**1. Introduction**

General physical fitness, defined as the effect of the externalization of motor skills, is the basis for athletic training regardless of the sport [1–3]. By properly developing its level, it is possible to implement more precise and effective specialized training forms, which are a prerequisite for athletes' performance during tournaments [4–6]. Research also shows that inadequate levels of fitness may increase the likelihood of injury [7]. The development of the athlete's motor skills depends on sport [1]. Kickboxing is a combat sport that is complex in technique, tactics, and movement structure, and requires an adequate level of motor skills as a foundation for activities during competitions [8–10].

Research on the effectiveness of combat sports training indicates that it is highly dependent on general motor skills [11–14]. For kickboxers, the basis for high competitive

**Citation:** Ambrozy, T.; Rydzik, Ł.; ˙ Kwiatkowski, A.; Spieszny, M.; Ambrozy, D.; Rejman, A.; Koteja, A.; ˙ Jaszczur-Nowicki, J.; Duda, H.; Czarny, W. Effect of CrossFit Training on Physical Fitness of Kickboxers. *IJERPH* **2022**, *19*, 4526. https://doi.org/10.3390/ ijerph19084526

Academic Editor: Luca Paolo Ardigo

Received: 9 February 2022 Accepted: 7 April 2022 Published: 8 April 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

performance is a high level of strength, dynamics, anaerobic capacity, and aerobic capacity, with the latter considered the necessary foundation for all activities. Therefore, physical preparation should be based on improving upper limb strength and speed (punches and combinations during offensive actions and blocks and dodges during defensive actions) and developing high levels of anaerobic power (dynamic kicks and punches) [15,16].

Training efficiency, which is the ability to endure loads based on sufficient physical capacity, is also an important element that increases the competitive performance of athletes [17–20]. The specifics of training at the elite level should be tailored to the individual athlete's needs. In search of opportunities to improve work efficiency during training, modern forms of training are implemented and verified, often differing from the classic training methods commonly used in kickboxing, which include repetition methods (at maximal or submaximal work intensity, full rests) and interval methods (low, medium, or high intensity, and incomplete rests) based on technical elements using special coaching devices such as focus mitts, kick shields, punching bags, etc. [5,21].

The growing popularity of CrossFit is of interest to researchers in the context of implementing this training modality, with its hallmark being competing during exercise [22,23]. This type of training, based on exercises derived from the arsenal of training means used in gymnastics, weightlifting, and strength and functional training, is focused on the continuous improvement of the athlete's performance in individual exercises through competition with a training partner or themself [24]. This model of strength and conditioning training can also be applied in the kickboxing environment.

The implementation of new forms of training and the detailed verification of their effects on the physical capabilities of athletes are widespread in combat sports [25–29]. Through the implementation of experimental training plans, it becomes possible to improve the quality of training, which can, in turn, affect the competitive performance of athletes [30]. The implementation of modern experimental forms of training in kickboxing is popular, and the novelty of our study is the introduction of the author's form of training based on CrossFit training with exercises commonly used in kickboxing. Contemporary analyses in the context of the implementation of CrossFit training in the structure of kickboxing include the effect of CrossFit exercises on the body posture of kickboxers [31], and a comparison of CrossFit training and other forms of resistance training to maintain an optimal level of fitness [32].

To determine the athlete's level of preparation for the competition, an analysis of a fight is performed [16,33,34] by determining the technical and tactical indices or by evaluating special physical fitness using tests corresponding to the structure of a kickboxing bout [35]. This type of analysis was conducted in the present study.

The aim of this study was to determine the effect of the author's training according to the principles of CrossFit on the development of general and special physical fitness in a group of kickboxers compared to a control group. The choice of general fitness tests was based on examinations performed in previous studies [16,33]. In this study, an attempt was made to find if the proposed experimental training program could be successfully implemented into the regular strength and conditioning programs in kickboxing.

