Zacca, R, Fernandes, RJP, Pyne, DB, and Castro, FAdS. Swimming training assessment: the critical velocity and the 400-m test for age-group swimmers. J Strength Cond Res 30(5): 1365-1372, 2016 - To verify the metabolic responses of oxygen consumption (Vo 2), heart rate (HR), blood lactate concentrations [La], and rate of perceived exertion (RPE) when swimming at an intensity corresponding to the critical velocity (CV) assessed by a 4-parameter model (CV 4par), and to check the reliability when using only a single 400-m maximal front crawl bout (T 400) for CV 4par assessment in age-group swimmers. Ten age-group swimmers (14-16 years old) performed 50-, 100-, 200-, 400- (T 400), 800-, and 1,500-m maximal front crawl bouts to calculate CV 4par. Vo 2, HR, [La], and RPE were measured immediately after bouts. Swimmers then performed 3 × 10-minute front crawl (45 seconds rest) at CV 4par. Vo 2, HR, [La], and RPE were measured after 10 minutes of rest (Rest), warm-up (Pre), each 10-minute repetition, and at the end of the test (Post). CV 4par was 1.33 ± 0.08 m·s -1. Vo 2, HR, [La], and RPE were similar between first 10-minute and Post time points in the 3 × 10-minute protocol. CV 4par was equivalent to 92 ± 2% of the mean swimming speed of T 400 (v400) for these swimmers. CV 4par calculated through a single T 400 (92%v400) showed excellent agreement (r 0.30; 95% CI: -0.04 to 0.05 m·s -1, p 0.39), low coefficient of variation (2%), and root mean square error of 0.02 ± 0.01 m·s -1 when plotted against CV 4par assessed through a 4-parameter model. These results generated the equation CV 4par 0.92 × v400. A single T 400 can be used reliably to estimate the CV 4par typically derived with 6 efforts in age-group swimmers.