et al., 2019).Indeed, force platforms offer a wider array of measurable variables, delivering invaluable biomechanical insights (Gathercole, Sporer, Stellingwerff, & Sleivert, 2015), such as the rate of force development (RFD) (Kavvoura et al., 2018).Hence, this study aims to ascertain the associative and predictive strength of CMJ and SJ tests concerning bandal-chagi performance.If a connection emerges between the jump variables and the kick's kinematics, coaches could implement this study's findings to steer athlete selection, discern competitive tiers, project sports performance, and design training regimens.Given this context, our primary objective is to determine the associative and predictive strength of standard biomechanical variables from CMJ and SJ tests, and the spatiotemporal aspects of the bandal-chagi kick executed with the rear dominant lower limb in elite TKD athletes.We posit that both tests will successfully predict the performance of spatiotemporal components of the bandal-chagi as measured by kinematic analysis.

Experimental approach
The present study follows a cross-sectional design in which a group of elite TKD athletes were analyzed in a biomechanics laboratory.They underwent anthropometric measurements, two jump tests, and a kick monitored by a kinematic device.We initially reached out to the National Federation and coaches to explain the objectives of this protocol.After obtaining permission from the relevant authorities, athletes meeting the inclusion criteria were contacted.Those who agreed and provided signed informed consent were included in the study.All measurements were performed in a single visit to the laboratory, following this sequence: 1) anthropometry, 2) warm-up, 3) CMJ, 4) SJ, 5) kinematic test.The ethics committee of the University approved this study (Protocol 71430923.4.0000.5546).

Participants
Elite athletes of both sexes participated in this study.The inclusion criteria were: a) ≥18 years old; b) ≥5 years of continuous competitive TKD training; c) black belt status; d) at least one National Championship title.Exclusion criteria included: a) injuries that could affect test performance; b) errors during signal capture; or c) withdrawal from the study.A total of 56 out of 120 athletes initially met the inclusion criteria.Among these, 28 (16 men and 12 female) agreed to participate.However, one athlete was excluded due to an error during the kinematic capture, resulting in a final sample of 27 athletes (15 men and 12 female).Their achievements include 11 national championships, 8 South American medals, and 8 Pan-American medals (from the year 2022).Table 1 presents other participant characteristics.

Anthropometric measures
Height was measured to the nearest centimeter, and body mass to the nearest 100 grams (using Detecto ® , 339, USA).Body fat was estimated using the Jackson and Pollock 7-skinfold protocol for men (Jackson & Pollock, 1978) and women (Jackson, Pollock, & Ward, 1980).

Jump tests
Participants warmed up for about 10 minutes, consisting of joint mobility exercises, moderate-intensity running, specific strength movements, and three practice attempts for each jump.After the warm-up, participants performed three CMJ attempts as described by Slinde, Suber, Suber, Edwén, and Svantesson (2008).After the CMJ, the participants performed three SJ attempts based on the method described by Markovic, Dizdar, Jukic, and Cardinale (2004).Both CMJ and SJ results were evaluated using the best result calculated by the ratio between flight time to contraction time (or time to take-off) and the reactive strength index modified according to McMahon, Lake, and Comfort (2018).All jumps were executed on two monoaxial force platforms (type: PS-2142; sampling: 50-1000Hz; Range: -1000 to 4400N over 6600N; Resolution: 0.34N; dimensions 350mm x 350mm, Pasco ® CI-6461, Pasco Scientific, USA).The sampling was set at 1000Hz, since it has shown good reliability in force-time variables (Dos ' Santos et al., 2018).

Kinematics of Bandal-chagi
Kinematic measurements utilized a photogrammetry video capture system (Vantage V5, VICON ® Motion Systems Ltd., Oxford, UK) with ten cameras capturing movements at a frequency of 200Hz.Prior calibration ensured residuals were adjusted to 2mm (Merriaux, Dupuis, Boutteau, Vasseur, & Savatier, 2017).Reflective and deformable sensors, 14mm in diameter, were placed at specific anatomical points (plug in gate lower limbs model), according the standard recommendations.All data were analyzed by Nexus Software 1.8.5 (VICON ® Motion Systems Ltd., Oxford, UK).To a measure in the software, the sensor's movement needs to be simultaneously captured by a minimum of two cameras.

