This study characterized the impact of high-intensity interval training on the kinetics of blood lactate and performance in trained boys and men. Twenty-one boys (11.4 ± 0.8 years) and 19 men (29.4 ± 5.0 years) performed a set of four 30-s sprints with 2-min of rest and a single 30-s sprint on 2 separate occasions (randomized order) with assessment of performance. Blood lactate was assayed after each sprint and during 30 min of recovery from both tests. The individual time-curves of blood lactate concentration were fitted to the biexponential function as [LA](t) = [LA](0) + A1(1-e^y1t) + A2(1-e^y2t), where the velocity parameters y1 and y2 reflect the capacity to release lactate from the previously active muscle into the blood and to subsequently eliminate lactate from the organism, respectively. In both tests, peak blood lactate concentration was significantly lower in the boys (four 30-s sprints: 12.2 ± 3.6 mmol/L; single 30-s sprint: 8.7 ± 1.8 mmol/L) than men (four 30-s sprints: 16.1 ± 3.3 mmol/L; single 30-s sprint: 11.5 ± 2.1; p < 0.001). The boys exhibited faster y1 (1.4531 ± 0.65 min; p < 0.001) and y2 (0.059 ± 0.023 min; p = 0.01) in the single 30-s sprint and faster y2 (0.049 ± 0.016 min; p = 0.01) in the four 30-s sprints. The worsening of performance from the first to the last of the four 30-s sprints was less pronounced in boys (9.2% ± 13.9%) than men (19.2% ± 11.5%; p = 0.01). In the present study boys, when compared with men, exhibited lower Peak blood lactate concentration; less pronounced decline in performance during the sprints concomitantly with more rapid release and elimination during the single 30-s sprint; and faster elimination of lactate following the four 30-s sprints.
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