The reliability of the Extra Load Index as a measure of relative load carriage economy

The aim of this study was to measure the reliability of the Extra Load Index (ELI) as a method for assessing relative load carriage economy. Seventeen volunteers (12 males, 5 females) performed walking trials at 3 km(.)h(-1), 6 km(.)h(-1) and a self-selected speed. Trial conditions were repeated 7 days later to assess test-retest reliability. Trials involved four, four-minute periods of walking, each separated by 5 minutes of rest. The initial stage was performed unloaded followed in a randomised order by a second unloaded period and walking with backpacks of 7kg and 20kg. Results show ELI values did not differ significantly between trials for any of the speeds (p = 0.46) with either of the additional loads (p = 0.297). The systematic bias, limits of agreement and coefficients of variation were small in all trial conditions. We conclude the ELI appears to be a reliable measure of relative load carriage economy. Practitioner Summary This paper demonstrates that the Extra Load Index (ELI) is a reliable measure of load carriage economy at a range of walking speeds with both a light and heavy load. The ELI, therefore, represents a useful tool for comparing the relative economy associated with different load carriage systems.