1. Introduction
Improper distribution of forces may cause abnormal movement, resulting in additional load on the foot tissues. It causes soft tissue damage and muscle dysfunction [1]. As the most modern martial art, Taekwondo combines the most accurate, scientific, most assertive, and fastest techniques among the traditional martial arts [2]. Excessive use of Yup-Chagi and Ap-Chagi kicks, asymmetric footwork, weight transfer on one leg (mainly on the non-dominant leg, such as left foot in individuals with only one dominant leg), and using this posture during exercise and on the contest area exacerbate the risk of developing a bow-leggedness complication [6]. 
Adi et al. (2016) stated that Dollyo Chagi and Ap-Chulugi exercises significantly improve the functional status of the bow-leggedness and functional instability of the lower limb among females taekwondo practitioners [7]. Malek-oghloo et al. (2019) acknowledged a high correlation between the incidence of bow-leggedness complication and the amount of participation in exercises with weight transfer on one leg and along and deviation of the knee from its normal position [8].
Studying and recognizing the ambiguous biomechanical aspects of performing movements and diagnosing the causes of injuries can, in addition to preventing their occurrence, help to improve the professional performance of taekwondo practitioners [12]. On the other hand, due to the limited studies on lower limb injuries and their relationship with knee kinematic parameters in professional taekwondo practitioners, the exact effect of taekwondo training on the incidence of bow-leggedness is still unknown. Therefore, this study tried to investigate the effect of eight weeks of Ap-Chulugi and Yup-Chagi exercises on the prevalence of bow-leggedness and changes in the knee’s kinematic parameters in professional males taekwondo practitioners.
2. Methods
A total of 32 taekwondo practitioners from different sports clubs in the northeastern part of Tehran city were selected as a statistical sample using G×Power software and were divided into “experimental” (16 people) and “control” (16 people) groups. According to gender, age, and membership in a homogeneous sport, these individuals were selected as subjects of this study. The experimental group performed predetermined exercises for eight weeks (3 one-hour sessions per week). At the end of eight weeks of exercise, the bow-leggedness incidence in the subjects was measured again. The control group did not perform any training during these eight weeks; only in the pretest and posttest, after warming up under instructors’ supervision, they were asked to perform the movements.
The training protocol of the experimental group consisted of eight weeks of Ap-chulugi and Yup-chagi exercises. The subjects practiced these two basic taekwondo techniques three times a week for 50 minutes. In each training session, 15 minutes were devoted to warming up and 5 minutes to cooling down. The control group did not have any specific activity during this period. Knee kinematic indices, including Navicular Drop Test (NDT), tibial torsion, Q angle, enterogen, internal and external hip rotation, and knee hyperextension, were tested in experimental and control groups in pretest and posttest. After descriptive analysis, the data were statistically analyzed using an independent t-test and SPSS V. 23 software.
3. Results
The results of the Kolmogorov-Smirnov test indicated the normal distribution of research data (Table 1).


4. Discussion and Conclusion
Analysis of the data in this study indicated that eight weeks of exercise in selective techniques of Ap-Chulugi and Yup-Chagi had a significant effect on the incidence of bow-leggedness in professional male taekwondo practitioners (Table 1). The results of comparing the data obtained from pretest to posttest regarding the effect of a period of Ap-Chulugi exercises on the development of bow-leggedness complication in taekwondo practitioners confirmed that the result of this study was consistent with the research of Adi et al. (2016), which revealed that six weeks of Ap-Chulugi training had a significant effect on the incidence of bow-leggedness complication.
In their study, Yu et al. (2016) examined 12 weeks of Bandae-Dollyo-Chagi and Yup-Chagi exercises on the prevalence of bow-leggedness complications in taekwondo practitioners and concluded that Yup-Chagi exercises had a significant effect on the development of bow-leggedness complication on taekwondo practitioners. The results of their research were consistent with this research. The results of this study revealed that eight weeks of practicing the selected technique of Yup-Chagi also had a significant effect on the incidence of bow-leggedness complications in professional male taekwondo practitioners. The results of this study were consistent with the research of Adi et al. (2016), Schultz et al. (2008), and Yu et al. (2016), who reported that Bandae-Dollyo-Chagi and Yup-Chagi exercises were effective in causing bow-leggedness complication in taekwondo practitioners.
In general, the results of this study confirmed that bow-leggedness complication as an essential variable might be practical on the negative performance of athletes and their injuries during exercise and competitions. Therefore, it seems necessary to pay attention to this issue in studies related to preventing possible injuries.

Ethical Considerations
Compliance with ethical guidelines
All ethical principles are considered in this article. The participants were informed about the purpose of the research and its implementation stages. They were also assured about the confidentiality of their information and were free to leave the study whenever they wished, and if desired, the research results would be available to them.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 

Authors' contributions
All authors equally contributed to preparing this article.

Conflicts of interest
The authors declared no conflict of interest.

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