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.

Performance of a commercial multi-sensor wearable (Fitbit Charge HR) in measuring physical activity and sleep in healthy children.

PURPOSE: This study sought to assess the performance of the Fitbit Charge HR, a consumer-level multi-sensor activity tracker, to measure physical activity and sleep in children. METHODS: 59 healthy boys and girls aged 9-11 years old wore a Fitbit Charge HR, and accuracy of physical activity measures were evaluated relative to research-grade measures taken during a combination of 14 standardized laboratory- and field-based assessments of sitting, stationary cycling, treadmill walking or jogging, stair walking, outdoor walking, and agility drills. Accuracy of sleep measures were evaluated relative to polysomnography (PSG) in 26 boys and girls during an at-home unattended PSG overnight recording. The primary analyses included assessment of the agreement (biases) between measures using the Bland-Altman method, and epoch-by-epoch (EBE) analyses on a minute-by-minute basis. RESULTS: Fitbit Charge HR underestimated steps (~11.8 steps per minute), heart rate (~3.58 bpm), and metabolic equivalents (~0.55 METs per minute) and overestimated energy expenditure (~0.34 kcal per minute) relative to research-grade measures (p< 0.05). The device showed an overall accuracy of 84.8% for classifying moderate and vigorous physical activity (MVPA) and sedentary and light physical activity (SLPA) (sensitivity MVPA: 85.4%; specificity SLPA: 83.1%). Mean estimates of bias for measuring total sleep time, wake after sleep onset, and heart rate during sleep were 14 min, 9 min, and 1.06 bpm, respectively, with 95.8% sensitivity in classifying sleep and 56.3% specificity in classifying wake epochs. CONCLUSIONS: Fitbit Charge HR had adequate sensitivity in classifying moderate and vigorous intensity physical activity and sleep, but had limitations in detecting wake, and was more accurate in detecting heart rate during sleep than during exercise, in healthy children. Further research is needed to understand potential challenges and limitations of these consumer devices.

Evaluation of Three Field Rewarming Techniques During Cold Weather Military Training.

INTRODUCTION: The purpose of this work was to evaluate the effectiveness of 3 rewarming techniques to determine how warfighters, and perhaps other populations in wilderness environments, should prioritize field rewarming options after a brief accidental immersion in cold water. METHODS: As part of a cold weather military training exercise, 31 military personnel (mean±SD age: 26±5 y, height: 180±10 cm, weight: 83.2±10.9 kg) completed a 10-min immersion in cold (0°C) water and subsequently rewarmed for 60 min using 3 different field rewarming techniques (sleeping bag, sleeping bag + warm fluids, or exercise). Heart rate, core and skin temperatures, thermal and shivering sensations, and manual dexterity (intravenous setup and insertion) were measured during the training exercise. RESULTS: Cold water immersion decreased core temperature (pre: 37.4±0.4; post: 36.4±1.0°C; P<0.001) and mean skin temperature (pre: 27.9±1.3; post: 15.6±1.8°C; P<0.001) and impaired manual dexterity (intravenous insertion time, pre: 71±12, post: 166±48 s; P<0.001). Recovery from mild cold stress was similar among all 3 rewarming techniques for all measurements. CONCLUSIONS: Findings suggesting similar rewarming responses in field settings are beneficial for the warfighter, and perhaps others, in that rewarming options exist and can be implemented with no compromise in recovery from cold stress.

Haemoconcentration, not decreased blood temperature, increases blood viscosity during cold water immersion.

INTRODUCTION: Prolonged cold-water immersion (CWI) has the potential to cause significant hypothermia and haemoconcentration; both of which have previously been shown to independently increase blood viscosity in vitro. The purpose of this study was to determine the effect of CWI on blood viscosity and examine the relative contribution of decreased blood temperature and haemoconcentration. METHODS: Ten healthy volunteers were immersed to mid-sternum in 10°C water for 90 minutes. Gastrointestinal (GI) temperature, haematocrit (Hct), and blood viscosity were measured pre- and post-CWI. RESULTS: CWI caused mean (SD) GI temperature to decrease from 37.5 (0.3)°C to 36.2 (0.7)°C (P < 0.05). CWI also caused mean Hct to increase from 40.0 (3.5)% to 45.0 (2.9)% (P < 0.05). As a result of the haemoconcentration and decreased GI temperature during CWI the mean blood viscosity increased by 19% from 2.80 (0.28) mPa·s⁻¹ to 3.33 (0.42) mPa·s⁻¹ (P < 0.05). However, when the pre-CWI blood sample was measured at the post-CWI GI temperature (36.2°C) there was no significant difference in the blood viscosity when compared to the pre-CWI (37.5°C) blood sample (2.82 (0.20) mPa·s-1 and 2.80 (0.28) mPa·s-1 respectively). Furthermore, the changes in Hct and blood viscosity during CWI were significantly correlated with an r = 0.84. CONCLUSION: The results of the current study show that prolonged, severe CWI causes a significant 19% increase in blood viscosity. In addition, the results strongly suggest that almost all of the increased blood viscosity seen following CWI is the result of haemoconcentration, not decreased blood temperature.

Cold Acclimation Does Not Alter Physiological or Perceptual Responses During Subsequent Exercise in the Heat.

INTRODUCTION: Warfighters often train and conduct operations in cold environments. Specifically, military trainees and divers that are repeatedly exposed to cold water may experience inadvertent cold acclimatization, which results in body heat retention. These same warfighters can quickly switch between environments (cold to hot or hot to cold) given the nature of their work. This may present a risk of early onset of hyperthermia when cold-acclimatized warfighters are subsequently exposed to physiological insults that increase body temperature, such as exercise and heat stress. However, there is currently no evidence that suggests this is the case. The purpose of this work, therefore, is to determine what impact, if any, repeated immersion in cold water has on subsequent exercise in the heat. MATERIALS AND METHODS: Twelve healthy subjects (values in mean ± SD: age, 25.6 ± 5.2 years; height, 174.0 ± 8.9 cm; weight, 75.6 ± 13.1 kg) voluntarily provided written informed consent in accordance with the San Diego State University Institutional Review Board. They first completed 120 minutes of moderate treadmill walking in 40°C and 40% relative humidity. During this trial, subjects' physiological and perceptual responses were recorded. Twenty-four hours later, subjects began a cold acclimation protocol, which consisted of seven, 90-minute immersions in cold water (10°C, water level to chest). Each immersion was also separated by 24 hours. Subjects then repeated a subsequent trial of exercise in the heat 24 hours after the final immersion of the cold acclimation protocol. RESULTS: Results from cold acclimation revealed no change in core temperature, a decrease in skin temperature, and attenuated shivering and lactate responses, which supports a successful insulative-hypothermic cold acclimation response. This type of cold acclimation response primarily results in heat retention with associated energy conservation. Findings for heat trials (pre-cold acclimation and post-cold acclimation) revealed no differences between trials for all measurements, suggesting that cold acclimation did not influence physiological or perceptual responses during exercise in the heat. CONCLUSION: Our findings indicate that military divers or trainees that are frequently exposed to cold water, and hence have the ability to experience cold acclimatization, will likely not be at greater risk of increased thermal strain when subsequently exposed to physical activity in hot environments. In this study, no physiological or perceptual differences were observed between trials before and after cold acclimation, suggesting that cold acclimation does not present a greater hyperthermia risk during subsequent exercise in the heat.