LAUSR

We thank you for your interest in our article “Influence of body position and axial load on spinal stiffness in healthy young adults” [1], and we thank the editor for giving us the opportunity to clarify the perceived issues. The reader points out that the exclusion criteria for healthy subjects investigated in the study were very limited and that specifically, asymptomatic inflammatory spondyloarthropathies, asymptomatic congenital disorders, disorders of the hip and knee as well as respiratory and abdominal disorders may influence the spinal stiffness. We fully agree with this point; however, we think it is highly unlikely that the results of the study were influenced by any of these disorders. Any potential participant suffering from a known, symptomatic disorder was excluded from study participation. Of course, we cannot be certain that none of the participants had a yet undetected, asymptomatic disorder such as an asymptomatic inflammatory spondyloarthropathy or a congenital spinal disorder. However, the incidence of such disorders is very low (e.g. for hemivertebra: 9.1 per 100,000 [2]). Therefore, it seems reasonable to neglect a theoretically possible influence of an undetected asymptomatic disorder on the study results. We agree with the reader that it would be interesting to integrate the findings of the study with additional information regarding spine biomechanics, such as spinal alignment obtained using x-ray or other imaging techniques. We deemed these additional examinations beyond the scope of this study and ethically and economically inappropriate for pure research purposes in the context of our work. We suggest to consider our study, in which the measurement procedure was of course standardized (as explained in the manuscript), as a starting point upon which other research groups can build their own study designs in which they reevaluate the risk–benefit ratio. The reader also asks why only 50 percent of body weight was chose as additional axial load. We appreciate the question and are happy to provide the rationale: In a previous study, a change in lumbar spinal stiffness during hypergravity (g = 1.8) was found [3]. To achieve approximately the same load on the lumbar spine as during hypergravity, we chose a load of 80% of the human trunk, which is about 50% of the total body weight [4]. The reader further suggests that spinal stiffness should be examined in future studies using a wider range of additional loads. In fact, we have continued this line of research and have recently submitted a manuscript with the results of the effects of different additional axial loads on spinal stiffness. Hopefully, this article will be published soon so that the reader can refer to this publication.