Altered Sympathoadrenal Activity Following Cold-Water Diving.

INTRODUCTION: Little data exist on the effect of extremely cold-water diving on thermo-metabolic hormone secretion. Moreover, the impact of repetitive dives on the stress response is unknown. The purpose of this study was to determine the effects of two daily bouts of cold-water diving on the hormonal and metabolic profile of elite military personnel and to measure the stress response. METHODS: Healthy, male, Norwegian Special Forces operators (n = 5) volunteered for this study. Physiological and hormone data were analyzed prior to and following twice-daily Arctic dives (3.3°C). RESULTS: Core temperature was maintained (p > .05), whereas skin temperature was significantly reduced over the course of each dive (p < .01). Pairwise comparisons revealed adrenocorticotropic hormone (ACTH) and cortisol concentration significantly decreased across both dives and days (p < .001). Adrenaline and noradrenaline significantly increased across both time and day (p < .001). Leptin, testosterone, and IGF-1 significantly decreased over time but recovered between days. CONCLUSION: The main findings of this effort are that there is a rapid sympathetic-adreno-medullary (SAM/SNS) response to cold-water diving and a suppression of the hypothalamic-pituitary-adrenal (HPA) axis and hormones related to repair and recovery. While the sample size was too small to determine the role of SAM/SNS, HPA, and thyroid hormone effect on thermoregulation, it addresses a gap in our understanding of physiological adaptions that occurs in extreme environments.

Efficacy of closed cell wet-suit at various depths and gas mixtures for thermoprotection during military training dives.

Purpose: To evaluate a closed-cell wet-suit for thermal protective capability during extreme cold water exposure at various depths. Methods: Thirteen (n = 13) elite military divers who were tasked with cold-water training, participated in this study. To mimic various depths, the Ocean Simulation Facility (OSF) at the Navy Experimental Diving Unit (NEDU) was pressurized to simulate dive depths of 30, 50, and 75fsw. Water temperature remained at 1.8-2.0°C for all dives. Four divers dove each day and used the MK16 underwater breathing apparatus with gas mixes of either N202 (79:21) or HeO2 (88:12). Mean skin temperature (TSK) (Ramanathan, 1964), core temperature (Tc), hand and foot readings were obtained every 30 min for 30 and 50fsw and every 15 min during the 75fsw dive. Results: TC was significantly reduced across all dives (p = 0.004); however, was preserved above the threshold for hypothermia (post dive Tc = 36.5 ± 0.4). There was no effect of gas mix on TC. TSK significantly decreased (p < 0.001) across all dives independent of depth and gas. Hand and foot temperatures resulted in the termination of three of the dives. There were no significant main effects for depth or gas, but there were significant main effects for time on hand temperature (p < 0.001) and foot temperature (p < 0.001). Conclusion: Core temperature is maintained above threshold for hypothermia. Variatioins in TC and TSK are a function of dive duration independent of depth or gas for a closed-cell wet-suit in cold water at various depths. However, both hand and foot temperatures reached values at which dexterity is compromised.

Exercise training with blood flow restriction has little effect on muscular strength and does not change IGF-1 in fit military warfighters.

OBJECTIVE: Aerobic exercise with blood flow restriction (aBFR) has been proposed as an adjunctive modality in numerous populations, potentially via an enhanced growth factor response. However, the effects of aBFR on highly trained warfighters have yet to be examined. The purpose of this study was to determine if adjunctive aBFR as part of a regular physical training regimen would increase markers of aerobic fitness and muscle strength in elite warfighters. In addition, we sought to determine whether the changes in blood lactate concentration induced by aBFR would be associated with alterations in the insulin-like growth factor (IGF) axis. DESIGN: Active-duty US Naval Special Warfare Operators (n=18, age=36.8 ± 2.2 years, weight=89.1 ± 1.2 kg, height=181.5 ± 1.4 cm) from Naval Amphibious Base Coronado were recruited to participate in 20 days of adjunctive aBFR training. Peak oxygen consumption (VO2 peak), ventilatory threshold (VT), and 1-repetition max (1-RM) bench press and squat were assessed pre- and post-aBFR training. Blood lactate and plasma IGF-1 and IGF-binding protein-3 (IGFBP-3) were assessed pre-, 2 min post-, and 30 min post-aBFR on days 1, 9, and 20 of aBFR training. RESULTS: Following aBFR training there were no changes in VO2 peak or VT, but there was an increase in the 1-RM for the bench press and the squat (5.0 and 3.9%, respectively, P<0.05). Blood lactate concentration at the 2-min post-exercise time point was 4.5-7.2-fold higher than pre-exercise levels on all days (P<0.001). At the 30-min post-exercise time point, blood lactate was still 1.6-2.6-fold higher than pre-exercise levels (P<0.001), but had decreased by 49-56% from the 2-min post-exercise time point (P<0.001). Plasma IGF-1 concentrations did not change over the course of the study. On day 9, plasma IGFBP-3 concentration was 4-22% lower than on day 1 (P<0.01) and 22% lower on day 9 than on day 20 at the 30-min post-exercise time point (P<0.001). CONCLUSIONS: Our data suggest that aBFR training does not lead to practical strength adaptations or alterations in the IGF axis in a population of highly trained warfighters.

