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 degrees 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 degrees 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

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

PurposeThis 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.Methods59 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.ResultsFitbit 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.ConclusionsFitbit 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

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

PurposeThis 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.Methods59 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.ResultsFitbit 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 (pConclusionsFitbit 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