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Article

Acute Effects of a Simulated Karate Bout on Muscular Strength Asymmetries of the Lower Limbs in Elite Athletes of Different Age Categories

by
Eliza Gaweł
*,
Miłosz Drozd
,
Adam Maszczyk
and
Adam Zając
Institute of Sport Sciences, The Jerzy Kukuczka Academy of Physical Education in Katowice, 40-065 Katowice, Poland
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(2), 888; https://doi.org/10.3390/app15020888
Submission received: 7 December 2024 / Revised: 6 January 2025 / Accepted: 9 January 2025 / Published: 17 January 2025
(This article belongs to the Section Applied Biosciences and Bioengineering)
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Abstract

:

Featured Application

Featured Application: A short-lasting sport-specific physical effort can lead to relevant changes in inter-limb strength asymmetries, which may indicate an early onset of adaptation.

Abstract

This study aimed to assess the acute impact of a simulated kumite bout (WKF formula) on peak isometric strength performance of the dominant and non-dominant lower limbs in elite karate athletes of different age categories (U16, U18, Senior), in the context of inter-limb asymmetry. Sixty-one elite male and female athletes (age = 17.48 ± 3.26 [years], body height = 1.72 ± 0.08 [m], body mass = 63.79 ± 10.00 [kg]) participated in this study, which included a randomized crossover design (two experimental sessions under two different conditions). Inter-limb asymmetry was assessed based on the isometric mid-thigh pull test. Friedman’s test indicated significant differences in the mean values of the peak vertical force (PVF) between the assessed limbs (test = 10.8; p = 0.013; Kendall’s W = 0.059). Elite karate athletes, regardless of the age category, tend to have inter-limb strength asymmetries in the lower extremities; however, the impact of a simulated kumite bout was not fully confirmed. A kumite bout seems to have a favorable impact on bilateral asymmetries in U16 and U18 athletes, but not in Seniors, who seem to be at increased risk of injury after completing the bout (asymmetry > 15%). Limb dominance is not necessarily related to greater values of PVF.

1. Introduction

Combat sports and martial arts can be categorized into the following styles: (a) grappling, (b) striking, and (c) clinch [1,2]. Karate, in the WKF (World Karate Federation) formula, belongs to striking martial arts and is characterized by intermittent high-intensity actions, which include fighting in the stand-up position, within which punches and kicks are performed on the opponent’s body (kumite), or in the form of a structured routine (kata) [3]. However, although karate is believed to be a complex sport, a kumite bout is characterized by asymmetrical and unstructured movement patterns [4].
The analysis of the technical actions that are performed during kumite bouts, including the statistical data of the WKF World Championships [5,6], indicate that even though a precise high kick is rewarded with an ippon (three points), kumite athletes tend to score a yuko (one point) with a straight jab or a reverse punch much more frequently. This may be related to the mass of the upper limb (5%), which is much lower when compared to the lower limb (20%) [7]. This allows an athlete to reach the opponent in a straight line within a shorter period of time than various other kinds of kicks [6,8]. During a competition, two basic punches are mostly used to score the yuko, i.e., kizami-zuki—a jab punch (approximately 110 ms), and gyaku-zuki—a reverse punch (approximately 150 ms) [6,9,10]. According to the biomechanical analyses of the above-mentioned karate punches [7,11], in order to perform an accurate strike, an athlete has to displace and accelerate his body mass by lunging towards the opponent and extending the punching limb forward [11], which increases the load on the front lower limb up to approximately 70% compared to the back limb (30%) [12]. Similarly, an uneven biomechanical load occurs when performing the roundhouse kick (Mawashi-geri) [13].
Considering the above, the specifics of kumite stances, which are combined with striking punches, require lateralization of the performed actions [11]. Because of that, kumite athletes tend to have a preferable side (left or right) to fight on, the selection of which is related to several factors, e.g., neuromuscular control [14]. This is also related to lower limb dominancy, i.e., the front lower limb is believed to be the dominant one, as it always performs the first lunge to punch the opponent’s body [11], and it is usually used for kicking and throwing. Thus, long-term, unbalanced, one-sided loads may predispose kumite athletes to lower limb strength and functional asymmetries, including the occurrence of footedness and/or handedness, due to pre-existing limb preferences [15]. It has also been shown that an inter-limb strength imbalance (greater than 15%) increases the risk of non-contact injuries compared to individuals who are under the above-cited threshold [16].
Although each kumite bout may require a diverse effort–pause ratio and various technical actions [17], studies have demonstrated that different strength components can impact the effectiveness of karate strikes [18,19,20]. However, adaptations resulting from selected strength variables seem to contribute to an athlete’s karate performance in different ways. One example of this is maximal isometric strength, which is believed to be an objective indicator of the level of performance, especially in combat sports [21,22,23] such as karate. This is mainly because the level of maximal isometric strength allows one to assess an athlete’s ability to produce force, including the generation of force–velocity curves [24] that are significant during athletic tasks conditioned by strength, such as explosive strikes. Moreover, it also provides insights into the identification of inter-limb differences [25,26]. Since it has been acknowledged that inter-limb asymmetries can impact sports performance both (a) advantageously, e.g., by enhancing the change in direction speed [27], and (b) disadvantageously, e.g., by increasing the risk of injury [28], this issue has become an area of great interest for sport scientists [29,30].
To date, scientific research that has addressed the issue of lower limb strength asymmetry in karate athletes is limited [30,31,32,33,34,35]. Moreover, to the best of the authors’ knowledge, only two studies have evaluated this issue in karate in the Olympic formula [31,35]. Nevertheless, the cited studies mostly did not provide an experimental approach and included a small sample size; therefore, further research is needed. Despite scientific evidence regarding the impact of simulated karate fights on several aspects of physiological performance [36,37,38], it is difficult to find a single study that analyzed the impact of a kumite bout on inter-limb asymmetry. Given the gap in the scientific literature, it seems justified to conduct additional research that would provide reliable data according to the presented research issue. Accordingly, this experiment included a large study sample of athletes that represent an elite level of kumite performance (WKF formula), which may allow for general interference according to the impact of karate on inter-limb asymmetry. It may also contribute to the implementation of adequate training strategies, which, in turn, will allow us to not only balance and identify asymmetries but also to minimize the risk of non-contact injuries.
Thus, this study aimed to assess the acute impact of a simulated kumite bout (WKF formula) on the peak isometric strength performance of the dominant (front) and non-dominant (back) lower limbs in elite karate athletes of different WKF age categories (U16, U18, Senior) with regard to inter-limb asymmetries. It was hypothesized that elite kumite athletes have inter-limb strength asymmetries in the lower limbs, which are induced by the kumite bout and are derived from a self-made preference that is influenced by the dominant lower limb. Secondly, it was assumed that the dominant lower limb is characterized by higher values of peak vertical force compared to the non-dominant lower limb. In addition, it was hypothesized that inter-limb asymmetries increase within consecutive WKF age categories, which may be related to both greater strength adaptations in the dominant side/limb and the longer duration of the obligatory fight time (3 min) during a kumite bout.

