PurposeThe aim of this study was to examine the goodness of fit, prediction accuracy, and stability of general and individual relationships between velocity loss and the percentage of completed repetitions out of maximum possible (VL-%repetitions) in the free-weight back squat exercise. The effects of sex, training status and history, as well as personality traits, on the goodness of fit and the accuracy of these relationships were also investigated.MethodsForty-six resistance-trained people (15 females and 31 males) performed a one-repetition maximum (1RM) test, and two repetitions to failure (RTF) tests, 72 h apart. RTF tests were performed with 70, 80, and 90% of 1RM with 10 min inter-set rest.ResultsThe findings question the utility of using general and individual VL-%repetitionsrelationships to prescribe training volume with free-weight back squats as (1) the agreement in the %repetitionscompleted until reaching a given velocity loss threshold across two consecutive testing sessions was unacceptable, regardless of the load used; and (2) the ability of general and individual VL-%repetitionsrelationships to predict %repetitionsin a subsequent testing session were poor (absolute errors > 10%). Sex, training status and history, and personality traits did not affect the goodness of fit of general and individual VL-%repetitionsrelationships or their prediction accuracy, suggesting potential generalisability of those findings among resistance-trained populations.ConclusionsVL-%repetitionsrelationships do not seem to provide any additional benefits compared to costless, traditional methods and hence should not be used for monitoring and prescribing resistance training with a free-weight back squat exercise.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00421-023-05155-x.