Variables measured during the test
For the bandal-chagi analysis, we adopted the kinematic division proposed by Kim, Kwon, Yenuga, and Kwon (2010), dividing the kick into three phases: a) Toe-off; b) Maximum Knee Flexion; and c) Impact.Athletes practiced ten bandal-chagi kicks, with at least two at maximum intensity.All kicks targeted a specific TKD mitt, with each athlete choosing the distance for the mitt.They delivered three kicks with a 14-second interval, which reflects the average attack time in international competitions (Santos et al., 2014).All kicks utilized the rear leg, based on data from Gutiérrez-Santiago, Pereira-Rodríguez, and Prieto-Lage (2020) indicating its effectiveness in scoring during competitions.Figure 1 provides a model derived from the kinematic analysis of a kick by a female athlete.

Statistical analysis
Data normality was initially assessed using the Kolmogorov-Smirnov test.One-way ANOVA was used for comparisons among men, women, and the total group.Pearson's correlation was performed to assess the strength of association between jump and kicking performance.This evaluated potential associations between CMJ and SJ variables with kinematic measures.A linear regression was also conducted using the stepwise method to determine the predictive power of the jumps.All analyses were executed using the SPSS software program (version 25.0).A significance level of p ≤ 0.05 was set for all tests.

Results
Table 1 shows the anthropometric measures of the participants and performance of sit-and-reach test.In comparing males, females, and the total participants, no significant differences were observed in the means presented in Table 1 for the variables of age (F=0.823;p=0.445; ηp 2 =0.019) and experience (F=0.077;p=0.926; ηp 2 =0.01).Differences were noted for the other variables between the means presented by men and women; however, the averages for men did not differ from the overall average.These variables include body mass (F=8.436;p=0.001; ηp 2 =0.45), height (F=10.601;p≤0.001; ηp 2 =0.51), and fat percentage (F=17.958;p≤0.001; ηp 2 =0.064).The results of the CMJ are presented in Table 2.There were no significant differences (p>0.05) in comparing male, female and the total participants for the means in Table 2. Table 3 shows the results for SJ.In comparing males, females, and the total participants, significant differences in the means presented in Table 3 were observed for: peak takeoff acceleration (F=3.192;p=0.049; ηp 2 =0.29) and takeoff peak force/BM (F=3.233;p=0.48; ηp 2 =0.32).There were significant differences for these variables between males and females (p=0.044 for peak takeoff acceleration and p=0.042 for takeoff peak force/BM).Table 4 shows the results of the kinematic analysis of bandal-chagi performed by the athletes.No significant differences were observed (p>0.05) in comparing men, women, and the total sample in Table 4.
The correlations between typical CMJ variables and the kinematic analysis of bandal-chagi are presented in Table 5.
Only significant correlations are presented to better organize the results.Stiffness demonstrated a direct relationship with hip angular velocity in the second (r=0.527;p=0.05) and third phases of the kick (r=0.464;p=0.015), as well as with total speed (r=0.485;p=0.01).Stiffness showed an inverse relationship for angular acceleration during the first phase of knee acceleration (r=-0.414;p=0.032), and a direct relationship during the second (r=0.427;p=0.026) and third hip phases of the kick.Similar correlations were observed for acceleration in the third phase (r=0.458;p=0.016) and for total acceleration (r=0.474;p=0.012).The concentric phase of the jump had an inverse association with hip angular velocity during the kick's second phase (r=-0.404;p=0.037).Analyzing by D and ND lower limbs separately, Stiffness ND directly correlated with hip angular speed in the second (r=0.526;p=0.005) and third phases of the kick (r=0.471;p=0.013), and with total speed (r=0.504;p=0.007).Similar results were found for hip angular acceleration in the second (r=0.453;p=0.018) and third phases (r=0.463;p=0.015), and for total acceleration (r=0.486;p=0.01).Stiffness for the D lower limb directly correlated with hip speed in the second (r=0.537;p=0.004) and third phases (r=0.451;p=0.018), and with total speed (r=0.474;p=0.013).It also showed an inverse correlation with knee angular acceleration during the kick's first phase (r=-0.43;p=0.025).
