INTRODUCTION: Interval training sets may be applied in a varied sequence within a swimming session. The aim of this study was to investigate the effect of different sequences of training sets on performance and physiological responses in a training session. METHODS: Twelve swimmers (18.7±3.4 years) performed a 50 m sprint to evaluate maximal speed. On a subsequent day they completed an incremental intermittent protocol (5x200 m) to calculate the speed corresponding to the lactate threshold (sLT). Based on the preliminary tests, training sets of 8x200 m at speed corresponding to sLT (set A - heavy) and 8x50 m at 95% of the maximal speed (set B - extreme) were planned, both including 30 s of recovery between repetitions. In two randomized order sessions, swimmers completed the following set sequences: i) set A - set B (A-B), ii) set B - set A (B-A), with 10 min of passive recovery between sets. The time to complete each repetition and heart rate (HR) were recorded. Lactate concentration [La-] was determined at the start, middle and end of each set and the mean [La-] was calculated. Blood pH, base excess (BE) and bicarbonate (HCO3) were determined at the start and end of each set. Plasma volume changes (dPV) after each training set were calculated based on hematocrit and hemoglobin changes. R-R intervals were recorded the night after each session and heart rate variability was calculated. The logarithm of root mean square successive difference (LnRMSSD) was used to examine the effect of the entire session on the autonomic nervous system. Individualized training impulse (TRIMPi) was also determined using individual HR and lactate profiles determined during the incremental protocol. RESULTS: Performance in each set separately was not different between sessions regardless of sets sequence (p>0.05). Mean HR and mean [La-] were not different between sessions (p>0.05). However, mean pH of the entire A-B session was higher compared to the reverse sequence, while the reduction on BE and HCO3 was greater in B-A compared to A-B session (pH: A-B: 7.35 ± 0.04, B-A: 7.31 ± 0.04, BE: A-B: -5.1 ± 3.6, B-A: -8.4 ± 2.5 mmol/L, HCO3, A-B: 20.5 ± 2.9, B-A:17.9 ± 1.9 mmol/L, p<0.05). Set A or set B separately showed similar physiological response independent of the sequence they were applied (p>0.05). dPV differed between sessions (A-B: -4.9 ± 7.7, B-A: -0.9 ± 7.2%, p<0.05), was decreased after set A (-3.6 ± 9.0%) in A-B session but was increased in B-A session (9.4 ± 8.9%, p<0.05). However, dPV after set B was not affected by the sequence applied (p>0.05). Nocturnal LnRMSSD was not different between sessions (p>0.05) while TRIMPi was increased in B-A session (A-B: 66.8 ± 36.0, B-A: 82.5 ± 30.0 a.u., p<0.05). DISCUSSION: Training sets sequence does not affect performance and physiological responses in each set separately. However, the overall effect of training session consisting of a near-maximum intensity set followed by a set at lactate threshold leads to a greater metabolic and acid-base disturbance and induce higher internal training load compared to the reverse order.
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