Physiological factors of importance for load carriage

The energy expenditure during carrying no load, 20, 35 and 50 kg at two walking speeds, 3 and 5 km/h, was studied in 36 healthy participants, 19 men (30 ± 6 yrs, 82.5 ± 7.0 kg) and 17 women (29 ± 6 yrs, 66.1 ± 8.9 kg). Anthropometric data, leg muscle strength as well as trunk muscle endurance and muscle fibre distribution of the thigh were also obtained. To load the participant a standard backpack filled with extra weight according to the carrying weight tested was used. Extra Load Index (ELI), the oxygen uptake (VO2) during total load over no-load-exercise, was used as a proxy for load carrying ability. In addition to analyzing factors of importance for the ELI values, we also conducted mediator analyzes using sex and long term carrying experience as causal variables for ELI as the outcome value. For the lowest load (20 kg), ELI20, was correlated with body mass but no other factors. Walking at 5 km/h body mass, body height, leg muscle strength and absolute VO2max were correlated to ELI35 and ELI50, but relative VO2max, trunk muscle endurance and leg muscle fibre distribution were not. Sex as causal factor was evaluated in a mediator analyses with ELI50 as outcome. ELI50 at 5 km/h differed between the sexes. The limit for acceptable body load, 40% of VO2max (according to Astrand, 1967), was nearly reached for women carrying 35 kg (39%) and surpassed at 50 kg at 3 km/h, and for men carrying 50 kg at 5 km/h. This difference was only mediated by difference in body mass. Neither muscle fibre distribution, leg muscle strength, trunk muscle endurance and body height nor did absolute or relative VO2max explain the difference. Participants with long term experience of heavy load carrying had significant lower ELI20 and ELI50 values than those with minor or non-experience, but none of the above studied factors could explain this difference. The study showed that body mass and experience of carrying heavy loads are important factors for the ability to carry heavy loads.