The concentric peak force of the ND lower limb positively correlated with foot acceleration during the first (r=0.388;p=0.045) and third phases (r=0.441;p=0.021), and total acceleration (r=0.395;p=0.042).The D lower limb had correlations with foot acceleration during the first (r=0.423;p=0.028) and third phases (r=0.456;p=0.017) and total hip acceleration (r=0.397;p=0.04).Eccentric braking RFD rate negatively correlated with the first phase of hip angular velocity for both the nondominant (r=-0.458;p=0.016) and dominant (r=-0.415;p=0.031) legs.The relative RFD-100ms of the ND lower limb negatively correlated with the foot's linear acceleration during the third (r=-0.496;p=0.008) and total phases (r=-0.479;p=0.011).Positive correlations for peak force during takeoff were observed with foot acceleration in the first phase for both ND (r=0.407;p=0.035) and D (r=0.429; p=0.026).The peak takeoff force for both D and ND positively correlated with the hip's angular acceleration during the third and total phases.Table 6 shows the correlations between the SJ variables and the kinematics of the bandal-chagi.Jump height directly correlated with the duration of the third phase of the kick (r=0.385;p=0.048).Peak Power/BM positively correlated with the duration of the third phase of the kick (r=0.44;p=0.022) and negatively with the angular velocity of the knee in the second phase (r=-0.407;p=0.035).The peak acceleration at takeoff positively correlated with the time spent kicking in the first (r=0.407;p=0.035), second (r=0.493;p=0.009), and the total kick duration (r=0.558;p=0.003).This variable also negatively correlated with the total linear foot speed (r=-0.387;p=0.046), foot acceleration in the third phase (r=-0.411;p=0.033), and total acceleration (r=-0.417;p=0.031).The peak force during takeoff positively correlated with the time spent kicking in the first (r=0.406;p=0.035), second phase (r=0.494;p=0.009), and the total duration (r=0.558;p=0.002).This variable also negatively correlated with the total linear foot speed (r=-0.39;p=0.044), foot acceleration in the third phase (r=-0.411;p=0.033), and total acceleration (r=-0.418;p=0.03).The vertical speed at takeoff showed a direct correlation with the duration of the kick in the third phase (r=0.4;p=0.039).
For analysis separated by D and ND: the Concentric RFD of the ND lower limb had a positive correlation with the time in the second phase of the kick (r=0.609;p=0.001) and negative correlations with foot speed in the third phase (r=-0.556;p=0.003), total speed (r=-0.473;p=0.013), foot acceleration in the third phase (r=-0.659;p≤0.001), and total acceleration (r=-0.558;p=0.002).Similar results were observed for the Concentric RFD of the D lower limb.The Force at Peak Power of the D lower limb directly correlated with hip acceleration in the third phase of the kick (r=0.432;p=0.024).The Takeoff Peak Force for both D and ND limbs showed a positive correlation with hip acceleration in the third phase of the kick (r=0.424;p=0.027 for ND and r=0.5; p=0.008 for D).Only the D limb showed a correlation in total acceleration (r=0.448;p=0.019).Table 7 shows the linear regression for CMJ and SJ measures of the participants and performance in bandal-chagi.In linear regressions where CMJ variables were used to predict bandal-chagi kinematic measurements, Peak Power/BM was found to be predictive of total knee angular speed.Stiffness ND exhibited predictive power either on its own (p=0.048)or in conjunction with eccentric breaking RFD (p=0.031) for total hip angular speed.The total foot linear acceleration was predicted by the eccentric breaking RFD-100ms ND (p=0.011).Additionally, Stiffness ND served as a predictor for total angular hip acceleration (p=0.01).
Regarding the measurements obtained from the SJ, Takeoff peak force/BM predicted total kick time (p=0.002).Concentric RFD D was a predictor for total linear foot speed (p=0.005).Concentric RFD/BM predicted total linear foot acceleration both independently (p=0.001) and in combination with vertical speed at takeoff (p=0.042).Finally, takeoff peak force D was predictive for total hip angular acceleration either on its own (p=0.019)or in combination with peak power/BM (p=0.041).

