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Introduction: Military personnel must manage a multitude of competing physiological and cognitive stressors while maintaining high levels of performance. Quantifying the external workload and cognitive demands of tactical military field exercises closely simulating operational environments, will provide a better understanding of stressors placed on personnel to inform evidence-based interventions., Methods: Thirty-one soldiers completing a dismounted 48 hours tactical field exercise, participated in the study. External workload was quantified using a wrist-worn triaxial accelerometer, with cognitive function (Go-/No-Go, N-back, psychomotor vigilance task and subjective workload ratings (NASA-TLX) assessed pre-exercise, mid-exercise and postexercise. Physical activity was described using Euclidian Norm Minus One (m g ), with moderate vigorous physical activity (MVPA) and sedentary light physical activity (SLPA) as ≥ or <113 m g , respectively. Changes in general cognitive performance (total accuracy-speed trade-off (ASTO) % change) and function outcome variables (overall mean reaction time, ASTO and number of correct and missed responses) were calculated for each assessment from pre-exercise, to mid-exercise and postexercise., Results: For the exercise duration (50:12±02:06 hh:mm) participants spent more time completing SLPA compared with MVPA (1932±234 vs 1074±194 min; p <0.001), equating to 33% of the time spent completing MVPA. Overall cognitive performance decreased over the exercise (pre-to-post: -249). However, the largest decrement was observed pre-to-mid (-168). Perceived mental demand associated with the cognitive assessments significantly increased over the duration of the exercise (pre-: 33; mid-: 38 and post-: 51; χ 2 F(2) = 26.7, p = <0.001, W=0.477) which could suggest that participants were able to attenuate a further decline in cognitive performance by investing more effort/mental resources when completing assessments., Conclusion: The study successfully quantified the physical activity, and subsequent impact on cognitive function, in soldiers completing a 48 hours tactical field exercise. Further research is needed to better understand how physiological stressors interact with cognitive function during military operations., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2024. No commercial re-use. See rights and permissions. Published by BMJ.)