2. Materials and Methods

2.1. Experimental Approach to the Problem

The experiment was carried out according to a randomized crossover study design. All subjects participated in two experimental sessions under different conditions (set 24h apart). An isometric mid-thigh pull (IMTP) test was performed in order to evaluate the acute effects of a simulated kumite bout (WKF formula) on the peak vertical force (PVF) in the dominant (front) and non-dominant (back) lower limbs based on the isometric mid-thigh pull (IMTP) asymmetry results. The study participants were randomly assigned to the following conditions: (1) an experimental condition (EXP)—2/3 min of effective fight time of a simulated kumite bout (WKF formula) and (2) a control condition (CTRL)—2/3 min of treadmill walking at moderate speed (4.0 km/h) [39]. All measurements were performed at two time points, i.e., (1) before and (2) after the simulated kumite bout. During the CTRL condition, measurements were also performed at the same time points; however, a treadmill walk was applied instead of the simulated kumite bout. A detailed description of the experimental procedures is provided in Section 2.3, while a flowchart of the study design is shown in Figure 1.

2.2. Study Participants

A priori, the sample size was calculated within the statistical software G*Power, (Dusseldorf, Germany) [version 3.1.9.2]. The calculation was based on the following statistical variables: ‘ANOVA, repeated measures, and within–between interaction’ (three groups of study participants, two experimental conditions, two measurements for each condition) including the statistical power of 0.9, a significance level of 0.05, and a d Cohen’s effect size of 0.25. Based on the calculations, a minimum sample size of 54 individuals was obtained.
Initially, sixty-four karate athletes agreed to participate in this study; however, three of them withdrew from the experiment. A total of 61 (female = 28; male = 33; mean age = 17.48 ± 3.26 [years]; mean body height = 1.73 ± 0.08 [m]; mean body mass = 63.79 ± 10.00 [kg]) elite WKF kumite athletes from the Polish national team participated in this study. Several of them were medalists of the European Championships, World Championships, or World Senior’s League—Karate 1—Series A. Moreover, all study participants were experienced in strength and conditioning training (Table 1). The inclusion criteria were as follows: (1) kumite athlete (WKF formula) of the Polish national team who has valid permission from a sports physician to participate in competition (karate WKF), (2) males and females in the following WKF age categories: U16 (14–15 years old), U18 (16–17 years old), and Seniors (18–34 years old), (3) at least 5 kyu (blue belt), (4) at least 3 years of kumite training experience at an elite level, and (5) at least 2 kumite training sessions per week. The exclusion criteria were as follows: (1) an occurrence of an injury in the last six months that resulted in exclusion from training for a period longer than 14 days, (2) withdrawal from this study, (3) WKF kumite athletes with disabilities, and (4) pregnancy (with regard to female athletes). Table 1 provides a detailed description of the study participants.
All measurements were performed during a 3-day training camp of the Polish national karate team at the Academy of Physical Education in Katowice, Poland. Before starting the experimental procedures, the participants were informed about the benefits and potential risks of this study. They were also allowed to withdraw from this research at any moment, without indicating a specific reason. Moreover, informed consent was obtained from all the study participants and from the parents/legal guardians of those who were under 18 years of age. Furthermore, the study participants were instructed not to exercise 24 h before testing in order to prevent fatigue and to maintain their regular dietary and sleeping habits. They were also encouraged to abstain from using caffeine-containing supplements or beverages 12 h prior to the examination. The research protocol was approved by the Bioethics Committee for Scientific Research at The Academy of Physical Education in Katowice, Poland (No. 1/V/2024), and met the ethical standards of the Declaration of Helsinki 2013 [40].

2.3. Procedures

The study participants arrived at the certified Strength and Power Laboratory in the morning at 7–8 a.m. At first, they filled out a self-made interview questionnaire that included data about their athletic career, medical history, and training. Next, body mass and body composition were evaluated by the bioelectric impedance method, using the InBody 770 (In Body, Cerritos, CA, USA) analyzer. The assessment was performed on an empty stomach, without shoes, and in a light national uniform (t-shirt and shorts—total mass 0.5 kg). A stadiometer with a centimeter scale (Charder HM-200P, Taichung City, Taiwan) was used to evaluate body height. Moreover, the body mass index (BMI) was calculated following the standard formula: (body mass [kg]/body height2 [cm]) [41]. The main testing procedures were performed between 11 a.m. and 2 p.m. in order to avoid the possible impact of the circadian rhythm on strength performance. The 3 h interval between the anthropometric measurements and main testing procedures was used to serve breakfast and to perform a familiarization session of the IMTP test, during which athletes were informed about the testing procedures, and both their body position and optimal height of the power rack were assessed (in random order). Prior to the beginning of the main testing session, the athletes performed a general warm-up according to the following protocol: 5 min of treadmill running (BH Fitness LK6800, Vitoria-Gasteiz, Spain) at an intensity of 8 km/h, inclination 0.0, followed by a series of kumite techniques, which were performed in pairs (athletes were selected according to the fighting draws of the EXP condition). The techniques were performed alternately, on an inactive opponent, starting from the dominant and followed by the non-dominant side (left/right): 5 giaku zuki chudan, 5 kizami zuki yodan, 5 mawashi-geri yodan, and 5 ura-mawashi-geri yodan (Figure 2). Such a solution enabled us to maintain the typical warm-up conditions during karate competitions.
After finishing the warm-up (approximately 15 min), the athletes were given 5 min of passive rest, after which the IMTP (pre-test) was performed. Next, the athletes who participated in the EXP condition had 5 min to change into a kumite uniform (including the required protectors) and to walk to the fighting arena. The athletes in the CTRL condition were given 5 min of passive rest before entering the treadmill. After completing the particular condition (EXP/CTRL), another 5 min rest interval was given to the athletes (Figure 1), after which the IMTP test was repeated (post-test). The duration of the rest intervals (5 min) was selected based on the official WKF competition rules [42]. Furthermore, this solution enabled us to maintain equal testing conditions for all athletes, regardless of the condition that was applied (EXP/CTRL).