#### **2. Material and Methods**

#### *2.1. Study Group*

The study was experimental in nature and was conducted in a group of 60 male kickboxers. Based on the sample size, as determined using G\*Power software 3.1.9.4, it was shown that, with the estimated moderate effect size, at least twenty participants were needed in each group (effect size f = 0.65, power = 0.95, *p* = 0.05) [36]. In the present analysis, 30 participants in each group were recruited to ensure a more accurate analysis. Participants were selected by purposive sampling, i.e., subjective non-random sampling based on clear criteria for selecting a group for the study such as training experience, sports skill level (minimum class 1 athletes; in Poland, athletes are classified by sports skill classes: there are classes 1, 2, and 3, with each class determined based on winning

during competitions), consent to participate in the experiment, and good health status. The inclusion criteria adopted were due to the high intensity of the proposed form of training that requires the participants to have bodies adapted to such a high level of exercise. The selected group was randomly divided into two (experimental and control) groups, each consisting of 30 athletes.

In the control group, training was conducted according to a standard general cycle. In the study group (experimental), training was modified (independent variable) by introducing an experimental program (CrossFit workout) into their standard training. The experiment required a high degree of accuracy and maximum commitment from the athletes. During the experiment, the athletes did not participate in sporting events or sparring at competitive intensity levels and did not report any injuries. Observation of the changes in the experiment and their quantitative evaluation were also performed. The dependent variable in this case was general and special physical fitness, viewed as differences in the fitness test results in both groups.

#### *2.2. Morphological Characteristics of the Subjects*

The mean training experience in all participants was 8.1 ± 4.24 years. They trained 1.5 to 2 h, 6 to 8 times per week. The mean age of study participants was 20.07 ± 1.46 years, body weight was 73.56 ± 8.13 kg, body height was 179.55 ± 0.45 cm, body fat was 14 ± 0.2%, and BMI was 23.63 ± 1.19 kg/m2. Body composition and body weight were measured using a Tanita BC-601 body composition monitor (Tanita, Tokyo, Japan) [37] in the morning in fasting conditions, whereas body height was measured using a SECA 2017 body height meter (Seca, Hamburg, Deutschland). The examination was performed in the standing position, as recommended by the manufacturer. The subjects did not consume alcohol the day before the measurement and avoided large meals and extreme physical exercise.

#### *2.3. Research Program and Methodology*

According to the principles of the implementation of pedagogical experiments, the researcher's interference involved manipulation of the training process in the experimental group. The first examination took place before the experimental training in both groups, in the control and preparation mesocycle of the preparation period. Next, the group of participants performed a modified workout (by introducing an experimental program based on CrossFit workout into their standard training programs) three times a week during each training session. During this time, the control group performed standard kickboxing training without modification. After eight weeks, another examination was conducted (effect control).

Furthermore, each participant was instructed not to use specialized diets and supplementation during the experiment due to the strong relationship between diets and results obtained during tests. Diets were monitored using notebooks in which the participants recorded the foods they consumed using home measures based on a photo album of foods and products. The recording procedure was continued for 3 days: 2 working days and 1 day off [38]. The analysis of diet observations revealed neither specific diets nor the use of performance-enhancing supplements in the training groups. The experiment was approved (No./309/KBL/OIL/2019) by the Bioethics Committee of the Regional Medical Chamber in Kraków, Poland.

#### *2.4. Principles of the Experimental Training*

Using the principles of CrossFit training methodology [39] and typical kickboxing training exercises based on the most frequently performed techniques [40], a training program was developed to improve the physical fitness of kickboxers (Table 1).


**Table 1.** Methodology of the experimental training program.

AMRAP—As many rounds as possible, a method of training that involves performing as many sets of efforts as possible at a fixed number of repetitions in a given time.

The experimental training program is based on the AMRAP method (Table 2), which consists of performing as many sets of efforts as possible with a fixed number of repetitions in a given time (10 min in this case) [39]. To avoid boredom and training routine, a different training unit was planned for each workout of the week. Each workout was preceded by a warm-up and consisted of 8 exercises including boxing punches, kickboxing kicks, and exercises typical of the CrossFit methodology such as burpees (transition from standing upright to squat position, kicking the feet back to a front support position, returning to squat position, and jumping up), or box jumps. The training program was designed so that it was simple to perform and accessible to every participant. Therefore, additional equipment was kept to a minimum.

