Seasonal Changes in Running Performance Across K League 1 and K League 2 in 2025

OBJECTIVES This study examined seasonal changes in running performance among outfield players who completed 90 minutes in the 2025 K League 1 and K League 2 seasons and assessed whether seasonal trajectories differed between leagues.METHODS The season was divided into Phase 1 (rounds 1-10), Phase 2 (rounds 11-20), Phase 3 (rounds 21-30), and Final Phase (rounds 31-38/39). A total of 5,629 player-match observations from 565 playerteam clusters and 500 matches were analyzed. Outcomes were total distance, very-high-intensity running (VHIR) count and distance, sprint count and distance, and the high-intensity activity distance ratio (HIADR). Marginal linear models used two-way cluster-robust standard errors for player-team and match. Holm correction was applied to six League × Phase interaction tests and to 13 planned contrasts within each outcome.RESULTS Mean values for all six outcomes were lowest in Phase 3 and increased in Final Phase in each league. In both leagues, VHIR count, VHIR distance, and HIADR decreased from Phase 1 to Phase 3 and increased from Phase 3 to Final Phase (adjusted <i>p</i> < 0.001). No League × Phase interaction was significant after Holm correction (adjusted <i>p</i> ≥ 0.249). The only significant adjusted between-league contrast was lower HIADR in K League 2 during Phase 2 (difference = −0.616 percentage points, 95% CI [−1.049, −0.183], adjusted <i>p</i> = 0.042). Between-league differences in changes from Phase 3 to Final Phase were not significant (adjusted <i>p</i> ≥ 0.151).CONCLUSIONS In both leagues, running performance was lowest in Phase 3 and rebounded in Final Phase, most consistently in VHIR and HIADR. Evidence was insufficient to establish different seasonal trajectories between leagues; This between-league inference was sensitive to the clustering structure. The leagueand phase-specific means provide practical references for monitoring high-intensity activity in full-match outfield players, particularly in Phase 3.