The effect of load distribution within military load carriage systems on the kinetics of human gait
Introduction
The effect that military load carriage has on ground reaction force (GRF) parameters has been examined previously in the literature (Birrell et al., 2007, Harman et al., 2000, Kinoshita, 1985, Lloyd and Cooke, 2000a, Polcyn et al., 2002, Tilbury-Davis and Hooper, 1999). However, less attention has been paid to the distribution of load on the body, particularly with respects to the biomechanical changes of gait. It has long been suggested that the most efficient way to load the body is by keeping it as close as possible to the body's CoM, while also utilising the larger muscle groups (Legg and Mahanty, 1985). However, due to various ergonomic reasons the backpack is the only really viable option for members of the military to carry their own equipment. Research has shown that placing load closer to the body's CoM results in a reduction in energy cost (Abe et al., 2008, Coombes and Kingswell, 2005, Datta and Ramanathan, 1971, Lloyd and Cooke, 2000b), with a more upright walking posture being adopted (Kinoshita, 1985, Harman et al., 1994). In terms of the kinetic effects a reduced maximum braking force and stance time, while increasing force minimum are reported outcomes as a result of distributing load around the trunk (see references below). The actual implications that these changes to basal gait patterns have to injury or energy cost is relatively unknown. However, we can postulate that a decrease in maximum braking forces may have a positive effect on blister development, due a reduction in the sheering forces applied to the foot–boot interface when walking. In addition, sports research has shown that a reduction in horizontal braking forces can facilitate the forward advancement of the body during running (c.f. Ciacci et al., 2009), this principle may also relate to long distance marches with heavy loads in military situations. High magnitudes or volumes of impact forces (or force produced at heel strike), like those experienced during load carriage or running, are a major risk factor for overuse injuries. In particular, stress fractures of the tibia and metatarsals and knee joint problems (Cavanagh and Lafortune, 1980, Polcyn et al., 2002). A reduction in either of these two parameters would have clearer implications on injury as a result of load distribution.
To the author's knowledge only five studies have investigated some aspects of load distribution on GRF parameters, including just three papers in peer-reviewed journal (Kinoshita, 1985, Lloyd and Cooke, 2000a, Hsiang and Chang, 2002), one military report (Harman et al., 2001) and one conference paper (Koulmann, 2006). These available studies have generally been restricted to between 4 and 6 load and carrying system combinations, with limited mechanisms put forward for the observed changes. The aim of this study is to build on and develop this current knowledge by investigating the effect that different distributions of carried load had on kinetic (specifically GRFs) parameters of human gait. This current study will use an increased number of load and backpack combinations (four different loads and three different backpack systems), and also military specific load carrying systems, while offering links to previous research to develop the implications of these changes to basal gait patterns.
Section snippets
Participants and equipment
Twelve male participants volunteered for the study (mass 81.3 kg ± 9.9 S.D., height 184.4 cm ± 6.2, age 29.2years ± 9.0). All participants volunteering for the study had previous experience carrying military style backpacks, all were right foot dominant and rear-foot strikers. A verbal and written explanation of the study was given, after which a health screen questionnaire was completed. Finally signed, informed consent was obtained from all participants before commencing the trial.
Kinetic
Results
Results from the study showed that only the thrust maximum (aka force produced at toe-off in the vertical axis) showed a significant main effect (p < 0.05) for the 3 LCS with the MANOVA (Table 2). The results indicate that the thrust maximum force produced during the Backpack LCS condition was significantly lower than those produced during the other two conditions (Standard and AirMesh LCS). In addition to this main effect the Tukey post-hoc test highlighted other significant differences
Discussion
This study utilised 12 different load and LCS combinations, and carried load in British military LCS. These factors make the current study both relevant and significant in aiding our understanding of the biomechanical effects of military load carriage. The most important findings from the study are described below.
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A significant change in the thrust maximum was observed between the 3 LCS. More specifically, the backpack LCS produce a lower force compared to the standard LCS at all loads. Also, a
Conclusions
Changing the distribution of load within the military LCS used in this study had limited effect on the GRF parameters of human gait. Despite this important findings were established, in particular the effect of heavy load carriage on maximum braking force. A 10% increase in maximum braking force was observed when carrying 32 kg in the backpack condition compared to the other two conditions used – the importance here lies in the development of blisters. The thrust maximum, or force produced at
Acknowledgements
The authors would like to thank Dr Robin Hooper for his help and advice, which were a major contribution to this study.
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