This study examined the effects of two different intensities of running between repeated sprints and compared them with passive recovery. Thirteen professional soccer players performed two sets of six 30 m sprints on three randomly assigned occasions. A 5 min passive rest period separated the two sets, while sprints were interspersed with either passive standing, running at 95% of the first lactate threshold (MOD) and running at maximum aerobic speed (HIGH). Performance decrements were greater in HIGH than MOD at the last sprint in both sets (set 1: 5.8 ± 4.2% vs. 2.6 ± 3.2%, p = 0.07; set 2: 9.1 ± 4.5% vs. 4.0 ± 6.1%, p = 0.016). Acceleration (0-15 m) was more affected than maximal-speed running (15-30 m) (condition × sprint interaction: p < 0.001). Mean and peak heart rate were higher in both running conditions than passive (p < 0.05), with no difference between MOD and HIGH. Blood lactate showed a significant set × condition interaction (p < 0.001), peaking at 13.6 ± 2.7 mmol·L-1 in HIGH, while blood lactate responses to passive and MOD were similar and peaked after the second set of sprints (10.7 ± 2.1 and 11.5 ± 2.8 mmol·L-1, respectively). Between-sprint running intensity markedly influenced fatigue development during repeated-sprint exercise. The HIGH condition elicited greater metabolic strain and performance decrements than MOD or passive conditions. Within the present protocol, passive standing was associated with smaller decrements in repeated-sprint performance despite high heart rate and blood lactate responses.
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