Objectives To examine mechanical alterations during interval-training treadmill runs in high-level team-sport players. Design Within-participants repeated measures. Methods Twenty high-level male field-hockey players performed six 30-s runs at 5.53 ± 0.19 m s−1 corresponding to 115% of their velocity associated with maximal oxygen uptake (vVO2max) with 30-s passive recovery on an instrumented treadmill. Continuous measurement of running kinetics/kinematics and spring-mass characteristics were performed and values were subsequently averaged over 20 s (8th–28th s) for comparison. Results Contact time (+1.1 ± 4.3%; p = 0.044), aerial time (+4.1 ± 5.3%; p = 0.001), step length (+2.4 ± 2.2%; p < 0.001) along with mean loading rates (+7.1 ± 10.6%; p = 0.026) increased from the first to the last interval, whereas step frequency (−2.3 ± 2.1%; p < 0.001) decreased. Both centre of mass vertical displacement (+3.0 ± 6.0%; p < 0.001) and leg compression (+2.8 ± 9.7%; p = 0.036), but not peak vertical forces (0.0 ± 4.1%; p = 0.761), increased with fatigue. Vertical stiffness decreased (−2.8 ± 6.9%; p = 0.012), whereas leg stiffness did not change across intervals (p = 0.149). Conclusions During interval-training treadmill runs, high-level team-sport players modified their mechanical behaviour towards lower vertical stiffness while preserving a constant leg stiffness. Maintenance of running velocity induced longer step lengths and decreased step frequencies that were also accompanied by increased impact loading rates. These mechanical alterations occurred early during the set.