Wildland Firefighter Load Carriage: Effects on Transit Time and Physiological Responses During Simulated Escape Route Evacuation.
B. Ruby, University of Montana; G. Ledbetter, Mesa State College; D. Armstrong, National Naval Medical Center.

Finger stick for blood lactate
analysis at end of simulated
escape route evacuation.

This investigation was to determine the effects of carrying a load on the time required for evacuation along a simulated escape route (transit time). Subjects (eight males, weighing an average of 181 lbs [82.2 kg] and five females weighing an average of 145 lbs [65.8 kg]) completed two long field hikes in random order on two successive days; one hike with and one hike without a field pack. Field trials were conducted on Storm King Mountain near Glenwood Springs, CO (site of the 1994 South Canyon Fire). The dirt trail was 2,172 ft (660.5 m) long with a vertical gain of 450 ft (137 m), yielding an average grade of 20.75 percent. Subjects carried a calibrated portable metabolic system (Cosmed K4 or Aerosport VO2000), a fire shelter, and a Pulaski. During the pack trial, subjects also carried a 35-lb (15.9-kg) field pack. Blood samples collected before and 2 minutes after exercise were analyzed for lactate (Accusport lactate analyzer). Data were analyzed using a mixed-design ANOVA (analysis of variance) with repeated measures and planned comparisons (Table 2).

Men were 21.5 percent faster without a pack and women were 26.3 percent faster. For the males, mean and peak VO₂ were higher during the no-pack trial. The difference in blood lactate (at peak exertion and at rest) was significantly higher during the pack trial for the male subjects. Similarily, ratings of perceived exertion (RPE) were significantly higher for males during the pack trial (19.8) compared to the no-pack trials (18.7). In contrast there were no significant differences in ratings of perceived exertion for the females (20.0) for the pack trial compared to the no-pack trials (19.6). Simple regression analysis revealed a significant negative correlation between body weight and the slower times during the pack trials (r = -0.64), indicating that larger subjects were less affected by the pack. High correlations between peak VO₂ and transit rates were noted (r = 0.82 for the pack trial, r = 0.87 for the no-pack trial), indicating the contribution of aerobic fitness to quick escapes.

Subject nears end of simulated
escape route evacuation.

These data indicate that carrying a pack significantly impedes ground travel during a forced evacuation. The elevated blood lactate, heart rate, and ratings of perceived exertion responses coupled with the peak VO₂ levels indicate near-maximal effort and an emphasis on glycolytic energy production (data were collected at an elevation of about 7,000 ft). The data also show that larger individuals and those with higher levels of aerobic fitness are better able to negotiate the escape route. Finally, the results support the recommendation that wildland firefighters should abandon their packs during evacuations along emergency escape routes. (Supported by the Missoula Technology and Development Center and a collaborative research grant from Mesa State College, Grand Junction, CO).

Note: This study supplements previous estimates of the effect of carrying a load on escape route evacuation. Earlier estimates based on moderate loads and hiking speeds indicated a 15 to 20 percent faster time without pack and tool. With higher speeds or heavier loads, dropping the pack could increase speed by up to 30 percent, based on those estimates. Subjects in this study on Storm King Mountain were instructed to hike as fast as possible. Results indicated they could travel 21.5 to 26.3 percent faster without the 35-lb pack, but with a tool that could be used to clear a site for shelter deployment. The data also provide clear support for requiring firefighters to maintain a minimum level of aerobic fitness for safety reasons.

This study compared two field tests commonly used to predict aerobic fitness in wildland firefighters with direct measures from laboratory testing. Forty-five males (average age of 25.2 years with a VO₂ max average of 53.6 mL/kg-min) completed a treadmill test for ventilatory threshold (VT) and VO₂ max test, the Leger 20-m shuttle run, and the pack test (3-mile [4.8-km] hike with 45-lb [20.5-kg] pack) on separate days. The mean highest shuttle run stage completed was 9.5 sec and the time to complete the pack test was 41.6 min. Heart rate (HR) and blood lactate concentration from the pack test were significantly lower than values observed at the ventilation threshold, but perceived exertion ratings (RPE) were similar. The heart rate and ratings of perceived exertion at the end of the shuttle run were not significantly different than at the end of treadmill test, but peak blood lactate was significantly lower after the shuttle run than after the treadmill test. Shuttle run performance was significantly correlated with both VO₂ at the ventilation threshold (r = 0.69) and VO₂ max (r = 0.91). Lower but significant correlations were found between pack test time and VO₂ at the ventilation threshold (r = -0.47) and VO₂ max (r = -0.65). While the shuttle run is a better predictor of aerobic fitness, the pack test involves other aspects of work capacity (such as muscular fitness) related to wildland firefighting tasks. (This paper, supported by Alberta Environmental Protection, was presented at the annual meeting of the Canadian Society for Exercise Physiology in 1999).

Note: Dr. Petersen reports that when 22 female subjects were added to the sample, analysis confirmed that a 45-min time on the pack test was equivalent to a VO₂ max (aerobic fitness) of 45 mL/kg-min. The correlation between the pack test and VO₂ max for this larger sample (n = 67) was -0.7. The VO₂ max is a measure of maximum intensity. Wildland firefighting is an endurance event that demands aerobic fitness (VO₂ max), muscular fitness, and the ability to sustain work output across long work shifts.