2.4. Isometric Mid-Thigh Pull Test

The assessment of the strength performance in the lower limbs within isometric conditions was performed with the IMTP test, which is believed to be a gold standard to evaluate maximal isometric strength in elite combat sports athletes, including striking martial arts such as karate or taekwondo [43]. It is also recommended to evaluate the inter-limb asymmetries in athletes of various velocity-based sports [25,26,44] since it has been demonstrated that the level of peak force, evaluated via the IMTP test, impacts the magnitude of asymmetries and might also have a greater influence on the incidence of inter-limb strength differences when compared to other variables such as sex [25]. The measurements were performed on a force plate (ForceDecks, Vald Performance, Brisbane, Australia). The force plate is known to have good reliability (interclass correlation coefficient (ICC) = 0.78) for measurements performed with the IMTP test [45]. In addition, during a single testing trial, the platform allows for obtaining PVF results for both lower limbs and separately for the left and right lower limbs. The platform was set at 1000 Hz of sampling and was positioned on the floor, under a power rack (KEISER 3111, Fresno, CA, USA), which allowed us to position the barbell at any height through hydraulic jacks and pins. The IMTP test was performed with a standardized Olympic barbell (Eleiko International, Halmstad, Sweden; mass: 20 kg, diameter: 2.8 cm, length: 1.92 m), and its position on the power rack was established based on the recommendations proposed by Beckham et al. [46], i.e., 125° of the knee joint angle and 145° of the hip joint angle (upright torso) (Figure 3A,B). The above-mentioned testing position was chosen because it has been shown that forces generated in the cited angles of the hip and knee joints are the highest [46]. This enables the optimization of the force–time characteristics during the IMTP test [23]. Both the foot position on the platform and the hand placement on the barbell were volitional [23]. Moreover, a hand-held electronic goniometer (LIMIT, QC1932, Rzeszów, Poland) was used in order to assess the hip and knee joint angles before each test trial. The measurements were performed by two experienced researchers (EG, MD).
The study participants performed two testing trials according to the following protocol: 3 s of maximal isometric pull with a 2 min passive rest interval. Before the first trial, they were instructed to ‘pull the barbell as hard as possible while pushing their feet into the platform’. Moreover, prior to each trial, the participants were asked to apply the minimum amount of pre-tension that was needed upon the immovable barbell [23] in order to remove the ‘slack’ in their bodies immediately before the commencement of the IMTP trial. The trial started on the researcher’s (MD) command ‘Three, two, one, go!’, after which the athletes pulled the barbell while the second researcher (EG) controlled the time of the trial via electronic software and was simultaneously responsible for measuring the time of the passive rest interval (via an electronic timer). Additionally, during the trial, the athletes were encouraged by the researchers’ (EG, MD) verbal incentive to maximize their physical effort. They were also informed about their peak result after each trial. The obtained data were automatically recorded on a laptop with IMTP software (version 2.0.8899) immediately after finishing the task, with an accuracy of up to one scalar value. Similar to other research dealing with the IMTP test [45], the best trial was taken for further analyses.

2.5. Simulated WKF Karate Bout

In order to evaluate the acute effects of a simulated WKF karate bout (Figure 4B) on PVF performance and inter-limb strength asymmetries, a kumite bout was performed (EXP condition) in accordance with the official rules [42] of the World Karate Federation, i.e., 2 min of an effective fight time for the study participants from the U16 and U18 age categories and 3 min for Seniors. The only modification that was applied involved the duration of the fight—the bout did not end when one of the participants received an eight-point advantage or the ‘Hansoku’ command [42]. This solution enabled us to maintain equal physical effort among all study participants, according to the particular age category.
Prior to beginning the testing procedures, an event entitled ‘Scientific research of the Polish national team kumite athletes—Katowice 2024’ on the Sportdata.org platform [47] was created within the license of the Polish Karate Union—Polish Karate Federation. As the Sportdata platform is an open-source platform for all sports events included in the WKF, in order to maintain Bioethical rules, the event was closed both for extrinsic registration and public access to any data related to this research. Only the project’s supervisor (EG) had access to import and export the participants’ data. The simulated WKF kumite bout was performed as a typical ‘OPEN’ category, according to which the study participants were grouped into their sex (male/female) and actual age categories (U16/U18/Seniors), without division on the weight categories. The Sportdata platform was operated by an experienced researcher (EG) (Figure 4A). The platform was used to (1) create the fighting draws, (2) describe the fighting color to all the study participants (AKA or AO), (3) control the remaining fighting time, and (4) present the given points or penalties during the simulated WKF kumite bouts, which were shown on a screen (Samsung 75″ QLED, UHD/4K, 3840 × 2160 px, Warsaw, Poland) that was connected to a laptop with Sportdata software.
The experimental condition was performed with the attendance of national team head coaches on a standard 8 × 8 m fighting square, which was built with attested WKF Tatami [42]. The simulated WKF kumite bout was judged by a panel of five licensed Polish referees (WKF/EKF or national license A). Moreover, in order to guarantee the safety of the experiment, the bout was performed in the presence of two paramedics. In addition, prior to the beginning of the experimental procedures, the athletes were instructed to exert their maximal effort and focus on the kumite bout.

