A contribution to understanding the impact of variations in body mass on fractionating the metabolic burden of military load carriage

Purpose: The oxygen cost associated with load carriage is dependent upon both its mass and its placement about the body. For occupations in which load carriage is routinely performed, and involves identical loads for all individuals, the relative metabolic cost varies inversely with body mass. However, whilst we understand the average impact of varying load placement, our appreciation of its impact on a morphologically diverse, contemporary workforce is very limited. Methods:The relationship between load placement and body mass was evaluated in 65 men (23.0 y [SD 3.0]; 80.5 kg [SD 1.7]: range 56.0-109.8 kg), matched for height-adjusted adiposity (59.3 mm [SD 25.4]) and height-adjusted body mass (65.9 kg [SD 22.0]). Participants were grouped into mass categories (55-65 kg [N = 12]; 66-76 kg [N = 15]; 77-87 kg [N = 19]; 88-98 kg [N = 12]; 99-110 kg [N = 7]) and walked at 4.8 km h−1 (0% gradient) for five, 15-min stages, separated by 5-min rests. Each stage involved a unique load configuration presented in a balanced order: unloaded (battle dress and running shoes); head loading (1.38-kg helmet); torso loading (25-kg weighted vest); hand loading (2 kg each wrist); and foot loading (2 kg each foot [boot plus ankle]). Data were collected using open-circuit respirometry, sampled over the last 5 min. Results: Within each condition, gross oxygen consumption increased in parallel with body mass (p < 0.05). However, when net data were normalised for the external load (change score divided by the added mass), a body mass-dependency of the oxygen cost was not realised within any condition. When averaged across the mass categories, the oxygen consumed per kilogram of mass carried was greater for foot loading (44.8 mL kg−1 min−1 [±2.0]; p < 0.05) compared to all other locations (head: 7.0 mL kg−1 min−1 [±4.9]; torso: 8.6 [±0.3]; hands: 8.0 [±1.3]). The gross oxygen costs of torso (1211.8 mL min−1 [±21.0]) and foot loading (1179.8 mL min−1 [±20.7]) were both greater than the other three conditions (unloaded: 999.2 mL min−1 [±20.5]; head: 1005.8 mL min−1 [±19.2]; hands: 1038.4 mL min−1 [±19.0]; p < 0.05), but did not differ from one another (p > 0.05). Conclusion:Whilst the relative impact of fixed load carriage was greater for smaller individuals, no relationship was found between body mass and the nett mass-specific, ambulatory oxygen cost of these added loads, regardless of their location. This is important, for it shows that whilst loads have greater impact on smaller individuals, that impact appears not to be position dependent.