Acute Effects Of Ruck March On Task Switching And Associated Brain Activity

Ruck marches consist of walking for extended periods carrying approximately 50% of body weight and are essential during military training and operations. Nevertheless, there is little research examining the impact of ruck marches on cognitive and brain function. Understanding how military-relevant fatigue impacts warfighter cognition and brain activity is necessary to characterize and predict performance decrements. PURPOSE: We investigate the acute effects of a loaded ruck march on task-switching performance and associated brain activity. METHODS:16 (1 woman) active-duty U.S. Army Soldiers performed a two-hour ruck march on a treadmill under loaded condition (RL) (carrying approximately 50% of their body weight) and unloaded condition (RUL). Heart Rate (HR), oxygen consumption (VO2), and rate of perceived exertion (RPE) were measured periodically during both rucks. Cognitive performance was measured by reaction time (RT) and associated brain activity (mixed block/event-related) on the prefrontal cortex was measured by the hemodynamic response function (HRF) using near-infrared spectroscopy (fNIRS) before and after each march. Two-way repeated-measures ANOVA was performed with load (RL vs. RUL) and time (PRE vs. POST) as within-participants factors. RESULTS: HR (BPM) (RL 142.9 ± 15.9 vs. RUL 118.1 ± 6.31), VO2 (ml/kg/min) (RL 21.6 ± 2.32 vs. RUL 16.9 ± 1.13), and RPE (RL 13.0 ± 3.5 vs. RUL 8.6 ± 2.3) were higher (p < 0.001) during RL than RUL. There were no differences (p > 0.1) in RT (ms) between conditions (POST_RL 0.82 ± 0.095 vs. POST_RUL 0.80 ± 0.90) and time points (PRE_RL 0.824 ± 0.12 vs. POST_RL 0.82 ± 0.095 and PRE_RUL 0.834 ± 0.10 vs. POST_RUL 0.80 ± 0.90). However, there was a main effect of time, condition, and interaction (p < 0.01) for brain activity. HRF (mmol) post-exercise increased for the RL (PRE_RL 8.24x10-6 ± 7.08x10-6 vs. POST_RL 1.49x10-5 ± 8.79x10-6) and did not change for the RUL (PRE_RUL 2.00x10-5 ± 1.34x10-5 vs. POST_RUL 1.86x10-5 ± 1.10x10-5). CONCLUSIONS: A two-hour loaded ruck march efficiently promoted higher exertion. Despite no differences in cognitive performance, the loaded march elevated brain activity in the PFC post-exercise. These results may have implications for neural efficiency theory in military personnel. ACKNOWLEDGED: APPROVED FOR PUBLIC RELEASE