2.6. Assessment of Lower Limb Dominancy

As the intrinsic nature of a kumite bout includes unexpected and unstructured technical and tactical actions, in order to guarantee reliable data for the statistical analyses, each study participant was observed during the simulated WKF kumite bout for lower limb dominancy. Selection of the dominant lower limb was performed based on the following criteria: (a) the total time of the effective fight time spent in the left/right position and (b) the athlete’s subjective statement about limb dominancy based on the question: ‘Which lower limb is your dominant during the kumite bout (left/right)?’. In all study participants, both criteria were consistent. Thereafter, each athlete’s limb dominancy was adequately assigned to the PVF results obtained via the IMTP test.

2.7. Treadmill Walk

A total of 2/3 min of treadmill walking at a speed of 4 km/h (inclination 0.0) was performed as the CTRL condition for several reasons. As a kumite bout can be characterized by different effort–pause ratios, aerobic metabolism represents the main energy system [17]. Thus, it was assumed that a low-intensity aerobic activity, which is based on a cyclic movement pattern, would create optimal CTRL conditions in comparison to the high-intensity, acyclic actions that are related to the kumite bout. Moreover, walking is known to be a basic movement pattern that is performed during various activities of daily living, and it is related to gate biomechanics [48]. Therefore, applying this kind of physical activity might indicate if there are any functional asymmetries that could possibly occur in the lower limbs, which might also allow us to compare the effect of karate-specific effort on the incidence of asymmetries.
The speed of the treadmill walking was set at 4 km/h because this walking speed is included in the ‘normal’ range (4.0–6.4 km/h) [39]. Moreover, it was assumed that the above-mentioned speed would adequately reflect each athlete’s typical walk during activities of daily living. It has also been shown that the muscle activity ratio can be diverse depending on the treadmill walking speed [49]. According to the present research, it was assumed that 4 km/h, which mainly activates the tibialis anterior muscle, medial gastrocnemius, and biceps femoris muscles, would be an adequate solution when compared to higher speeds, e.g., 5 km/h, which cause higher activation of the semitendinosus and lower shoulder muscles [49].
The study participants exerted constant physical effort at low intensity on the treadmill (BH Fitness LK6800, Vitoria-Gasteiz, Spain). Prior to the beginning of this study, the athletes were instructed to maintain their typical posture during walking. Before starting the trial, the participants entered the treadmill and placed both feet on its sides. Next, the treadmill’s speed was increased up to 4 km/h. When the requested speed was reached, each participant received the verbal command from one of the researchers (MD) ‘Three, two, one, go!’, after which the athlete stepped on the treadmill’s track and the trial started. The trial’s time was measured by one of the researchers (EG) via a hand-held timer. The end of the trial was announced with the researcher’s verbal command ‘Stop!’, after which the treadmill was paused.

2.8. Statistical Analyses

All statistical analyses were performed using jamovi (version 2.2.5) statistical computer software and were expressed as means with standard deviations (±SDs). Statistical significance was set at p < 0.05. The Shapiro–Wilk test, Mauchly’s test, and Leven’s test were used to verify the normality, sphericity, and homogeneity of the samples’ data variances, respectively. Moreover, the skewness and kurtosis of the analyzed data were verified. A two-way repeated measures ANOVA (2[EXP; CTRL] × 2 time points [pre-test; post-test] × between subject effects [WKF age category]) (or Friedman’s Test for nonparametric variables) was used to investigate the influence of the simulated WKF kumite bout on the PVF parameters of the dominant and non-dominant lower limbs. Effect sizes for the main effects and interactions were estimated by calculating the partial eta squared (η2), which was classified as small (0.01 to 0.059), moderate (0.06 to 0.137), and large (>0.137) [50]. For Friedman’s two-way repeated measure ANOVA by ranks Kendall’s coefficient of concordance was calculated to estimate the ESs. Post hoc tests with Bonferroni correction (for parametric variances) (or nonparametric equivalent test) were used to analyze pairwise comparisons if a significant main effect or interaction was found. A one-way ANOVA (or a Kruskal–Wallis H test for nonparametric variables) was used to verify differences between the analyzed groups (U16, U18, Seniors). If statistical significance was obtained, Tukey’s (for equal variances) post hoc test or pairwise comparisons (for unequal variances) were calculated. The within-group data of the PVF were analyzed using the paired samples t-test. The magnitude of mean differences was expressed with a standardized Cohen’s d effect size (ES) for the parametric variables or with the ES given by the Wilcoxon signed-rank test for the nonparametric variables. Thresholds for qualitative descriptors of Cohen’s d were defined as follows: 0.01 to 0.19 as a very small effect [51], 0.20 as a small effect, 0.21 to 0.50 as a moderate effect, 0.51 to 0.80 as a large effect, and greater than 0.80 as a very large effect [50]. The 95% confidence intervals for mean values were also calculated. Moreover, the reliability of the battery of the performance tests was assessed via ICC analysis using a two-way mixed, absolute agreement parameter. The obtained values of reliability were interpreted as follows: poor (below 0.5), moderate (between 0.5 and 0.75), good (between 0.75 and 0.9), and excellent (greater than 0.90) [52].

3. Results

In the present study, the measurements performed with the force plate (ForceDecks, Vald Performance, Brisbane, Australia) were characterized with an interclass correlation coefficient of ICC = 0.95 (CI = 0.93 to 0.97).

3.1. Dominant Lower Limb

The results of the descriptive statistics of the PVF [N] with the particular conditions (EXP; CTRL) and different time points (pre-test; post-test) are presented in Table 2.
The one-way ANOVA revealed a lack of statistically significant differences in the PVF of the dominant lower limb (p > 0.05) between the analyzed age categories (U16, U18, Seniors) for both conditions and time points.
A two-way ANOVA with repeated measures indicated neither a statistically significant interaction (condition × time-point × WKF age category) (F = 0.511; p = 0.602; η2 = 0.017), the main effect of condition (F = 0.317; p = 0.576; η2 = 0.005), nor the main effect of time point (F = 1.791; p = 0.186; η2 = 0.03).

