1.    Introduction
One of the key physical requirements in Taekwondo is achieving 180-degree flexibility, particularly in joints such as the hips, which is essential for executing effective kicks and defensive maneuvers. This martial art demands a multidirectional range of motion (ROM) in the hip joint, allowing athletes to perform advanced techniques and rapid movements with precision and efficiency (2). The 180-degree middle split is a fundamental flexibility technique in Taekwondo that enhances hip ROM, increases muscular flexibility, and strengthens the adductor muscles. Mastery of this technique enables athletes to deliver more powerful and accurate kicks, while also improving functional balance and postural stability during combat—factors that collectively reduce the risk of injury. Enhanced hip flexibility and neuromuscular control can translate into improved speed, precision, and power, facilitating the execution of complex and high-speed techniques (3). Stretching methods used to improve these attributes include static, dynamic, passive, active, isometric, and proprioceptive neuromuscular facilitation (PNF) stretching (2). Given the technical and tactical demands of Taekwondo, as well as the competitive rules, high levels of lower-limb flexibility are required, often necessitating dedicated flexibility training to expand the ROM of the hip’s intricate joint structure (3).
Although the effects of stretching on athletic performance may vary depending on the type and timing of the stretch, PNF is widely recognized as one of the most effective techniques for enhancing flexibility, with limited research disputing its efficacy (12). Typically, static and PNF stretching are implemented separately within training sessions or on alternate days. Furthermore, most previous research has focused on static stretching and balance control, with relatively few studies directly comparing the effects of multiple stretching modalities. This study aims to address this gap by comparing two commonly used stretching protocols—static and PNF—and examining their effects on flexibility (as measured by the 180-degree middle split) and functional balance in Taekwondo athletes.
2.    Methods
Given the nature and objectives of the study, this research employed an applied, quasi-experimental design with a pre-test–post-test approach and a control group over an eight-week intervention period. The target population consisted of male and female Taekwondo practitioners aged 15 to 25 years. Participants were recruited from local sports clubs and were required to have at least three years of Taekwondo experience, train two to three sessions per week, and be physically capable of completing all performance tests independently. A total of 39 participants were selected using purposive and convenience sampling. They were assigned to one of three groups—static stretching, PNF stretching, or control (no stretching)—based on age, gender, and body mass index (BMI). Two female participants (one from the static stretching group and one from the control group) withdrew from the study due to lack of interest.
The aim of the study was to evaluate the effects of different stretching protocols on 180-degree middle split flexibility and functional balance. Participants were instructed to refrain from intense physical activity for at least 24 hours prior to testing, and no injuries occurred during the intervention period.
The PNF protocol was based on the contract-relax-agonist-contract (CRAC) technique, while static stretches were performed with 30-second holds. Both protocols were implemented over an eight-week period. Outcome measures included the 180-degree flexibility test, Single-Leg Stance Time (SLST), and Hop Tests (HT), assessed before and after the intervention. Statistical analysis was conducted using analysis of covariance (ANCOVA) followed by Bonferroni post hoc tests, with the significance level set at p ≤ 0.05.
3.    Results
Table 1 presents the variables examined in the three groups: static stretching, PNF stretching, and control (no stretching), including 180-degree flexibility and balance (single-leg stance time and hop tests). The results showed that athletes who performed PNF stretching demonstrated significantly better performance in the 180-degree flexibility test (p < 0.001), single-leg stance time (p < 0.001), and hop tests (p < 0.05). Bonferroni post hoc comparisons of the groups in the post-test are presented in Fig. 1, adjusted using ANCOVA.


4.    Conclusion
The findings of this study indicate that PNF stretching exercises led to greater improvements in flexibility among Taekwondo athletes. While static stretching also resulted in an average improvement of approximately 4.4 degrees compared to the control group (no stretching), this change was not statistically significant. Stretching exercises are believed to modify muscle structure and enhance the extensibility of elastic components. Thus, the observed increases in range of motion (ROM) are likely attributable to improved flexibility within these elastic elements (15). Several physiological mechanisms may underlie flexibility gains following stretching. Evidence suggests that short-term increases in ROM are more closely associated with enhanced stretch tolerance rather than reductions in muscle-tendon unit stiffness. Notably, PNF stretching has been shown to be more effective than static stretching in improving stretch tolerance (14). For long-term improvements, researchers propose that sustained stretching may lead to an increase in the number of sarcomeres in series, effectively lengthening the muscle through chronic adaptation to stretch-induced stress (17). In addition to flexibility, ROM plays a key role in postural control and balance performance. Limited flexibility can impair the body's ability to regain a stable posture after external perturbations. For instance, restricted ankle dorsiflexion ROM may lead to compensatory subtalar joint pronation, increasing postural sway due to reduced stability during pronation and supination (19).
From a neural perspective, it has been suggested that PNF stretching may reduce joint oscillations mediated by spinal reflexes, thereby facilitating a shift in control to higher motor centers responsible for coordination and balance (20). Overall, the results of this study suggest that PNF stretching is an effective method for enhancing both flexibility and functional balance in Taekwondo athletes. By improving joint ROM and reducing muscle stiffness, this technique may support the execution of complex movements and rapid directional changes—key elements of performance in sports such as Taekwondo.

Ethical Considerations
Compliance with ethical guidelines
The Ethics Committee of Qazvin University of Medical Sciences approved the study (IR.QUMS.REC.1402.011).
Funding
This research did not receive any financial support from government, private, or non-profit organizations.
Authors' contributions
All authors contributed equally to preparing the article.
Conflicts of interest
The authors declare that there are no conflicts of interest associated with this article.