Discussion
This study measures the associative and predictive power of biomechanical variables of the CMJ and SJ for spatiotemporal variables of the bandal-chagi kick performed with the dominant rear lower limb in elite athletes.The main results indicated that both jumps can predict the technical performance of bandal-chagi, thereby corroborating our hypothesis.Stiffness (specially of ND lower limb) was the variable which showed the most correlations and was the main predictor variable for the CMJ.In turn, Concentric RFD/BM and Takeoff Peak Force (D) were the main predictor variables for SJ.The capture system enables precise movement analysis of the kicking technique performed by the athlete (Ha et al., 2009), however, it is expensive equipment and requires the use of a specific laboratory (Fiorentino et al., 2013), Thus, it is interesting that there are indirect indicators that can predict the athlete's technical performance.The results of the present study can be used by coaches to evaluate athletes through a simple jump test and that have predictive the kick performance.
CMJ is not specific to combat sports, however this jump is capable of estimating the specific performance of TKD athletes (Chiodo et al., 2012;da Silva Santos et al., 2016).In our study, the main predictor variable of CMJ was Stiffness of the ND lower limb.Stiffness is associated with the ability to accumulate elastic energy in the lower limbs, which enhances jumping performance (Struzik & Zawadzki, 2013).In this sense, the energy transfer from the lead limb (i.e., ND) appears to be decisive for bandal-chagi performance, as this variable is directly associated with hip speed when performing the kick.Estevan et al. (2013) observed that the efficiency of the bandal-chagi is associated with the push of the support leg against the ground, in the opposite direction to the proximal segment of the lead leg.Furthermore, studies have shown that a higher Stiffness increases the performance of muscular strength and power (Papla, Ewertowska, & Krzysztofik, 2023;Struzik & Zawadzki, 2013).In fact, speed, reaction time and kick power are physical strengths that determine success in combat (Moreira, Goethel, & Gonçalves, 2016;Wąsik, Mosler, Ortenburger, Góra, & Cholewa, 2021).
The main predictor variables for SJ were Concentric RFD/BM and Takeoff Peak Force.The force development rate is an important indicator of lower limb power (Maffiuletti et al., 2016), as previous studies had already measured this variable in TKD athletes (Kavvoura et al., 2018;Moreira et al., 2016;Moreira, Paula, & Veloso, 2015).In a kinematic analysis study of bandal-chagi by Moreira et al. (2016), the RFD/ BM (calculated during the preparation phase for the kick) is capable of distinguishing elite and sub-elite athletes, with RFD being higher in elite athletes.However, our data contradict these previous findings, as the RFD/BM measured in the SJ was inversely associated with the linear foot speed during the kick.A possible explanation for this difference in results is possibly associated with the body mass of the lower limbs.When analyzing the body mass of the lower limbs of TKD fighters, Kavvoura et al. (2018) observed that the lean mass of the lower limbs is directly related to RFD.Another point to be considered is the diversity of methods used to measure the RFD, which generates doubts in the interpretation of the results (Wąsik et al., 2021).Moreira et al. (2015) observed a strong correlation between the RFD obtained in an isometric leg-press test and the peak hip velocity when performing the bandal-chagi kick (r=0.89).
Takeoff Peak Force expresses the maximum force applied at the moment the feet lose contact with the force platform, and constitutes an important variable in the jump test to determine the efficiency of muscle strength and power (Amasay & Suprak, 2022).Our results show that a higher the reactive ground force against the platform results in a higher hip angular speed.Similar results were observed by Moreira et al. (2015), which showed that the reactive ground force in the CMJ showed a strong association (r=0.9)with the hip peak force during the bandal-chagi.A previous study with isokinetic analysis showed that the ability to produce force with the hip combined with the angular velocity of the knee are decisive in differentiating competitive levels in TKD athletes (Moreira et al., 2021).
For the present study we chose to only analyze the rear limb of bandal-chagi, which is due to the effectiveness of this type of attack (Gutiérrez-Santiago et al., 2020).When analyzing TKD combats, a higher frequency of attacks made with the bandal-chagi performed with the lead limb can be seen (Gutiérrez-Santiago et al., 2020;Kwok, 2012), and a higher frequency of using the front leg has been observed after the insertion of electronic protectors in sport since the 2016 Olympic games (Márquez, López-Gullón, Menescardi, & Falcó, 2022).However, the rear leg is more effective in generating points (Casolino et al., 2012;Gutiérrez-Santiago et al., 2020).In this line, a study of technical-tactical analysis in university athletes by Falco, Estevan, Álvarez, Morales-Sánchez, and Hernández-Mendo (2014) observed that winners perform fewer direct attacks, however they perform more counterattacks simultaneously and subsequent to the attacks and anticipatory counterattacks, which are predominantly performed with the rear leg.This fact reinforces our findings on the importance of transferring strength from the supporting leg to the rear leg when performing the bandal-chagi in a counterattack.
Although jump height is considered the most effective measure of CMJ and SJ (Struzik & Zawadzki, 2013), our results show that these variables do not have great predictive capacity for kicking performance.In fact, this has been the main variable when evaluating the performance of TKD fighters in jump tests (Chiodo et al., 2012;da Silva Santos et al., 2016;Norjali Wazir et al., 2019).This limitation is possibly associated with the equipment used, as most studies have used mats to measure performance in the SJ and CMJ (Bridge, Ferreira da Silva Santos, Chaabene, Pieter, & Franchini, 2014;Ojeda-Aravena et al., 2020).Therefore, based on our results, it is recommended to use force platforms to obtain kinetic and kinematic measurements (McErlain-Naylor, King, & Pain, 2014), as these may have greater predictive capacity for the fighter's competitive performance.The results of the present study were observed in elite athletes, and so future studies can analyze whether the same predictors occurred in non-elite athletes.It should be noted that elite athletes perform better in CMJ and SJ when compared to non-elite athletes (Norjali Wazir et al., 2019).Another limitation of the present study is the single analysis between men and women, although no differences were observed in terms of kinematic variables (Table 4); thus, it would be interesting that future studies explore this analysis separately.

