Extreme External Loading Increases The Exertion Required To Meet The Metabolic Demands Of Walking

Abstract

Although the effort required for a given activity is typically proportional to the metabolic demands, recent evidence suggests that external loading may skew this relationship. Understanding how external loads influence perceived exertion and metabolic expenditure is essential for ensuring optimal health and performance in dismounted warfighters, who often carry extremely heavy equipment. PURPOSE: To examine the effect of varying military rucksack loads on the ratio between perceived exertion and metabolic rate. METHODS: Nineteen healthy volunteers (age, 23.8 ± 7.0 years; body mass, 75.1 ± 13.5 kg; height, 173.4 ± 7.5 cm) walked at three treadmill speeds (0.45, 0.89, and 1.34 m·s-1) while carrying four different load conditions (0, 22, 44, and 66% body mass) in the Modular Lightweight Load-Carrying Equipment (MOLLE 4000) rucksack. Ratings of perceived exertion were recorded with 30 seconds left in each trial and metabolic measurements were obtained using a laboratory metabolic cart. Planned comparisons were conducted to compare the ratio of net rating of perceived exertion to net metabolic rate (RPEnet:Ṁnet) between loads at each speed. RESULTS: RPEnet:Ṁnet was significantly higher when walking at 0.45 m·s-1 with the 66% load (2.03 ± 0.88) versus only the 0% (1.32 ± 1.04), and 22% (1.49 ± 0.80) loads (p < 0.018 for both) and not the 44% (1.67 ± 0.88) load (p = 0.092). For the 0.89 m·s-1 walks, RPEnet:Ṁnet was significantly higher with the 66% load (1.52 ± 0.50) versus the 0% (1.08 ± 0.78), 22% (1.06 ± 0.58), and 44% (1.22 ± 0.64) loads (p < 0.034 for all). Similarly, RPEnet:Ṁnet was significantly higher during the 1.34 m·s-1 walks with the 66% load (1.14 ± 0.35) versus the 0% (0.84 ± 0.55), 22% (0.85 ± 0.42), and 44% (0.91 ± 1.14) loads (p < 0.009 for each). CONCLUSION: Light-to-moderate rucksack loading causes similar increases in perceived exertion and metabolic rate during walking. However, extremely heavy loads (>44% body mass) require a disproportionate level of effort for a given level of metabolic demand. The views expressed in this abstract are those of the authors and do not reflect the official policy of the Department of Army, Department of Defense, or the U.S. Government.