Repeated sprint training induces prolonged residual fatigue compared to other high-intensity interval training modalities in middle-distance runners

This study aimed to monitor the kinetics of neuromuscular fatigue and inflammation indices in middle-distance runners following exposure to three different high-intensity interval training (HIIT) modalities: short-interval HIIT, long-interval HIIT, and repeated sprint training (RST). A crossover repeated-measures design was used involving 33 male middle-distance runners (19.6 ± 2.3 years) who completed short-interval HIIT, longinterval HIIT, and RST. Neuromuscular performance was assessed using the countermovement jump (CMJ) and isometric mid-thigh pull (IMTP), while inflammatory (salivary IL-6) and perceptual markers (delayed onset muscle soreness, DOMS and perceived recovery scale, PRS) were measured at rest, immediately post-exercise, and at 24 and 48 hours post-exercise to evaluate changes over time. All variables showed significant main effects of time and modality, as well as interactions (p < 0.001). Immediately post-exercise, RST consistently induced the greatest acute fatigue, evidenced by larger declines in CMJ (9.6%) and IMTP (6.7%), an increase in IL-6, and a 56.8% drop in PRS. At 24 hours post-exercise, RST still showed significantly reduced performance (CMJ and IMTP) and elevated inflammation (IL-6) compared to short and long-HIIT. DOMS peaked at 24 hours in the RST group, which also reported lower perceived recovery. By 48 hours, performance and inflammatory markers largely returned to baseline across all groups. However, RST continued to show higher DOMS and lower PRS than both short and long-HIIT (p < 0.001). These findings suggest that RST induces a more pronounced and prolonged recovery period compared to both short- and long-interval HIIT. Coaches should consider that recovery following RST sessions may require up to at least 48 hours, which is longer than for short- or long-interval HIIT.