DEET insect repellent: effects on thermoregulatory sweating and physiological strain.

Insect repellents (e.g. N,N-diethyl-m-toluamide or DEET) applied to the skin can potentially interfere with sweat production and evaporation, thus increasing physiological strain during exercise-heat stress. The purpose was to determine the impact of 33% DEET lotion on sweating responses, whole body thermoregulation and thermal sensation during walking exercise in the heat. Nine volunteers (2 females, 7 males; 22.1 ± 4.9 years; 176.4 ± 10.0 cm; 79.9 ± 12.9 kg) completed 5 days of heat acclimation (45°C, 20% rh; 545 watts; 100 min/day) and performed three trials: control (CON); DEET applied to forearm (DEET(LOC), 12 cm(2)); and DEET applied to ~13% body surface area (DEET(WB),). Trials consisted of 30 min walking (645 watts) in 40°C, 20% rh environment. Local sweat rate (SR), onset and skin wettedness were measured in DEET(LOC), and heart rate (HR), rectal temperature (T (re)), skin temperature (T (sk)), RPE, and thermal sensations (TS) were measured during DEET(WB). No differences (p > 0.05) were observed between DEET(LOC) versus CON, respectively, for steady state SR (1.89 ± 0.44 vs. 2.09 ± 0.84 mg/cm(2)/min), SR area under the curve (46.9 ± 11.7 vs. 55.0 ± 20.8 mg/cm(2)), sweating onset, or skin wettedness. There were no differences (p > 0.05) in HR, T (re), T (sk), Physiological Strain Index, RPE or TS between DEET(WB) versus CON. DEET did not impact measures of local forearm sweating and when applied according to military doctrine, did not adversely impact physiological responses during exercise-heat stress. DEET can be safely worn during military, occupational and recreational activities in hot, insect infested environments.

Prior heat stress: effect on subsequent 15-min time trial performance in the heat.

UNLABELLED: The impact of prior heat stress on subsequent aerobic exercise-heat performance has not been studied. PURPOSE: To determine whether prior heat stress degrades subsequent aerobic exercise performance in the heat. METHODS: Eighteen nonheat acclimated males were trained (four practice trials) on an aerobic exercise performance test in 22 degrees C and then divided into two (n = 8) groups. One group (EUHPH; (.)VO2peak = 44 +/- 7 mL x kg x min(-1)) was tested after 90 min of recovery (in 22 degrees C) from 3 h of intermittent light-intensity (<30% (.)VO2peak) exercise-heat (50 degrees C) stress, where sweat losses were matched with fluid intake (3.5 +/- 0.5 L) to maintain euhydration. The other group (EUH; (.)VO2peak = 45 +/- 5 mL x kg x min(-1)) was tested while euhydrated without prior exercise-heat stress. Aerobic performance was determined from a 30-min cycling preload (50% (.)VO2peak) followed by a 15-min time trial in 40 degrees C. Total work during the 15-min performance time trial in EUH and EUHPH was compared, as were the percent changes from the best practice trials. RESULTS: Volunteers were euhydrated (plasma osmolality < 290 mOsm x kg(-1)) and normothermic before each exercise-heat trial. Heart rate and core temperature were not different (P > 0.05) between groups at any time point during exercise. Total work was not different (P > 0.05) at baseline or between EUH (150.5 +/- 28.3 kJ; 2.0 +/- 0.3 kJ x kg(-1)) and EUHPH (160.3 +/- 24.0 kJ; 1.8 +/- 0.2 kJ x kg(-1)). The percent change in total work relative to baseline was not different (P > 0.05) between EUH (-18.7% +/- 9.2%) and EUHPH (-15.0% +/- 7.8%). CONCLUSIONS: If hydration and body temperatures recover, prior exercise-heat stress does not result in a greater degradation in aerobic time trial performance in the heat compared with heat exposure alone.