Objective: The purpose of this investigation was to characterize the effects of self-selected work activity on energy expenditure, water turnover, and thermal strain during wildland fire suppression. A secondary aim was to contrast current data with data collected 15 years ago using similar methods to determine whether job demands have changed. Methods: Participants (n = 15, 26 ± 3 years, 179 ± 6 cm, 78.3 ± 8.6 kg) were monitored for 3 days for total energy expenditure, water turnover, core and chest skin temperature, physical activity, and heart rate. Participants arrived to the mobile laboratory each morning, submitted a nude weight, ingested a temperature transmitter, provided a urine sample, and were equipped with a physiological and activity monitor. Participants completed live wildland fire suppression during their work shifts. Results: Mean core temperature was 37.6° ± 0.2°C, mean chest skin temperature was 34.1° ± 1.0°C, mean heart rate was 112 ± 13 beats/min, and the mean physiological strain index score was 3.3 ± 1.0. Wildland firefighters spent 49 ± 8%, 39 ± 6%, and 12 ± 2% in the sedentary, light, and moderate-vigorous intensity categories, respectively. The mean total energy expenditure was 19.1 ± 3.9 MJ/d, similar to 1997 (17.5 ± 6.9 MJ/d). The mean water turnover in 2012 was 9.5 ± 1.7 L/d, which was higher ( P < .05) compared with 1997–98 (7.0 ± 1.7 L/d). Conclusions: Wildland firefighters do not induce consistently high cardiovascular and thermal strain while completing arduous work in a hot environment despite fairly high chest skin temperatures. The total energy expenditure in the current study suggests job demands are similar to those of 15 years ago, while the increased water turnover may reflect a change in drinking habits.

Introduction: The objective of this study was to document characteristics of hiking during wildland firefighter (WLFF) training and wildfire suppression. For the first time, the overall physical demands during wildland firefighting were evaluated in the field using global positioning systems coupled with wireless physiological monitoring and load carriage prediction models. Methods: Male (n=116) and female (n=15) interagency hotshot crew and type II WLFFs on wildfires volunteered for this direct observation study. Participants’ heart rate, internal temperature, speed, and elevation gain were monitored throughout training and during wildfire suppression. The Pandolf and Santee equations were used to predict metabolic rate to estimate oxygen consumption of uphill and downhill hiking. Results: Equipment weight varied by crew type (type II: 24±9 kg and interagency hotshot crew: 28±6 kg; P<0.05). Grade of terrain was steepest during training hikes, and ingress hikes were statistically different from egress and training hikes (ingress: 4±9%, shift: 4±9%, egress: 1±8%, training hikes: 10±9%; P<0.01). Estimated oxygen consumption was highest during ingress hikes and was significantly different from all other hike types on fire assignments (ingress: 22±12, shift: 19±12, egress: 19±12 mL·kg^-1·min^-1; P=0.01). Oxygen consumption was higher during training hikes (34±14 mL·kg^-1·min^-1) than during job-related hikes ( P<0.01). Conclusions: The greatest metabolic demand during wildfire assignments occurred during ingress hikes. On average, this was close to the estimated metabolic demand of the job qualification arduous pack test. However, greater metabolic demand occurred for periods during both shift (on the job) and training hikes. These data quantify the demands associated with actual wildland performance of WLFFs and can help define future work capacity testing and training procedures.

Objectives In order to make recommendations to protect wildland firefighters (WLFFs) from heat-related illness, the role physical exertion plays on core body temperature (Tc) in the work environment needs to be considered. We aimed to estimate the association of time-varying job tasks with differing exertion levels on change in Tc, among WLFFs engaged in fire management activities, while controlling for ambient conditions and individual characteristics. In addition, we examined whether duration of tasks modified the relationship between task and change in Tc. Methods We used data from The Wildland Firefighter Heat Related Illness study, a repeated measures study with individual-level data on 301 WLFFs from 2013 to 2016. Job tasks over one full shift were recorded and categorised into four levels (sedentary, light, moderate and high) based on estimated exertion levels. Tc was measured every 15 s using an ingestible sensor and personal monitors measured ambient conditions. The analysis used generalised estimating equations with an autoregressive correlation structure. Results We found statistically significant associations between job tasks with increased physical demand and increases in Tc. When compared with sedentary tasks, the estimated increases in Tc for light, moderate and high activity tasks of average duration were 0.07°C (95% CI 0.02 to 0.11°C), 0.19°C (95% CI 0.15 to 0.23) and 0.44°C (95% CI 0.39 to 0.49°C), respectively. In addition, we found evidence of interaction between task type and duration. Conclusions This study supports the hypothesis that occupational exposure to physical exertion increases Tc, highlighting the need to update recommendations to protect WLFFs from heat-related illness.

