High Intensity Exercise Compromises Prefrontal Cortex Oxygenation and Executive Function in Reserved Officer Training Corps Cadets

Abstract

Military performance often depends on high level cognition specifically, executive function (EF) while simultaneously performing strenuous exercise. Studies of EF during exercise have provided varied results (enhanced vs diminished function). Similarly, cognitive change models suggest that changes in EF (positive or negative) during exercise stem from a reallocation of substrates (e.g. blood flow, O2) from the prefrontal cortex (PFC). PFC oxygenation increases during low and decreases at high exercise intensities. Thus, understanding PFC oxygenation responses during concurrent EF and exercise provides insight into EF and exercise performance which is applicable to the military.PurposeTo determine PFC oxygenation and EF performance over a range of exercise intensities.Methods15 Reserve Officer Training Corp (ROTC) cadets (19.6 ± 2 yrs, 5 women) completed 4 visits separated by ≥24hrs. Visit 1) familiarization trial 2) treadmill maximal graded exercise test (GxT): 3.5 mph, 0% grade increasing 2% every 2‐min until 16%, at which speed increased by 0.5 mph every 2‐min until exhaustion. Visits 3) & 4) Executive function and exercise test (EFET, data from visits 3 & 4 average into a singular response). EFET was combined comprehensive iPad based EF test (Cedar Operator Workload Assessment Tool, fixed to the treadmill) and GxT. Heart rate (HR), respiration rate (RR), arterial blood saturation (SpO2) (Equivital Life monitoring system) and PFC oxygenation (Near infrared spectroscopy, NIRS) were continuously recorded. The EF score for each intensity was analyzed for accuracy of correct responses and termed % Hit Rate = correct responses, (correct responses+ false positives + incorrect response). Heart Rate Reserve was calculated to normalize exercise intensity (%HRR). NIRS variables, oxygenated hemoglobin (O2Hb), deoxyhemoglobin (HHb), and total hemoglobin (tHb) were used to calculate Tissue Saturation Index (TSI).ResultsData are mean ± SD, effect size Ω2. During the EFET, % Hit Rate was unchanged from baseline until 70% HRR (99.0 ± 7.0 vs 85.0 ± 6.0 %, p < 0.01) where it steadily declined with intensity to a nadir at 100% HRR (51± 20 %, p < 0.01). TSI followed a similar pattern to Hit Rate, only declining ≥70%HRR and did so at a greater rate during EFET compared to GxT (p<0.05, Ω2=0.02), reaching a nadir in both conditions at 100% HRR (59.3 ± 2.0 vs. 61.7 ± 3.5 %, p<0.05). Other variables, RR, SpO2, and HR did not differ in response in the EFET compared to GxT (p>0.05).ConclusionEF is only affected during high intensity exercise (above 70%HRR), where it declines linearly to maximal exercise. This is mirrored by PFC oxygenation, declining 3 70%HRR and to a greater extent than just exercise alone (vs GxT) at very high intensity. Systemic variables are not different between GxT and EFET. Therefore, EF decline is dependent on exercise intensity, as is TSI. These data suggest reductions in EF during high intensity exercise are at least in part related to attenuated PFC oxygenation.