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Introduction
Short-intervals high-intensity interval training (HIIT) consists of repeated brief (15-45s) exercise bouts at supra-maximal intensities, usually ranging between 100 and 120% of the minimal running velocity eliciting maximal oxygen uptake (vVO2max), with rest periods <1-2min.1 In endurance (e.g., middle/long distance runners and cyclists),2 team- (e.g., football)3 and racket-sport (e.g., tennis)4 athletes, HIIT is commonly used as a time-efficient intervention to enhance sport-specific aerobic capacity. While it is generally accepted that HIIT elicits improvements in cardio-respiratory function and oxidative capacity within the muscle tissue (i.e., increases in mitochondrial enzyme activity, capillarization and mitochondrial biogenesis),1 it is also crucial to carefully manage anaerobic glycolytic contribution and neuromuscular load in order to avoid overload and maximize HIIT adaptations.5 Although physiological responses resulting from the completion of one acute bout of HIIT have largely been described,1,2 there is comparatively less data about the nature of accompanying mechanical alterations.5
In one study assessing stride mechanical adjustments to HIIT, Di Michele et al.6 reported no impairment of the running pattern of high-level endurance runners (VO2max 67mLmin-1 kg-1 ) between the first and the second exercise bout of a HIIT routine that consisted of two identical strength/aerobic circuits (i.e., six 30-s dynamic or explosive strength exercises, alternated with 200-1000m runs at 90-95% of vVO2max) interspersed by 5min of recovery. This study, however, is associated with several limitations that may confound the main conclusions reached by the authors: (i) the use of relatively low exercise intensities/long intervals, with also no strict control of the running velocity that fluctuated between 90 and 95% of vVO2max, (ii) an indirect evaluation of spring-mass model (SMM) characteristics (i.e., optoelectronic system) for a restricted number of steps (i.e., 4-5 consecutive steps every 400-m lap), and (iii) implementation of a rather complex high-intensity circuit alternating strength and HIIT running exercises that does not allow the specific effects of the run to be determined. Understanding the acute mechanical alterations to HIIT is of practical importance for individualizing team-sport training programs in order to maximize physiological adaptations of players while minimizing injury risks.5
Deteriorated spatio-temporal stride and SMM characteristics during fatigued running have been reported for runs of a few minutes (i.e., intensity >95% of vVO2max),7,8...