Introduction: The purpose of this study was to evaluate heat stress occurring in wildfire management activities with variable environmental conditions. Methods: Direct observation and real-time wireless physiological monitoring allowed for weather and physiological metrics, including heart rate, core temperature (T_c), skin temperature, and physiological strain index (PSI), of male (n=193) and female (n=28) wildland firefighters (WLFFs) to be recorded during wildfire management activities. Accelerometry data were used to categorize intensity level of activity. Results: Ambient temperature and relative humidity values were used to compute the heat index (HI; n=3891 h) and divided into quartiles (Q1: 13.3–25.1°C; Q2: 25.2–26.4°C; Q3: 26.5–28.9°C; Q4: 29.0–49.1°C). Activity levels remained relatively constant across all HI quartiles. The percentage of time spent performing moderate/vigorous activities was lowest during the hotter Q4 (Q1: 3%; Q2: 2%; Q3: 2%; Q4: 1%). Heart rate, T_c, PSI, and skin temperature associations with HI varied by resource type. Sixty-one percent of WLFFs (n=134) experienced a T_c ≥38.0°C, and 50% of WLFFs (n=111) experienced a PSI ≥6.0. Conclusions: Heat stress was prevalent as WLFFs performed job tasks of varying intensities in all ambient conditions. Spontaneous bouts of arduous labor, duration of work shifts, and other occupation characteristics present the possibility for substantial durations of hyperthermia, although no heat-related injuries occurred in this study. Despite chronic exposure to rugged sloped terrain, load carriage, and environmental conditions, self-regulation and individual attention to managing work:rest appears to be the primary management strategy in mitigating excessive accumulation of body heat in this occupation.

Introduction: Wildland firefighters (WLFF) work long hours in extreme environments, resulting in high daily total energy expenditure. Increasing work-shift eating episodes and/or providing rations that promote convenient eating has shown augmented self-selected work output, as has regular carbohydrate (CHO) consumption. It remains unclear how current WLFF feeding strategies compare to more frequent nutrient delivery. Our aim was to determine WLFFs’ self-selected field total energy intake (TEI), composition, and feeding patterns during wildland fire suppression shifts. Methods: WLFF were deployed to fire incidents across the United States throughout the 2018 fire season. Preshift food inventories collected at basecamp provided item-specific nutrient content (kilocalories, CHO, fat, protein). Work shift consumption (TEI, feeding frequency, episodic composition) was monitored in real time by field researchers on fireline via observational data capture using mobile tablets. Shift work output was also quantified via actigraph accelerometry. Results: Eighty-six WLFF (27.5±6.4 y; 16 female, 70 male) worked 14.0±1.1 h shifts, with a TEI of 6.3±2.5 MJ (1494±592 kcal) (51±10, 37±9, 13±4% for CHO, fat, and protein, respectively). WLFF averaged 4.3±1.6 eating episodes (1.4±1.3 MJ [345±306 kcal] and 44±38 g CHO·episode^-1). WLFF who consumed >20 kcal·kg^-1 averaged less sedentary activity than those consuming <16 kcal·kg^-1. Data are presented as mean±SD. Conclusions: Not including fire camp meals (breakfast, dinner), the present work-shift TEI approximates 33% of previously determined WLFF total energy expenditure and demonstrates that current WLFF consumption patterns may not deliver adequate nutrients for occupational demands.

Objective: To document the effects of wildland firefighter (WLFF) critical training (CT) on physiologic markers of muscle damage and acute overreaching. Methods: Eighteen male and three female WLFFs were studied during an 11-day CT. Upper-body (US), lower-body (LS) muscle soreness and body weight (BW) were collected daily. Blood was collected on days 1, 4, 8, and 11 to measure creatine kinase (CK), lactate dehydrogenase (LDH), cortisol, and testosterone. Skinfolds were taken on days 1 and 11 to estimate body fat (BF) and lean body weight (LBW). Results: CT resulted in a significant depression in BF and elevation in LBW, while main effects of time were seen in US, LS, CK, LDH, cortisol, and testosterone/cortisol ratio (P < 0.05). Conclusion: These data suggest WLFFs undergo significant physiological stressors resulting in muscle soreness, damage, and possible overreaching during CT.