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Mechanical Alterations to Repeated Treadmill Sprints in Normobaric Hypoxia

Abstract : Compelling evidence suggests larger performance decrements during hypoxic versus normoxic repeated sprinting. Yet the underlying mechanical alterations have not been thoroughly investigated. Therefore, we examined the effects of different levels of normobaric hypoxia on running mechanical performance during repeated treadmill sprinting. Methods: Thirteen team sport athletes performed eight 5-s sprints with 25 s of passive recovery on an instrumented treadmill in either normoxia near sea level (SL; FiO 2 = 20.9%), moderate normobaric hypoxia (MH; FiO 2 = 16.8%; corresponding to ~1800 m altitude), or severe normobaric hypoxia (SH; FiO 2 = 13.3%; ~3600 m). Results: Net power output in the horizontal direction did not differ (P 9 0.05) between conditions for the first sprint (mean T SD, pooled values: 13.09 T 1.97 WIkg j1) but was lower for the eight sprints in SH compared with SL (j7.3% T 5.5%, P G 0.001) and MH (j7.1% T 5.9%, P G 0.01), with no difference between SL and MH (+0.1% T 8.0%, P = 1.00). Sprint decrement score was similar between conditions (pooled values: j11.4% T 7.9%, P = 0.49). Mean vertical, horizontal, and resultant ground reaction forces decreased (P G 0.001) from the first to the last repetition in all conditions (pooled values: j2.4% T 1.9%, j8.6% T 6.5%, and j2.4% T 1.9%). This was further accompanied by larger kinematic (mainly contact time: +4.0% T 2.9%, P G 0.001, and +3.3% T 3.6%, P G 0.05, respectively; stride frequency: j2.3% T 2.0%, P G 0.01, and j2.3% T 2.8%, P G 0.05, respectively) and spring-mass characteristics (mainly vertical stiffness: j6.0% T 3.9% and j5.1% T 5.7%, respectively, P G 0.01) fatigue-induced changes in SH compared with SL and MH. Conclusion: In SH, impairments in repeated sprint ability and in associated kinetics/kinematics and spring-mass characteristics exceed those observed near SL and in MH (i.e., no or minimal difference). Specifically, SH accentuates the repeated sprint ability fatigue-related inability to effectively apply forward-oriented ground reaction force and to maintain vertical stiffness and stride frequency.
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Franck Brocherie, Grégoire P. Millet, Jean-Benoît Morin, Olivier Girard. Mechanical Alterations to Repeated Treadmill Sprints in Normobaric Hypoxia. Medicine and Science in Sports and Exercise, American College of Sports Medicine (ACSM), 2016, 48 (8), pp.1570-1579. ⟨10.1249/MSS.0000000000000937⟩. ⟨hal-01628542⟩

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