3.2. Non-Dominant Lower Limb

Table 3 shows a comparison of the PVF [N] with the applied conditions (EXP; CTRL) and time points (pre-test; post-test) in the elite karate athletes of different WKF age categories.
A Kruskal–Wallis H test showed that there was a statistically significant difference between the analyzed age categories in the PVF values obtained in the pre-test for the EXP condition (χ2 (2) = 7.010; p = 0.03). The pairwise comparison showed that significantly higher values of the PVF in the non-dominant lower limb were obtained in the Seniors in comparison to the athletes from the U16 group (p = 0.02, ES = 0.48).
Friedman’s test indicated statistically significant differences in the PVF for the non-dominant lower limb (test = 9.27; p = 0.026; Kendall’s W = 0.051). The pairwise comparison showed a statistically significant difference in the EXP condition between the pre-test and the post-test (p = 0.021; ES = 0.3) and between the EXP condition (pre-test) and the CTRL condition (post-test) (p = 0.004; ES = 0.34).

3.3. Inter-Limb Asymmetry

No statistically significant differences in inter-limb asymmetries were found between the analyzed WKF age categories.
The results of the descriptive statistics of the mean differences in the inter-limb PVF performance due to varied conditions (EXP; CTRL) and time points (pre-test; post-test) are presented in Table 4. It was found that in the athletes from the U16 and U18 age categories, the inter-limb PVF asymmetry significantly decreased after the kumite bout. This effect was not observed in the Seniors, in whom the analyzed value increased after the EXP condition (Table 4). On the other hand, the CTRL condition showed an increase in the inter-limb PVF asymmetry after treadmill walking in all the studied groups.
Statistical analyses performed with Friedman’s test indicated a statistical significance in the mean difference in the PVF between the dominant and non-dominant lower limbs (test = 10.8; p = 0.013; Kendall’s W = 0.059). The pairwise comparison between the analyzed data pointed to a significantly higher inter-limb asymmetry in the PVF in the pre-test for the EXP condition (p = 0.14; ES = 0.32) in comparison to the values obtained in the CTRL condition for the pre-test. Moreover, the results of the CTRL condition (post-test) were significantly higher in comparison with the EXP condition (post-test) (p = 0.002; ES = 0.41).
Figure 5 presents the frequency and diversity of the incidence of inter-limb PVF [N] asymmetries in athletes of different WKF age categories under varied conditions (EXP; CTRL) and time points (pre-test; post-test). The above-mentioned evaluation was performed based on the following criteria: inter-limb asymmetry (a) over fifteen percent (>15%) and (b) under fifteen percent (<15%).
It was found that in the pre-tests, the incidence of a bilateral asymmetry over fifteen percent (>15%) was observed in the athletes from all the studied groups. However, the frequency was not high, excluding the athletes from the U18 category (47%). Moreover, the analysis of the qualitative data showed that the frequency of the above-mentioned asymmetry (>15%) increased after the simulated WKF kumite bout in the Seniors and decreased in the athletes from the U16 (10% of the athletes) and U18 (21% of the athletes) categories. Simultaneously, in the CTRL condition, the inter-limb asymmetry (>15%) significantly increased in the majority of the athletes, i.e., U16 (50%), U18 (47%), and Seniors (36%).

