Response to “Letter to the Editor concerning ‘The Calf Raise App shows good concurrent validity compared with a linear encoder in measuring total concentric work’: Let us not compare apples to oranges”

Abstract

We are grateful for your efforts in helping us and the readers to gain a deeper understanding of our data [1]. With this response, we would like to clarify any uncertainties that remain and/or may have arisen.

First, we aimed to compare two different tests, like apples and oranges, not to claim superiority of the one over the other, but to provide insight on how the results from the two may differ when each is being used as intended (kindly see next paragraph). With the large amount of data already existing through the use of the linear encoder [3], and the emerging data through the Calf Raise app, we sought to bridge the two by direct comparison. We clearly state that the Calf Raise app has been tested for validity against gold standard (i.e., three-dimensional motion capture and force plates). In our study, we never claimed to compare the Calf Raise app against any gold standard. Concurrent validity is one of two basic approaches of criterion-related validity and seeks to establish the correspondence between the target test (in this case, the Calf Raise app) and a reference or gold standard: In our study, the reference standard was the linear encoder [4].

The calibration procedures described in your published videos were not strictly followed. The square shape of the tape, and its positioning, are honest mistakes made by us. For the remaining aspects, however, it is important to note that data collection was conducted before the summer of 2022. The videos referred to at the end of your letter were uploaded in early August 2022. The first mention of the distance between the device and the marker was, from what we can find, in your article published in Sports Biomechanics [5], accepted on 23 August of the same year. In other words, the calibration procedures were not available to us at the time of data collection. The marker diameter in fig. 1 in our paper [2] was, as stated, an example, and the picture was taken when drafting the manuscript. The diameter of the consistently square marker was consistently 25 mm.

The concerns about the linear encoder are highly relevant, and we share your thoughts. You write: ‘It appears that the linear encoder is the device introducing measurement errors in the vertical plane’. We address this, and your rationale, already in our discussion. What is not mentioned in our published article is that the position where the encoder is attached indeed defines zero.

Finally, we would like to address the comments on study registration. We state that our current study includes participants from an on-going and registered prospective study. This validity study under review, is registered in a local research database (https://www.researchweb.org/is/vgr/project/278701). The validity study was also included in the application to the Swedish Ethical Review Authority that was submitted in January 2022 (revised: February 2022).

To conclude, we thank you for your valuable input which is highly relevant to the calibration procedure of the Calf Raise app. You have provided proof that corroborates our findings in terms of systematically higher measurements obtained through the standard procedure of the linear encoder when compared to measurements obtained through the Calf Raise app. Together, we have elucidated pitfalls in the use of Calf Raise app that may be of interest for potential users within, and outside of, the scientific community. For future research, we agree that the results regarding intra-class correlations and variability of measurements should be interpreted with caution, until replicated or otherwise nuanced by independent trials. With respect to clinical implications, it would be interesting to see the inter- and intratester reliability between various clinicians without prior experience with the Calf Raise app.

The authors declare no conflict of interest.