BACKGROUND: Recovery duration during sprint interval training may influence metabolic stress, fatigue, and performance. However, its effects on energy system contributions during swimming remain unclear. This study aimed to compare the effects of 2- vs. 4-min recovery intervals during a 4 x 50 m (S4×50) sprint interval training on energy system contributions and performance parameters in competitive swimmers. METHODS: Thirteen swimmers completed two conditions in randomized order: S4×50 with 2-min (S2min) and 4-min (S4min) recovery intervals. Oxygen uptake was assessed using backward extrapolation, and blood lactate was measured to estimate aerobic and anaerobic contributions. Mean speed (vMEAN) and performance decrement (PD) were also calculated. RESULTS: vMEAN was higher (~0.91%, p = 0.008) and PD was lower (~49%, p =0.005) in S4min compared to S2min. No significant differences were observed in absolute or relative energy system contributions between conditions. However, vMEAN in both conditions was significantly associated with both anaerobic pathways, with the glycolytic contribution being the main predictor (R² = 0.67-0.89). Notably, only the S4min condition showed a significant association of performance with phosphagen pathway contribution (r = 0.64, p = 0.020). In S2min, PD was correlated with blood lactate (r = 0.61, p = 0.02). CONCLUSIONS: Longer recovery intervals (S4min) allowed a lower PD without altering overall energy system contributions, whereas shorter intervals (S2min) increased metabolic fatigue and PD. These findings highlight recovery duration as a key variable for modulating fatigue and prescribing sprint interval training in swimming. However, although recovery appears to be a determining factor in performance decrement, increasing the interval did not affect the metabolic contribution of the efforts.
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