Accuracy and reproducibility of a novel device for monitoring resistance training performed at self-selected movement velocity

Resistance training does not necessarily require repetition failure, whereas velocity-based training and “training not to failure” are available alternatives to know the optimal point to interrupt the sets. Nevertheless, velocity-based training require exclusively maximal intended velocities and training not to failure currently relies on subjectivity to estimate repetitions in reserve. This study evaluated the accuracy and precision of a linear encoder in estimating the maximum number of repetitions during sets performed until failure at self-selected movement velocity. Fifty-seven males were evaluated in three resistance exercises: close-grip lat pulldown, knee extension, and bench press. Accuracy was evaluated by comparing the mean and median of actual and estimated repetitions using t tests and Wilcoxon signed-rank tests, respectively. Additionally, the fatigue effect in consecutive sets was analyzed using two-way ANOVA for repeated measures. Levels of agreement were assessed through Bland–Altman analysis, and reproducibility was determined by calculating the Intraclass Correlation Coefficient (ICC). The results showed no significant difference between actual and estimated repetitions (t₁₇₈ = 0.307; p > 0.05; ES = 0.02; Z = − 0.45; p > 0.05; ES = − 0.02), even in the presence of fatigue between consecutive sets. The reproducibility for estimating maximal repetitions was good (ICC_3,2 = 0.88 [95% CI = 0.83–0.91], F_177,177 = 8.07, p < 0.001), with an acceptable degree of agreement. Errors of less than or equal to two repetitions occurred in over 90% of the series for the close-grip lat pulldown and bench press, with knee extension exhibiting a slightly lower frequency. Hence, practitioners and trainers should consider using this linear encoder for the evaluated exercises, especially when failure is not desired under self-selected velocity conditions.