Abstract
This letter critiques the data analysis of a study investigating peak oxygen uptake responses to cycling and running sprint interval training (Digby et al., 2023 J Strength Cond Res, 37(4), e313-e316). While the study effectively demonstrates the specificity principle in the context of sprint interval training, concerns arise regarding the methodology used to categorise participants as responders or non-responders. The letter highlights the disregard for the recommendations of a number of academics advocating for specific experimental designs and statistical analyses to examine inter-individual variability. Furthermore, the reliability of within-individual adaptive responses to training and the potential impact of measurement errors and biological fluctuations are considered. It is suggested that the (non-)responder categorisation adds nothing to the main findings of the study and should be avoided. The importance of using appropriate experimental designs and statistical analyses when investigating inter-individual variability is emphasised. An open-source beta-version simulator is introduced as an educational resource to demonstrate the limitations inherent in the responder counting approach.