Predictive Equations to Estimate Relative Load Based on Movement Velocity in Males and Females: Accuracy of Estimation for the Smith Machine Concentric Back Squat

Mendonca, GV, Fitas, A, Santos, P, Gomes, M, and Pezarat-Correia, P. Predictive equations to estimate relative load based on movement velocity in males and females: accuracy of estimation for the Smith machine concentric back squat. J Strength Cond Res 37(8): 1559-1565, 2023-We sought to determine the validity of using the Smith machine bar velocity to estimate relative load during the concentric back squat performed by adult male and female subjects. Thirty-two subjects (16 men: 23.3 ± 3.8 and 16 women: 26.1 ± 2.7 years) were included. The load-velocity relationship was extracted for all subjects individually. Mean concentric velocity (MCV), combined with sex, was used to develop equations predictive of relative load (% one repetition maximum [1RM]). Prediction accuracy was determined with the mean absolute percent error and Bland-Altman plots. Relative strength was similar between the sexes. However, male subjects exhibited faster concentric MCV at 1RM ( p < 0.05). Mean concentric velocity and the sex-by-MCV interaction were both significant predictors of %1RM ( p < 0.0001), explaining 89% of its variance. The absolute error was similar between the sexes (men: 9.4 ± 10.0; women: 8.4 ± 10.5, p > 0.05). The mean difference between actual and predicted %1RM in Bland-Altman analysis was nearly zero in both sexes and showed no heteroscedasticity. The limits of agreement in both men and women were of approximately ±15%. Taken together, it can be concluded that sex should be taken into consideration when aiming at accurate prescription of relative load based on movement velocity. Moreover, predicting relative load from MCV and sex provides an error of approximately 10% in assessments of relative load in groups of persons. Finally, when used for individual estimations, these equations may implicate a considerable deviation from the actual relative load, and this may limit their applicability to training conditions in which extreme accuracy is required (i.e., more advanced lifters and athletes).