BACKGROUND: The loads tactical athletes must carry have been shown to alter physical and cognitive performance, increase thermal load, and may induce dehydration. Decrements in performance could negatively influence mission outcomes. While load carriage and hydration are known to influence performance, they are rarely studied together in a controlled manner. The purpose of this study was to assess how a ruck march (RM) alters hydration, physical and cognitive performance. METHODS: 9 adults with load carriage experience volunteered for the study (m=6, f=3, age=33.2±7.1yrs, height=172.5±9.31cm, mass=75.7±18.2kg). Participants’ body composition and VO2max were assessed at least 2 days prior to the RM. Participants completed a 9.65-km RM with a 20kg ruck sack on a treadmill at 4.83-km/h in a temperature controlled laboratory(22.2 degrees C). Hydration, physical and cognitive performance were tested pre-, mid-, and post-RM. Hydration was tested via Urine Specific Gravity (USG). No water intake was allowed. Physical performance was tested via vertical countermovement jump, isometric mid-thigh pull (IMTP), and postural stability during a mental math task on force plates sampling at 1000Hz. Cognitive performance was assessed via psychomotor vigilance task reaction time. Wilcoxon signed ranks tests with Bonferroni correction were used to analyze changes between time points. Wilcox effect sizes (ES) were calculated. Alpha level was set a priori at 0.05. RESULTS: Participants were physically fit with VO2max of 48.1±7.5ml/kg/min, body composition of 15.7±4.7 percent body fat, and IMTP Peak Relative Force of 33.6±4.3N/kg. USG significantly increased from pre- to post-RM (p=0.012, ES=0.889) and mid- to post-RM (p=0.012, ES=0.889), indicating dehydration. There were no significant differences in physical measures, ranging from static postural stability to maximal dynamic and static strength, or cognitive measure of reaction time.Conclusion: A RM in a controlled environment induced dehydration but, contrary to previous research, had no effect on physical and cognitive performance. The participants in the study were physically fit and may not be representative of all initial entry soldiers. Future research should incorporate a more diverse sample and utilize regression models to more accurately detect the potential moderating role of physical fitness on performance fatigue under mild dehydration.

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