**Table 2.** The experimental training program used in the study.


A detailed video illustration of the exercises is provided in the Supplementary Materials.

*2.5. Physical Fitness Tests*

Physical fitness was evaluated using selected items of the International Committee on the Standardization of Physical Fitness Test (ICSPFT) [2] and Eurofit Physical Fitness Test (EUROFIT) [41]. The entire test battery consisted of the following tests:


exercise is repeated as many times as possible without a rest; the result is the number of complete pull-ups (chin over the bar) [2].


shields and punch shields, a protocol for recording the results, and a sport tester (heart rate monitor). In the first station, the athlete performs, from a fighting stance, a combination of punches to the shield held by the partner: left and right straight punches to the head, without stopping, for 30 s. After completing this part of the test, the athlete runs 10 m in a straight line to the next station (No. 2), where, from the fighting position, they perform roundhouse kicks to the shield held by the partner for 30 s: left high kick (high roundhouse kick) and right high kick to the head. Next, the athlete runs back to the first station with shields and performs a left straight-right hook combination for another 30 s to the head. After completion of this part of the test, the athlete runs 10 m to the partner holding the shield in station 2 and performs middle roundhouse kicks for 30 s alternately with the right and left leg to the body trunk. The total special exercise time during the test is 2 min (4 × 30 s). Correctly performed kicks and punches were counted in each of the four parts. Heart rate (HR, bpm) was measured directly after completion of the test and after 1 min rest. The Garmin HRM chest strap was used in the tests. The proposed special fitness test allowed for the evaluation of the technical level of athletes in terms of the most effective and most frequently used hand techniques (punches) and leg techniques (kicks), speed (number of punches and kicks performed per time unit), special endurance (response of the circulatory system and number of punches and kicks), coordination (combination of kicks and punches), and flexibility (kicking range). The 10 m running distance used to move between stations corresponds to the diagonal of the largest ring found in ring combat sports. The technical skills used in the test ensure the selectivity of the test, making it inaccessible to those who do not perform special training and do not have the appropriate level of technical proficiency. Furthermore, after the test, based on the results obtained, the index of special fitness was calculated using a specialized formula:

> Final HR(bpm) + HR1min(bpm) Kick + Punches (N)

where:

> Final HR—heart rate recorded immediately after completion of the test; HR1 min—heart rate recorded 1 min after completion of the test; Kicks—the number of kicks performed in the test; Punches—the number of punches performed in the test.

The special fitness index reflects the level of a fighter's special fitness, which means the effective interaction of the body's exercise capacity, general fitness, and the athlete's technical skills. The interpretation of the score is inversely proportional: the higher the level of special fitness, the lower the value of the kickboxing test index.


The tests were conducted in a three-day mode at noon: trials 1–5 on the first day, trials 6–9 on the second day, and special fitness tests on the third. All tests were performed before and after the experiment, except for tapping, standing long jump, and flexibility tests, which were performed twice, and the better score was recorded. The intervals between the tests were designed so that the participants rested completely. A 20 min warm-up was conducted with the athletes prior to testing.

#### *2.6. Statistical Analysis*

Statistical analysis of logarithmic data was performed using STATISTICA v13.1 PL software (Statsoft, Kraków, Poland). Basic descriptive statistics were computed: arithmetic means, standard deviations, and 95% confidence intervals. The data were tested for normal distribution using the Shapiro–Wilk test, and Student's *t*-test for dependent variables was used to assess the significance of differences. Student's *t*-test for independent variables was used to determine the significance of differences between the control and experimental groups. The effect size was calculated using Cohen's d index. When d ranges from 0 to 0.2, the effect is small, i.e., negligible; it is medium from 0.2 to 0.5, large from 0.5 to 0.8, and extremely large when over 1.4. The relationships between the data were verified using Pearson linear correlation. Correlation values were interpreted as weak for the ranges of −0.5 to 0.0 or 0.0 to 0.5, and strong for −1.0 to −0.5 or 0.5 to 1.0 [46,47]. The level of statistically significant differences was set at *p* < 0.05.
