1.    Introduction
Physical activity is essential, particularly for school-aged children and adolescents, with benefits that extend into adulthood (1). It improves cardiovascular health, supports healthy body composition, and reduces the risk of chronic conditions such as diabetes and obesity. Physical fitness is recognized as a key health indicator in this age group, and schools play a critical role in promoting active lifestyles through accessible and enjoyable exercise programs (2). Enhancing physical fitness and motor capabilities remains a primary focus in sports science, aiming to optimize performance and prepare students for skill acquisition in various sports (3, 4). A variety of training methods have been investigated in previous studies. For example, Kazemi et al. (2021) found that both rope jumping and elastic band exercises were effective in improving children's physical fitness (5). Similarly, Kokab et al. (2019) reported that resistance training positively influenced muscle strength, flexibility, body composition, and lung function in male students (6).
In recent years, suspension training has attracted attention as a strength training method suitable for individuals of all fitness levels (7). Its portability and adaptability have made it popular in diverse settings, including elderly fitness programs (8) and rehabilitation contexts (9). Suspension training systems like TRX can burn between 200 and 1,000 calories per hour (10) and are versatile across different age groups and fitness capacities. Despite its growing popularity, more research is needed to fully understand the benefits of suspension training—particularly in student populations—and how it compares to traditional exercise methods in improving muscular strength, endurance, flexibility, and balance.
2.    Methods
This semi-experimental study followed a pre-test–post-test design with two groups (training and control). A total of 40 tenth-grade male students from high schools in Mashhad were selected through convenience and purposive sampling. Inclusion criteria included the absence of any history of metabolic, musculoskeletal, neurological, or chronic conditions (e.g., cardiovascular or pulmonary diseases, asthma). Ethical approval was obtained from the Guilan University of Medical Sciences (Ethics Code: IR.GUMS.REC1399.252), and informed consent was collected from both participants and their guardians.
Participants were randomly assigned to either a training or control group (n = 20 per group). Physical fitness was evaluated using standardized field tests. (Static balance: Stork Balance Test, Upper body muscular endurance: Modified Pull-Up Test, Lower body strength: Standing Long Jump and Hamstring flexibility: Sit-and-Reach Test). The training group participated in a TRX suspension training program for six weeks, with three sessions per week, each lasting 60 minutes. The program included exercises targeting core stabilization, balance, muscular endurance, and flexibility. Prior to the intervention, all participants underwent three practice trials to ensure familiarity with the tests. Both pre-test and post-test scores were recorded using the same protocols. To ensure data quality, proper technique and consistency in execution were emphasized throughout the assessment. Data normality was confirmed using the Shapiro–Wilk test. Paired t-tests were used to compare within-group changes, while one-way ANCOVA was conducted to assess between-group differences, controlling for pre-test values. Statistical significance was set at p ≤ 0.05, and analyses were performed using SPSS version 24.
3.    Results
The results presented in Table 1 indicated no significant differences between the control and training groups in static balance, muscular endurance, muscle power, and flexibility during the pre-test stage, suggesting that the groups were homogeneous at baseline. However, the one-way analysis of covariance (ANCOVA), using pre-test scores as a covariate, revealed significant differences in the post-test scores between the two groups in the variables of static balance, muscular endurance, and flexibility. No significant difference was found between the groups in terms of muscle power. Furthermore, paired t-test results demonstrated that the training group experienced significant improvements in static balance, muscular endurance, and flexibility from pre-test to post-test. In contrast, no significant changes were observed in the control group across these measures.

4.    Conclusion
This study aimed to examine the effects of six weeks of TRX suspension training on selected physical fitness components in high school students. The results demonstrated significant improvements in static balance, muscular endurance, and flexibility in the training group compared to the control group, while no significant difference was found in muscle power. The improvement in static balance may be attributed to the instability introduced by suspension exercises, which constantly challenge postural control and activate proprioceptive receptors. These exercises stimulate the neuromuscular system, leading to better coordination and enhanced control of movement patterns. By engaging the core and lower limb stabilizing muscles, TRX training may help reprogram motor control strategies via increased sensory input from joint and muscle receptors (15). Increased muscular endurance observed in the training group can be explained by the nature of TRX training, which involves bodyweight resistance, repeated movement patterns, and sustained muscular engagement. The combination of isometric holds, dynamic repetitions, and variable angles of resistance challenges both muscular strength and endurance simultaneously. The repetitive effort required to stabilize the body in a suspended state further promotes endurance adaptations. Flexibility improvements may be linked to the increased range of motion required during TRX movements, especially in lower body exercises. The constant engagement of antagonist muscle groups and dynamic stretches during training could contribute to enhanced hamstring flexibility over time. Although no significant changes were observed in muscle power, this may be due to the relatively short duration of the intervention or the specificity of the exercises, which may not have provided sufficient overload to stimulate power development.
Overall, the findings suggest that TRX suspension training is an effective, accessible, and versatile method for improving several key physical fitness components in adolescents. It holds promise for school-based physical education programs aiming to enhance balance, endurance, and flexibility.

Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be addressed in this research.
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.