The purpose of this study was to investigate the involvement of neutrophil dynamics and function in exercise-induced muscle damage (EIMD) and delayed-onset muscle soreness (DOMS), and the effect of molecular hydrogen (H₂) intake on these parameters. Nine healthy and active young men performed H₂ and placebo bath trial in a crossover design. They carried out downhill running (−8% slope) for 30 min at a speed corresponding to 75~85% of peak oxygen uptake (VO₂peak). Subsequently, they repeated bathing for 20 min per day for one week. Degree of muscle soreness (visual analogue scale: VAS), peripheral leukocyte counts, neutrophil dynamics and function, muscle damage, and inflammation markers were measured. Plasma interleukin (IL)-6 concentration was significantly correlated with peripheral neutrophil count, VAS, and serum creatine kinase activity, respectively, after downhill running. Peripheral neutrophil count and serum myoglobin concentration were also significantly correlated. Conversely, there were no effects of H₂ bath. These results suggest that IL-6 may be involved in the mobilization of neutrophils into the peripheral blood and subsequent EIMD and DOMS after downhill running; however, it is not likely that H₂ bath is effective for the inflammatory process that is centered on neutrophils after downhill running.

Background: Estimating race times for open-water swimming based on pool swimming times could be useful for talent identification and training optimisation. We aimed to compare the swimming speeds of the world’s top and other swimmers in the 2023 Aquatics Championship men’s 10 km OWS race. Methods: Sixty-five swimmers were divided into four groups: G1 (1st–10th positions), G2 (11st–30th positions), G3 (31st–47th positions), and G4 (48th–65th positions). Swimming speed, stroke frequency (SF), and stroke length (SL) for each lap (laps 1–6) were recorded. Critical speed (CS) was calculated from each participant’s personal best times in the 400, 800, and 1500 m freestyle events in the pool. Swimming speed against CS was calculated (%CS). Results: The top performance group (G1) maintained their swimming speed from beginning (lap 1, 1.53 m/s) to end (lap 6, 1.50 m/s), at 92.7 ± 1.9% of CS, characterised by longer SL (1.26 m) and lower SF (72.86 rpm). G3 and G4 were unable to maintain their swimming speed, which decreased from G3: 97.64 ± 1.62% and G4: 96.10 ± 1.96% of CS at lap 1 to G3: 88.39 ± 3.78% and G4: 85.13 ± 5.04% at lap 6. This reduction in swimming speed is consistent with the increased reliance on anaerobic metabolism reported in previous studies under similar conditions. Conclusions: Race pacing for maintaining speeds of 92%CS throughout the race could be an important resilient index in open-water swimming. %CS might be a useful index for estimating the athletic performance level in open-water swimming.