4. Discussion

Empirical data about the acute effects of a karate bout on muscular strength asymmetries is scarce. Therefore, this study aimed to evaluate the acute impact of a simulated kumite bout (WKF formula) on the peak isometric strength performance of the self-preferred dominant (front) and non-dominant (back) lower limbs in elite athletes of different age categories (U16, U18, Senior) with regard to inter-limb asymmetry. The main finding of this study was that elite kumite athletes, regardless of their age category, tend to have bilateral strength asymmetries in the lower extremities, which differed due to the applied condition and time point. Moreover, the results of our study indicate that limb dominance is not necessarily related to higher values of the PVF, as the non-dominant lower limb (back) was found to be stronger in the U16 group (CTRL, pre-test and post-test), in the U18 group (all measurements), and in the Seniors (EXP, post-test; CTRL, pre-test). Moreover, the simulated kumite bout partially impacted the isometric strength performance (p = 0.021; ES = 0.3). Furthermore, the inter-limb asymmetry was found to significantly decrease after the EXP condition (excluding the Senior category) and to increase after the CTRL condition (especially >15%) (Figure 5.) in all age categories (Table 4). The above-mentioned findings show that even a short-lasting sport-specific physical effort can lead to relevant changes in the inter-limb strength asymmetry, which may indicate an early onset of adaptation.
The results of our study are partially in line with the findings of several authors [14,32,33]. For instance, it has been shown that karate athletes (U18) tend to demonstrate specific adaptations in certain strength measurements [33], while Obradović et al. [32] indicated the impact of long-term karate training on chronic muscular adaptations in karate athletes. Similarly, Mala et al. [15] suggested that both the unilateral movement patterns observed in karate and limb preference may induce tissue adaptations, which result in inter-limb asymmetries. This was also confirmed in our study. However, our analysis indicates that the acute changes in inter-limb asymmetries can differ depending on the WKF age category, as we found a considerable decrease in the inter-limb strength differences in the U16 and U18 athletes. Moreover, an increase in the isometric strength asymmetry was observed in the Seniors (Table 4). It seems that in the case of younger athletes (U16-U18), a kumite bout could improve muscle tension, which may compensate for the uneven loads in the lower limbs. This thesis can be confirmed by Kotrljanovic et al. [35], who indicated that karate training symmetrically stimulates the development of the human body. Similar conclusions were drawn by Trajković et al. [33], who pointed out that karate athletes (U16, U18, and Seniors) do not differ with regard to the flexor/extensor strength ratio between the opposite lower extremities. Moreover, Scattone-Silva et al. [31] suggested that regular competitive karate training is not necessarily a bilateral strength difference-inducing factor in elite Senior athletes. Contrary to the research by Scattone-Silva et al. [31], our results suggest that kumite-specific efforts can induce muscular strength asymmetries, which was found in the Senior group. It seems that Senior athletes might be predisposed to higher intracellular adaptations when compared to younger athletes, which can be related to longer kumite training experiences (Table 1). However, the longer duration of the obligatory effective fight time (3 min vs. 2 min) could also impact the asymmetry results that were found in our study. Thus, future studies are needed in order to deeply evaluate this research issue, which would enable generalization.
It should be noted that the self-made preference of the dominant limb is part of an adaptive process that is desirable when performing sport-specific actions [15]. This effect could be particularly relevant while performing karate-specific asymmetrical movements such as a reverse punch (gyaku zuki), during which the body mass is displaced from the back lower limb to the front (zenkutsu-dachi) stance [12], allowing for an accurate strike. Therefore, the results of the present research can in some way explain the increase in the inter-limb asymmetry after the simulated kumite bout, which may result from intramuscular adaptations. However, to the best of the authors’ knowledge, currently, there is no data with which to compare the results of our study. Thus, in order to address this issue, further studies are needed to evaluate the impact of a simulated kumite bout on asymmetries in the kinematic variables of the lower extremities and kumite performance. Moreover, to fully understand the complexity of interactions between the characteristics of sports performance in karate (WKF formula) and its multifaceted effects on the musculoskeletal system, future studies should also assess the mechanical properties of muscle fibers.
Considering the impact of limb dominancy on maximal isometric strength performance, our study indicated that the dominant lower limb is not necessarily characterized by higher values of the PVF, as this effect was mainly observed in the U16 athletes during the EXP condition. On the other hand, the non-dominant lower limb was found to be stronger in the U16 group (CTRL, pre-test and post-test), the U18 group (all measurements), and the Seniors (EXP, post-test; CTRL, pre-test). However, in the case of the Senior and U16 athletes, the differences were not large. Our results partly correspond with the findings of Mekic et al. [34], who indicated that karate athletes achieved better results during isokinetic testing in the dominant lower limb. However, as a kumite bout is characterized by unexpected and unstructured technical actions, it seems possible that the higher PVF in the non-dominant lower limb may be related to a greater frequency of kicks (especially with the front lower limb) compared to punches, which requires a higher load on the back lower limb [13], the tactical strategy (especially defensive actions) or the athlete’s habitual fighting position. Nevertheless, according to the authors’ knowledge, this is the first study that assessed the acute effects of a simulated kumite bout on the inter-limb isometric strength asymmetry in the lower limbs; thus, it is difficult to compare our results within the current body of scientific literature.
Changes in the percentage of inter-limb asymmetries differed depending on the condition. For instance, due to the kumite bout, the frequency of asymmetries over fifteen percent (>15%) increased in the Seniors and decreased in the athletes from the U16 and U18 age categories (Figure 5). On the other hand, the CTRL condition (post-test) indicated a significant increase in the frequency of asymmetries (>15%) in all age groups; however, the Seniors achieved a lower percentage of the frequency in comparison to the EXP condition. This indicates a favorable impact of the kumite-specific activity on the musculoskeletal system in U16 and U18 athletes; however, this cannot be fully confirmed in Seniors, as in this age group, the kumite bout seems to increase the risk of non-contact injury (asymmetry > 15%), which occurred in 50% of the studied athletes. Our findings are partly in line with the study conducted by Scattone-Silva et al. [31], who found that the inter-limb asymmetry in the lower limbs in karatekas differs from less than 10% to up to 15%. On the other hand, Kotrjanovic et al. [35] indicated that karate athletes may not present an increased risk of knee injury due to certain training adaptations. Nevertheless, the current literature provides insufficient evidence regarding injury risk due to karate training. Therefore, this issue should be investigated in future research.

Limitations

This study has several limitations that need to be addressed. First of all, as the study participants were elite karate athletes who represented the highest national level, the extrapolation of our research results to other groups of karate athletes should be carried out with adequate caution. Secondly, we investigated only the PVF variable; thus, it is unknown whether other kinematic variables, such as the RFD at different time points, would not be effective in investigating the issue of lower limb asymmetry. Moreover, we did not analyze the statistics of the conducted techniques during the simulated kumite bout; therefore, we were unable to indicate which movement pattern (punch vs. kick) underlined the results of the present study. Furthermore, the results of our study are limited to the acute effects of a simulated WKF kumite bout; therefore, future studies should include longitudinal research to evaluate the chronic impact of kumite-specific activity on inter-limb asymmetry and injury risk. Lastly, even though we included a large study sample (n = 61), we did not evaluate sex and kumite weight categories as factors differentiating inter-limb asymmetries. Therefore, it would be interesting to investigate these aspects in future studies.

5. Conclusions

  • The results of our study indicate that elite kumite athletes, regardless of the age category, tend to have inter-limb strength asymmetries in the lower extremities; however, the impact of a simulated kumite bout was not fully confirmed.
  • A kumite bout seems to have a favorable impact on bilateral strength asymmetries in the lower limbs in U16 and U18 athletes but not in Seniors, who simultaneously seem to be at increased risk of injury after completing the bout (asymmetry > 15%).
  • Limb dominancy is not necessarily related to greater values of the PVF, as other factors such as tactical strategy, an athlete’s habitual fighting position, or the kick-to-punch ratio can also impact the IMTP test results.
  • A short-lasting sport-specific physical effort (a single kumite bout) can lead to relevant changes in inter-limb strength asymmetries, which may indicate an early onset of adaptation.

Author Contributions

Conceptualization, E.G.; methodology, E.G. and A.Z.; validation, E.G. and A.Z.; formal analysis, E.G., A.M., and A.Z.; investigation, E.G. and M.D.; resources, E.G.; data curation, E.G.; writing—original draft preparation, E.G.; writing—review and editing, E.G.; visualization, E.G. and A.Z.; supervision, A.Z.; project administration, E.G.; funding acquisition, A.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

This study was conducted in accordance with the Declaration of Helsinki 2013 and approved by the Bioethics Committee for Scientific Research at The Academy of Physical Education in Katowice, Poland (No. 1/V/2024), date of approval: 9 May 2024.

