## 4 Discussion
More research is needed to better understand the relationship between these trunk endurance test scores, which will allow us to determine the extent to which a single trunk endurance test may be generalizable to other measures of trunk endurance. In this regard, static tests could be applied in environments or sports that require isometric trunk endurance demands, such as maintaining a more upright sitting posture in educational settings over long days or hockey, gymnastics, and cycling. Dynamic tests could be used in daily life situations or in sports requiring continuous trunk flexion or extension movements, such as kayaking, volleyball, handball, dressage, or soccer, for example. This research also has some limitations that should be stated. First, the results are limited to Spanish children and adolescents, which may not be representative of students of the same age around the world. Second, the sample size of each sub-group varies according to the sub-group under investigation, and the large variability in test results (i.e., the Ito test) may have affected the comparison between sub-groups. In addition, special attention would be required when performing trunk endurance tests due to the high inter-individual variability, not only from differences in participants' muscle endurance but also from the challenges in maintaining a standardized posture. Even minor variations in the initial position or in balance maintenance can significantly affect the results [64]. It is then crucial to follow standardized protocols, provide clear instructions, involve expert evaluators, and conduct several familiarization sessions to ensure consistency and reliability [23; 32]. In any case, the standardization of protocols will not entirely eliminate the inherent variability and validity concerns associated with certain trunk endurance tests. In fact, it has been shown that female SB test performance may be influenced by factors beyond trunk lateral flexor endurance, including contributions from the external oblique and deltoid muscles, body mass, and trunk height [65]. These factors, along with the activation of shoulder musculature and fatigue levels, may affect the validity of the SB test as a measure of lateral flexor endurance. Similar concerns, though potentially to a lesser extent, may also apply to other trunk endurance tests used in this study, highlighting the complexity of isolating trunk muscle performance in field-based assessments. Therefore, the results obtained in this study regarding muscle strength and endurance should be interpreted with caution as they are subject to variability and validity limitations inherent to the methods used. Another limitation of this study that should be mentioned is that the maturity offset was calculated using an equation based on the leg length, sitting height, age, body height, and body weight of the subjects, which may not be as accurate as using the gold standard method (x-ray exams). The leg length was also obtained by the difference between the standing and sitting heights, and for future research, it would be advisable to directly measure the leg length to offer greater accuracy and reduce the likelihood of error due to postural variations, measurement inaccuracies, and individual differences in body proportions. However, to minimize the sub-group assignment error derived from the equation, participants with a maturational offset between \(-1\) to \(-0.5\) and \(+0.5\) to \(+1\) were removed from the dataset, as were students who fell outside of \(-3\) or \(+3\) years. As a result, this decision led to a smaller sample size in the post-PHV sub-group compared to the other sub-groups. Finally, personal factors, such as motivation, may influence field test performance, even if participants received full information about the tests as well as encouragement during performance. Additionally, factors like physical activity levels could significantly impact trunk endurance, potentially affecting test results independently of the CA or MAT.