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METABOLIC COST AND PERCEIVED EXERTION WHEN ADDING 9 KG TO WILDLAND FIREFIGHTER PACKS

Abstract

Wildfire suppression includes arduous tasks such as uphill hiking, line digging, and lifting for prolonged periods. The U.S. Forest Service asked us to evaluate the effect of adding 9kg to a standard 16 kg wildland firefighter (WLFF) pack on task perceptions and work rates. PURPOSE: To examine the metabolic cost and perception of task difficulty between a 16 vs. 25 kg WLFF pack to simulate the addition of an enhanced fire shelter, and how this will affect work rates of WLFFs. METHODS: Twenty-Five WLFFs (28.0±5.8 years) participated and had VO2max values of 53.5±5.7 ml*kg-1*min-1 and VO2 vt= 37.1±6.1 ml*kg-1*min-1. Experimental sessions included: 1) 10 minutes carrying each pack (16 or 25kg) at 3 mph up 2.5% and 7.5% grades. 2) Line digging tests consisted of a standardized 10 minute “Pullaski Treadmill” test with each pack, and 3)10 minutes simulated lifting of 7 and 18 kg objects. Average VO2, HR, perception of effort scales and other data were recorded for each task. Statistical differences were evaluated using a dependent t-test, p<0.05. RESULTS: Heavier packs resulted in significantly increased VO2 during uphill hiking at both 2.5% (16kg=20.0±1.7 vs 25kg = 22.32±1.79 ml*kg-1*min-1, +11.8%, p<0.01) and at 7.5% (16kg=23.0±1.4 vs 25kg = 25.3±1.59 ml*kg-1*min-1, +10.1%, p<0.01) but not for lifting tasks or line digging. When subjects were asked “How difficult would it be to maintain this work for 6-8 hours?” their perception was significantly greater for each mode of exercise when carrying the heavier pack. Hiking difficulty increased 23.2±18.0%, line digging difficulty increased 15.8±19.9%, and lifting difficulty increased 17.5±16.6%. CONCLUSION: Adding 9kgs to a 16kg WLFF pack results in increased metabolic cost for hiking but, because of how the pack is carried, does not increase VO2 cost for lifting or line digging. However, the heavier pack was perceived to make all work modes more difficult especially when the work duration increased. Additionally, less fit WLFFs had a much greater increase in VO2 cost and perceived difficulty with the heavier pack. There is no doubt that heavier packs will result in a greater safety risk and decreased work production for WLFFs and especially for lower fit firefighters.

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