The determinants of individual load carriage economy

Energy saving phenomena have been identified for load carriage on the head, the back, and evenly distributed between the back and front of the torso (back/front-loading), but the mechanics explaining these phenomena are unknown. This research aimed to identify the determinants of individual load carriage economy. Three empirical studies and the development of a theoretical deterministic model (TDM) are presented. Study 1 showed that the Extra Load Index (ELI), a measure of relative load carriage economy, and loaded walking gait kinematics have good test-retest reliability with 7 and 20 kg (e.g. largest coefficients of variation (CV) = 4.17%). Study 2 showed that there is no significant difference in ELI for head-, back- and back/front-loading across a range of load mass (3 – 20 kg) for experienced head-loaders. However, there were significant differences in gait kinematics between methods. For example, forward lean increased from 3 to 20 kg for back- (10.7°) and back/front-loading (2.4°) but decreased for head-loading (-2.2°). Study 2 also supported the existence of considerable inter-individual variation for both ELI (e.g. CV of up to 16%) and load carriage kinematics (e.g. change in forward lean from unloaded walking of +24% to –8% for back-loading with 20 kg). The TDM provides a framework to analyse the biomechanics of load carriage, as in study 3. Study 3 showed that a combination of reduced trunk movement and stride pattern perturbations from unloaded walking are associated with an improved economy for some load conditions (back/front-loading with 20 kg and head-loading with 12 kg), however this finding was not consistent across all load method and masses. In conclusion, a loaded walking gait closer to that of unloaded walking is beneficial for some load carriage conditions and may be an important determinant of load carriage economy. However, this does not explain individual load carriage economy variability.