Informed Consent Statement

Written informed consent was obtained from all subjects involved in this study and from the parents/legal guardians of those who were under 18 years of age.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on reasonable request.

Acknowledgments

The authors would like to acknowledge the authorities of the Polish Karate Union—Polish Karate Federation, i.e., Paweł Połtorzecki, Mariusz Pełka, and Piotr Koryczan, for making this research possible. We also would like to thank the head coaches of the Polish National Team and all athletes who participated in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Study design flowchart; IMTP—isometric mid-tight pull test.
Figure 1. Study design flowchart; IMTP—isometric mid-tight pull test.
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Figure 2. Study participants during the warm-up protocol (ura-mawashi-geri).
Figure 2. Study participants during the warm-up protocol (ura-mawashi-geri).
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Figure 3. The IMTP test on the ForceDesk (Vald Performance, Brisbane, Australia) platform. (A) Preparation for the trial and (B) performance of the trial (3 s).
Figure 3. The IMTP test on the ForceDesk (Vald Performance, Brisbane, Australia) platform. (A) Preparation for the trial and (B) performance of the trial (3 s).
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Figure 4. The EXP condition with the use of Sportdata software (A)—AKA (read colour); AO (blue colour) and simulated WKF kumite bout (B).
Figure 4. The EXP condition with the use of Sportdata software (A)—AKA (read colour); AO (blue colour) and simulated WKF kumite bout (B).
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Figure 5. The frequency and diversity of the incidence of inter-limb PVF asymmetries over (> 15%) and under (<15%) fifteen percent in the study participants of different WKF age categories due to different conditions (EXP; CTRL) and time points (pre-test; post-test) (created with Microsoft Excel).
Figure 5. The frequency and diversity of the incidence of inter-limb PVF asymmetries over (> 15%) and under (<15%) fifteen percent in the study participants of different WKF age categories due to different conditions (EXP; CTRL) and time points (pre-test; post-test) (created with Microsoft Excel).
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Table 1. The descriptive statistics and frequency tables of the study participants (n = 61), including particular WKF age categories (U16, U18, Seniors).
Table 1. The descriptive statistics and frequency tables of the study participants (n = 61), including particular WKF age categories (U16, U18, Seniors).
VariablesAll Study Participants
(nP = 61; nF = 28; nM = 33)
(Mean ± SD)		U16
(nP = 20; nF = 8; nM = 12)
(Mean ± SD)		U18
(nP = 19; nF = 7; nM = 12)
(Mean ± SD)		Seniors
(nP = 22; nF = 13; nM = 9)
(Mean ± SD)		Between-Group Comparison
(p-Value)
Age [years]17.48 ± 3.2614.5 ± 0.5116.63 ± 0.520.91 ± 2.910.001 *
(ES = 0.91)
Body mass [kg]63.79 ± 10.0060.43 ± 9.1663.72 ± 9.4466.91 ± 10.610.07
Body height [m]1.72 ± 0.081.7 ± 0.081.73 ± 0.061.74 ± 0.090.3
BMI [kg/m2]21.38 ± 2.6120.83 ± 2.8721.3 ± 2.4221.95 ± 2.410.15
Muscle mass [kg]30.68 ± 6.1129.32 ± 5.4130.52 ± 5.8732.05 ± 6.840.37
Fat mass [kg]8.89 ± 4.317.98 ± 4.719.34 ± 4.429.33 ± 3.870.56
Kumite training experience [years]10.31 ± 3.977.53 ± 1.939.82 ± 2.8113.27 ± 4.250.001 *
Number of kumite training sessions per week [n]4.43 ± 1.263.93 ± 1.244.71 ± 1.284.64 ± 1.180.1
Standard kumite training (hours)1.52 ± 0.241.52 ± 0.21.53 ± 0.261.5 ± 0.260.93
SC training experience (years)3.48 ± 2.612.58 ± 1.613.0 ± 2.04.7 ± 3.330.08
*—statistical significance between the groups based on the one-way ANOVA or equivalent nonparametric test; BMI—body mass index; ES—effect size; nP—total number of participants; nF—number of females; nM—number of males; SD—standard deviation; SC—strength and conditioning.
Table 2. A comparison of the PVF [N] in the dominant (front) lower limb with the applied conditions (EXP; CTRL) and time points (pre-test; post-test) in the study participants of different WKF age categories.
Table 2. A comparison of the PVF [N] in the dominant (front) lower limb with the applied conditions (EXP; CTRL) and time points (pre-test; post-test) in the study participants of different WKF age categories.
Condition
Time Point		EXP_PRE
(Mean, SD, 95% CI)		EXP_POST
(Mean, SD, 95% CI)		ES
(Pre vs. Post)		Δ [%]CTRL_PRE
(Mean, SD, 95% CI)		CTRL_POST
(Mean, SD, 95% CI)		ES
(Pre vs. Post)		Δ [%]
PVF [N]—Dominant Lower Limb
U16 (n = 20)813.0 ± 147.0
(748.0 to 877.0)		794.0 ± 156.0
(726 to 863)		0.44 ‡−2.34 ± 6.12794.0 ± 151.0
(726.0 to 863)		809.0 ± 191.0
(725.0 to 893)		0.09 ‡1.89 ± 26.49
U18 (n = 19)849.0 ± 254.0
(735.0 to 963.0)		832.0 ± 205.0
(740.0 to 924.0)		0.13 †−2.0 ± 19.29837.0 ± 226.0
(735.0 to 938.0)		813.0 ± 218.0
(715.0 to 910.0)		0.22 †−2.87 ± 3.54
Sen (n = 22)934.0 ± 206.0
(848.0 to 1020.0)		888.0 ± 204.0
(803.0 to 974.0)		0.25 ‡−4.93 ± 0.97913.0 ± 168.0
(843.0 to 983.0)		909.0 ± 175.0
(836.0 to 983.0)		0.22 ‡−0.44 ± 4.17
Δ—delta; †—Cohen’s d effect size; ‡—Wilcoxon’s rank effect size; CI—confidence interval for the mean; CTRL—control condition; ES—effects size; EXP—experimental condition; PVF—peak vertical force; n—number of study participants; POST—post-test; PRE—pre-test.
Table 3. A comparison of the PVF [N] in the non-dominant (back) lower limb with different conditions (EXP; CTRL) and time points (pre-test; post-test) in the study participants of different WKF age categories.
Table 3. A comparison of the PVF [N] in the non-dominant (back) lower limb with different conditions (EXP; CTRL) and time points (pre-test; post-test) in the study participants of different WKF age categories.
Condition
Time Point		EXP_PRE
(Mean, SD, 95% CI)		EXP_POST
(Mean, SD, 95% CI)		ES
(Pre vs. Post)		Δ [%]CTRL_PRE
(Mean, SD, 95% CI)		CTRL_POST
(Mean, SD, 95% CI)		ES
(Pre vs. Post)		Δ [%]
PVF [N]—Non-Dominant Lower Limb
U16 (n = 20)769.0 ± 182.0
(689.0 to 849.0)		772.0 ± 158.0
(702.0 to 841.0)		−0.05 †0.39 ± 13.19809.0 ± 179.0
(730.0 to 887.0)		815.0 ± 185.0
(734.0 to 896)		0.02 ‡0.74 ± 3.35
U18 (n = 19)891.0 ± 231.0
(787.0 to 995.0)		848.0 ± 166.0
(773.0 to 923.0)		0.31 †−4.83 ± 28.14870.0 ± 192.0
(783.0 to 956)		861.0 ± 224.0
(761.0 to 962.0)		0.08 †1.03 ± 16.67
Sen (n = 22)926.0 ± 218.0
(834.0 to 1017.0)		893.0 ± 212.0
(804.0 to 981.0)		0.35 †−3.56 ± 2.75921.0 ± 232.0
(824.0 to 1018.0)		897.0 ± 210.0
(809.0 to 985.0)		0.31 †−2.61 ± 9.48
Δ—delta; †—Cohen’s d effect size; ‡—Wilcoxon’s rank effect size; CI—confidence interval for the mean; CTRL—control condition; ES—effects size; EXP—experimental condition; PVF—peak vertical force; n—number of study participants; POST—post-tests; PRE—pre-test.
Table 4. A comparison of mean differences in the inter-limb PVF [N] asymmetries due to the applied condition (EXP; CTRL) and time point (pre-test; post-test) in the study participants of different WKF age categories.
Table 4. A comparison of mean differences in the inter-limb PVF [N] asymmetries due to the applied condition (EXP; CTRL) and time point (pre-test; post-test) in the study participants of different WKF age categories.
Condition Time PointEXP_PREEXP_POSTESΔ [%]CTRL_PRECTRL_POSTESΔ [%]
Inter-limb
PVF Difference [N]
(Mean, SD, 95% CI)		Inter-limb
PVF Difference [N]
(Mean, SD, 95% CI)		Pre vs. PostPre vs. PostInter-limb
PVF Difference [N]
(Mean, SD, 95% CI)		Inter-limb
PVF Difference [N]
(Mean, SD, 95% CI)		Pre vs. PostPre vs. Post
U16 (n = 20)111.0 ± 91.2
(70.7 to 151)		94.7 ± 83.7
(58.0 to 131)		0.18 †−14.68 ± 8.22121.0 ± 124.0
(66.1 to 175.0)		154.0 ± 81.8
(118 to 189)		−0.38 ‡27.27 ± 34.03
U18 (n = 19)163.0 ± 153.0
(94.4 to 232.0)		91.9 ± 62.8
(63.3 to 120.0)		0.42 †−43.62 ± 58.9595.1 ± 86.0
(56.4 to 134.0)		168.0 ± 129.0
(110.0 to 225.0) *		−0.63 †76.66 ± 50.0
Sen (n = 22)155.0 ± 101.0
(113.0 to 197.0)		172.0 ± 122.0
(121.0 to 223.0)		−0.15 †10.97 ± 20.79103.0 ± 107.0
(57.9 to 148.0)		125.0 ± 118.0
(75.9 to 175.0)		−0.26 †21.36 ± 10.28
* statistical significance within the group; Δ—delta; †—Cohen’s d effect size; ‡—Wilcoxon’s rank effect size; CI—confidence interval for the mean; CTRL—control condition; ES—effects size; EXP—experimental condition; n—number of study participants; POST—post-test; PRE—pre-test; PVF—peak vertical force.
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MDPI and ACS Style

