Additional load by body armour changes the ground reaction force pattern in early stance

INTRODUCTION: Military or police personnel must often carry additional load in form of body armour and weapons, which impacts the biomechanics of human locomotion resulting in increased injury risk [1] and increased energy cost [2]. The effect of load carriage on walking gait has been extensively investigated, whereas only little is known about the effect of carrying loads on the mechanics of slow running. Therefore, the aim of this study was to expand the empirical evidence of the effect of load carriage on the kinetics of running. METHODS: 28 physically active subjects (1.87 ± 7.1 m; 77.1 ± 9.5 kg; 24.9 ± 4.3 yrs.) ran on a single-belt treadmill at 2.4 ms-1 both with (w PPE; 20.9 kg) and without body armour (w/o PPE). The vertical ground reaction force (GRFv) was measured through 4 triaxial force transducers (1000 Hz, MC3A-3-500-4876, AMTI Inc., Watertown, USA) which were embedded into the treadmill. The data were averaged over 20 steps per subject and normalized to body weight. Differences in GRFv between the two conditions were tested for significance using statistical parametric mapping. Additionally, the duration of the stance time was tested using the Mann-Whitney-U test. RESULTS: Wearing PPE affected the GRFv while running at 2.4 ms-1 on a treadmill. Statistically significant differences (p < 0.001) were found for the time period from initial touchdown (TD) until 14 % of the stance phase and from 19 % of the stance phase until toe-off (TO). The maximum GRFv w/o PPE amounted to 2.19 ± 0.17 times body weight and was significantly (p < 0.001) increased to 2.55 ± 0.22 times body weight when the subjects wore PPE (Fig. 1.). Additionally, the stance time was significantly (p = 0.003) longer w PPE (328 ± 23 ms vs. 310 ± 23 ms). CONCLUSION: The maximum GRFv was reached at 40 % of the stance phase for both conditions. It is noteworthy that the relative increase of the maximum GRFv was lower than the increase of body weight by the additional load (16.4 % vs. 27.1 %) which is in accordance with [3]. This disproportional increase might be a result of the prolonged stance times in the loading condition. The period from TD until 14 % of the stance phase showed a higher loading rate and a distinct impact peak w PPE which is not identifiable w/o PPE. Since the impact peak is associated with rearfoot-running [4], carrying additional load might have forced the subjects to strike the ground with the heel. This foot strike pattern would move the point of force application at TD posteriorly which would decrease the external moment arm of the GRFv and thereby increase the mechanical advantage of the plantarflexors. Future research should therefore investigate whether a strengthening of the plantarflexors enhances the ability to cope with additional load during running.