Wildland Firefighters benefit from additional fire resistant clothing; however this may increase metabolic demand. PURPOSE: This study evaluates the effects of a flame resistant synthetic base undershirt on the metabolic demand while exercising in a hot environment. METHODS: Ten recreationally active male subjects (age: 25 ± 6.1, 55.0 ± 7.1ml·kg-1·min-1 VO2max, 11.1 ± 5.3 %fat) completed two walking trials clothed in either a Cotton (C) or Synthetic (S) undershirt. Additionally subjects were equipped with Forest Service issued Person Protective Equipment consisting of a 35 lb pack, Nomex shirt, Nomex pants, gloves, and hard hat. Trials consisted of 180 minutes of walking (2.5 mph, 4% grade) in a climate-controlled chamber (35oC and 40% relative humidity). Each 180 minute trial was divided into three 50 minute exercise bouts followed by 10 minute seated rest. Expired gases were collected at time intervals 5, 40, 100, 160 minutes during exercise to determine oxygen consumption (VO2) and Respiratory Exchange Ratio (RER). Heart Rate (HR), Rate of Perceived Exertion (RPE) Core temperature (Tc), and skin temperature (Tsk) were collected every 10 minutes. Repeated measures of ANOVA’s were performed using SPSS 22.0. RESULTS: HR showed an increase over time (p<0.001), but did not have a significant difference between S and C (p=0.27). RPE displayed significant effects in time (p=0.001) but no significant in trials (p=0.55). Tc and Tsk also increased over time (p<0.001), but did not show a difference between the two under shirts (p=0.73). RER increased over time (p=0.01) but did not differ between trials (p=0.92). VO2 demonstrated a time x trial interaction (p=0.048), with S being greater than C by the end of the trial (18.2 ± 0.5 ml·kg-1·min-1 vs 17.7± 0.6 ml·kg-1·min-1). CONCLUSION: While the data does not show differences between the two shirts in HR, Tc, Tsk, and RPE, there was an increase in VO2 suggesting that a flame resistant synthetic undershirt elicited an increase in energy expenditure.

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