Gaweł, E.; Drozd, M.; Maszczyk, A.; Zając, A. Acute Effects of a Simulated Karate Bout on Muscular Strength Asymmetries of the Lower Limbs in Elite Athletes of Different Age Categories. Appl. Sci. 2025, 15, 888. https://doi.org/10.3390/app15020888

AMA Style

Gaweł E, Drozd M, Maszczyk A, Zając A. Acute Effects of a Simulated Karate Bout on Muscular Strength Asymmetries of the Lower Limbs in Elite Athletes of Different Age Categories. Applied Sciences. 2025; 15(2):888. https://doi.org/10.3390/app15020888

Chicago/Turabian Style

Gaweł, Eliza, Miłosz Drozd, Adam Maszczyk, and Adam Zając. 2025. "Acute Effects of a Simulated Karate Bout on Muscular Strength Asymmetries of the Lower Limbs in Elite Athletes of Different Age Categories" Applied Sciences 15, no. 2: 888. https://doi.org/10.3390/app15020888

APA Style

Gaweł, E., Drozd, M., Maszczyk, A., & Zając, A. (2025). Acute Effects of a Simulated Karate Bout on Muscular Strength Asymmetries of the Lower Limbs in Elite Athletes of Different Age Categories. Applied Sciences, 15(2), 888. https://doi.org/10.3390/app15020888

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