Influence of graded hypercapnia on endurance exercise performance in healthy humans.

Military and/or emergency services personnel may be required to perform high-intensity physical activity during exposure to elevated inspired carbon dioxide (CO2). Although many of the physiological consequences of hypercapnia are well characterized, the effects of graded increases in inspired CO2 on self-paced endurance performance have not been determined. The aim of this study was to compare the effects of 0%, 2%, and 4% inspired CO2 on 2-mile run performance, as well as physiological and perceptual responses during time trial exercise. Twelve physically active volunteers (peak oxygen uptake = 49 ± 5 mL·kg-1·min-1; 3 women) performed three experimental trials in a randomized, single-blind, crossover manner, breathing 21% oxygen with either 0%, 2%, or 4% CO2. During each trial, participants completed 10 min of walking at ∼40% peak oxygen uptake followed by a self-paced 2-mile treadmill time trial. One participant was unable to complete the 4% CO2 trial due to lightheadedness during the run. Compared with the 0% CO2 trial, run performance was 5 ± 3% and 7 ± 3% slower in the 2% and 4% CO2 trials, respectively (both P < 0.001). Run performance was significantly slower with 4% versus 2% CO2 (P = 0.046). The dose-dependent performance impairments were accompanied by stepwise increases in mean ventilation, despite significant reductions in running speed. Dyspnea and headache were significantly elevated during the 4% CO2 trial compared with both the 0% and 2% trials. Overall, our findings show that graded increases in inspired CO2 impair endurance performance in a stepwise manner in healthy humans.

Wet Military Uniforms Pose Low Risk of Hypothermia while Static in Mild Cold Air.

Wet clothing is less insulative than dry clothing and increases heat loss in cold air. Tactical necessity can render removal of wet clothing impossible and/or require Warfighters to remain static to avoid detection, limiting heat production and posing a threat of hypothermia (core temperature <35°C). This study aimed to characterize body temperatures and evaluate hypothermia risk while statically exposed to 5°C air wearing three wet military uniforms. Further, low-speed loaded walking was evaluated as a strategy to raise end-static temperatures. Twelve adults (11M, 1F) randomly completed three wet-cold trials wearing either the Improved Hot Weather Combat Uniform (IHWCU), Army Combat Uniform (ACU), or ACU with silk-weight base layer (ACU+). Each trial involved 180 minutes of cold air (5.3±0.3°C, 0.8 m·s-1) exposure after a clothed 2-minute head-out immersion (34.0 ± 0.2°C). Volunteers were static for 60 minutes followed by 120 minutes of loaded walking. Rectal temperature (Tre) area under the curve did not differ among the three wet uniforms when static (p=0.431) with Tre increasing, rather than decreasing, across the 60 minutes (IHWCU: +0.26±0.19°C, ACU: +0.37±0.21°C, ACU+: +0.36±0.20°C). Hypothermia risk with 60-minute static wet-cold exposure therefore appears minimal, regardless of the military uniform worn. End-static finger temperatures (IHWCU: 9.48±2.30°C, ACU: 9.99±1.82°C, ACU+: 9.27±1.66°C, p >0.999) were reduced by ~20-23°C posing a considerable dexterity concern. Heat production of ~210 W·m2 appeared sufficient to begin to reverse negative cumulative heat storage and initiate slight elevation of rectal and peripheral temperatures, although finger temperatures increased < 2°C after 120 minutes. ClinicalTrials.gov ID:NCT05409937.