Technical Note on the Reliability of the PowerLift App for Velocity-Based Resistance Training Purposes: Response

Thank you for the opportunity to respond to the points raised in this letter from Mr. Balsalobre regarding our recent article. We appreciate the interest shown in our study. However, it seems that this author has misunderstood some points about the reliability analyses that we conducted and, therefore, this needs to be clarified. Our study aimed to analyze the agreement between five bar velocity monitoring devices, currently used in resistance training, to determine which was the most reliable device based on the results of reproducibility (between-device agreement for a given trial) and repeatability (between-trial variation for each device) analyses. Among the five different technologies assessed, only the data regarding the PowerLift smartphone app have been questioned by the author. It should be noted that this author is the main developer of the PowerLift app and the main author of the paper in which it was allegedly validated. The arguments put forward by the author to question our results are based on the assumption that ‘‘the observers who performed the video analysis did not use the app properly, because, following their data, the raw difference in frames would be higher that 20–30 frames’’. As clearly detailed in our manuscript, ‘‘Intradevice reproducibility was assessed by comparing the velocity outcomes for trial 1 simultaneously obtained by each pair of the two (same brand and model) devices’’. Thus, the results we presented here were obtained using two same version PowerLift apps (v 4.0 iOS) installed on two iPhone 6 units running iOS 11.3, and analyzed by the same examiner. Notwithstanding the above, in the letter, the author noted ‘‘Specifically, in powerlifting exercises like the bench-press, observers need to manually select the frames in which the bar takes-off the chest (beginning of the lift) and stops its vertical ascent (end of the lift). Because of that, previous research analyzing the validity and reliability of VBS [video-based systems] compared to scores of two observers analyzing the same videos in order to detect the inter-observer variability’’. Thus, it seems that the author has confused the concept of ‘‘intra-device agreement’’ (i.e., comparing the outcomes simultaneously obtained by two devices for a given trial) with the concept ‘‘inter-observers’ agreement’’ (i.e., comparing the outcomes obtained by two observers for a given video file in the same smartphone). It should be noted that we were fully aware of the need of counting both with experienced examiners and reproducible procedures to ensure the quality of the measures, particularly in manually operated systems such as the PowerLift app. In our study, the examiner showed an excellent reliability when he analysed the same video file, in the same smartphone, in two separate occasions (ICC > 0.997, 95% ICC 0.984–1.000; CV < 3.02%), which are values higher than those reported in previous investigations where the smartphone was hold in the hands. We therefore agree that the PowerLift is very easy to use and reliable in terms of intra-observers’ agreement. However, this was clearly not the hypothesis tested in our investigation, which leads us to think that the author has not read the study carefully. The two main goals of our work were: (1) to identify the errors arising from current velocity monitoring techAddress correspondence to Jesús G. Pallarés, Human Performance and Sports Science Laboratory, Faculty of Sport Sciences, University of Murcia, C/Argentina S/n., Santiago de la Ribera, Murcia, Spain. Electronic mails: alejandro.mcava@gmail.com, jgpallares@um.es Annals of Biomedical Engineering, Vol. 48, No. 1, January 2020 ( 2019) pp. 6–8 https://doi.org/10.1007/s10439-019-02305-1 BIOMEDICAL ENGINEERING SOCIETY