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Reply to the Letter to the Editor on: “Effectiveness of Multi-activity, High-intensity Interval Training in School-aged Children.”
We would like to thank you for the opportunity to respond to the issues raised in the letter by Mans and colleagues and to clarify aspects of our study in relation to their concerns. We would also like to thank Mans and his colleagues at Erasmus MC Medical University, Rotterdam, for their interest in our paper and for taking the time to express their concerns.
In their letter to the editor, Mans and colleagues  note that a CONSORT flow diagram was missing, causing a lack of information on the number of participants excluded or lost to follow-up. We agree that a flow chart is missing and acknowledge the importance of reporting this information. As the intervention was incorporated into the schools’ physical education classes and was thus mandatory, we lost no participant throughout the intervention. We realize that we should have stated this more precisely in the manuscript.
Furthermore, Mans et al.  address the main findings of our study showing that HIIT is associated with a significant decrease in systolic blood pressure, arterial pressure, and aortic pulse wave velocity in school-aged children. They note the potential concern that the relatively small differences in blood pressure (BP) found may not be of clinical importance. This is a valid and challenging question. Mans et al. already provide an answer in their letter by stating that the minimal clinically important difference in systolic blood pressure is unknown.
Although, the reductions in peripheral BP may appear relatively small, it is recognized that already modest reductions in BP are associated with a significantly reduced risk of cardiovascular complications  . Epidemiological studies in adults indicate that a decrease of 2 mmHg in systolic BP is likely to reduce the mortality associated with stroke by 6% and coronary heart disease by 4%, whereas a reduction of 5 mmHg is likely to reduce the risk of these diseases by 14% and 9%, respectively  . Unfortunately, similar studies in children are missing. However, knowing that BP values track into adulthood , we would argue that the 4 mmHg decrease found in our study  could be of clinical relevance. However, based on the results of our study, we are not in the position to answer this question conclusively.
Mans and colleagues  also address the fact that we presented the aerobic fitness results in z-scores only, without any additional information on how to interpret these differences. We must agree that we should have included more information on these z-scores. To evaluate the results of the 6-Minute-run, the individual test score (in meters) was put in relation to the mean test performance of the corresponding age norm from Bös et al. .
Mans and colleagues  also question the generalizability of our results, as they believe that the study sample might not correctly represent the general population of school-aged children. The question of the generalization of results is probably one of the main concerns in research. It is apparent that we cannot conclude that these results hold true for all children. Nevertheless, we think this concerns all studies, especially when cohort sizes are relatively small. However, we should have included that the children came from a relatively low socio-economic background, which may further specify the study sample.
We hope we were able to clarify some of the issues raised and are grateful for the valuable and constructive comments.
Sascha Ketelhut und Kerstin Ketelhut
04 January 2021 (online)
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