Early post-exercise physiological and perceptual responses following different sprint interval training...

Sprint interval training (SIT) is widely used in competitive swimming, yet early post-exercise physiological and perceptual responses to different SIT configurations remain incompletely understood. This study compared in highly trained national-level middle- and long-distance swimmers. This study compared early post-exercise rating of perceived exertion (RPE), heart rate (HR), and blood lactate concentration (BLa) responses following four composite SIT configurations in national-level competitive middle- and long-distance freestyle swimmers.

Fifteen highly trained national-level swimmers participated in this randomized crossover study. Each participant completed four SIT configurations in randomized order, with a 7-day washout period between sessions: 12 × 25 m with 1-min recovery, 10 × 35 m with 80-s recovery, 8 × 50 m with 2-min recovery, and 6 × 75 m with 3-min recovery. The work-to-rest ratio was maintained at 1:4 across configurations. Rating of perceived exertion (RPE), heart rate (HR), and blood lactate concentration (BLa) were measured before exercise and immediately post-exercise and at 1, 3, 5, 7, and 9 min of recovery. Baseline-adjusted linear mixed-effects models were used to examine protocol, time, and protocol × time effects, with post-exercise time treated as a categorical factor.

After adjustment for session-specific baseline values, significant main effects of protocol and time were observed for RPE, HR, and BLa (all p < 0.001). The protocol × time interaction did not reach statistical significance for RPE (p = 0.051) or HR (p = 0.209), A significant protocol × time interaction was observed for BLa (p = 0.005). BLa remained lower after SIT-1 than after SIT-2, SIT-3, and SIT-4 throughout recovery and decreased from 14.47 mmol·L^-1 immediately post-exercise to 9.96 mmol·L^-1 at 9 min, whereas BLa remained comparatively elevated after SIT-2 and SIT-3.

Among the four composite configurations, SIT-1 (12 × 25 m) produced the lowest early-recovery BLa, whereas SIT-2 (10 × 35 m) elicited the highest overall RPE and HR and, together with SIT-3, maintained the highest BLa. Coaches should therefore select the complete SIT set structure according to the desired acute metabolic and perceptual load and subsequent within-session recovery requirements, rather than considering repetition distance alone.