Conclusion
This study underscores the significant association between the biomechanical variables of the CMJ and SJ tests and the performance of the bandal-chagi kick in TKD.Remarkably, to our knowledge, this is the first study which concurrently analyzes the influence of both CMJ and SJ on bandal-chagi performance.Through our findings, we have identified predictors that offer valuable insights for TKD coaches and practitioners.These reliable indicators are consistent with the elite status of the participating athletes.From a practical standpoint, the emphasis lies on the stiffness, particularly of the ND lower limb in CMJ, and Concentric RFD/BM in SJ as pivotal in enhancing bandal-chagi kick performance.Rather than resorting to costly movement analysis tools, coaches can then leverage the simple CMJ and SJ tests to predict technical prowess in bandalchagi.Furthermore, it is advisable to tailor exercises that specifically focus on enhancing stiffness and accumulating elastic energy in the lower limbs, optimizing bandal-chagi performance.This study also highlights the strategic advantage of utilizing the rear limb for bandal-chagi, given its proven effectiveness in scoring points.Force platforms stand out as the preferred choice over mats for accurate jump test measurements.These insights not only help to distinguish between elite and sub-elite athletes for talent identification, but also pave the way for future studies that might explore potential gender-specific variations, even though none were observed in the current study.by understanding the biomechanics of CMJ and SJ, there is an opportunity to design training regimens which not only accentuate performance, but also prioritize athlete safety.In its entirety, our study serves as a comprehensive guide for coaches, shaping TKD training, assessment, and the strategic nuances of competition decisions.Acknowledgment
4 L 11 R and 1 L 22 R and 5 L Note: Data present by mean ± standard deviation.R -right lower limb domain.L -left lower limb domain.a p≤0.026 vs. female.

Table 1 .
Anthropometric characteristics of participants and the performance of sit-andreach test.

Table 2 .
Countermovement jump variables results by the athletes.

Table 3 .
Squat jump results by the athletes.

Table 5 .
Significant correlations between countermovement jump and kinematic measures of bandal-chagi performed by athletes.

Table 6 .
Correlation between squat jump and kinematic measures of bandal-chagi.SJ -squat jump.RFD -rating of force development.BM -body mass.D -dominant lower limb.ND -non-dominant lower limb.