The Use of Kinematic and Kinetic Analysis Measurement to Identify the Critical Factors in Optimising Dismounted Combatant Load Sharing Systems
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Armed forces throughout the world value maintaining effective and capable soldiers. That means having soldiers who can perform their job unhindered and without sustaining injury. Exposure to the physical demands of military service, including heavy load carriage, directly threatens soldier capability by impeding performance and increasing the risk of musculoskeletal injuries (MSI). Soldier attrition due to MSI has increased globally in the last 10 years, leading to reduced workforce capability and incurring substantial costs. Additionally, wearing heavy loads affects performance sustainability by increasing metabolic energy expenditure and decreasing time to fatigue compared to wearing lighter loads. To reduce the physical burden of load carriage, industry manufacturers have developed load sharing systems that re-distribute load from the shoulders to the hips via a hip belt, and claim that these systems decrease shoulder pressure and discomfort. However, these claims made by industry have not yet been substantiated by rigorous and impartial testing. Additionally, other potential effects of using these systems (e.g., altered walking patterns) have not been investigated. Changing the distribution of load may affect soldier performance in the field, or alter lower-limb joint loading in a manner that increases MSI risk. Therefore, the general purpose of this thesis was to comprehensively investigate biomechanical changes that occur when soldiers wear different load sharing systems compared to standard issue military armour, with implications for reducing MSI risk and/or improving soldier performance discussed throughout. The first study developed and evaluated performance of a new marker set to track trunk and pelvis motion during load carriage. This was essential as existing marker sets were validated for skin-surface placement or for use in obese populations, but not when using torso-borne load carriage systems. Two torso-borne load carriage armour conditions were examined by comparing joint angles measured from eight participants using a new marker set against those obtained using markers placed on the skin surface (i.e., without armour), and markers placed on body armour. Bland Altman analyses showed strong agreement between joint angles from the new marker set and markers placed directly on the skin. The agreement worsened with markers placed on top of body armour. Additionally, inter-protocol coefficient of multiple correlations (CMCs) comparing markers on body armour to the new marker set were poor to compared to CMCs between the skin-mounted markers and new marker set. Intra- and inter-session repeatability were high for the new marker set compared to placing marker over-top body armour. Given these positive results, the new marker set provides a viable alternative for researchers to reliably measure trunk and pelvis motion when equipment, such as body armour, obscures marker placement. The paper describing the new marker set was published as Lenton, G. K., Doyle, T. L. A., Saxby, D. J., & Lloyd, D. G., An alternative whole-body marker set to accurately and reliably quantify joint kinematics during load carriage. Gait and Posture, 54, 318-324, 2017. The second study evaluated the veracity of manufacturer claims of load sharing systems reducing shoulder pressure and discomfort. Twenty soldiers each wore twelve body armour variations: six armour types (one standard-issue armour, TBAS, and five prototype designs, cARM1-2, pARM1-3) and two carried loads (15 and 30kg) while walking on an instrumented treadmill at moderate (1.53 m⋅s-1) and fast (1.81 m⋅s-1) speeds for 10 minutes at each speed. Walking with hip belt compared to no hip belt armour resulted in decreased mean and maximum shoulder contact pressures, and 30% fewer participants experiencing shoulder discomfort in best performing hip belt designs. Additionally, regression analyses showed laterally concentrated shoulder pressure was associated with 1.34-times greater likelihood of experiencing shoulder discomfort (p=0.026). These results suggest body armour and backpack designs should integrate a hip belt and distribute load closer to the shoulder midline to reduce load carriage discomfort and, potentially, injury risk. The paper describing these results was submitted as Lenton G. K., Doyle T. L. A., Saxby D. J., Higgs J, Billing D, Lloyd D. G. Integrating a hip belt with body armour reduces the magnitude and changes the location of shoulder pressure and perceived discomfort in soldiers, Ergonomics. The purpose of study three was to determine the effects of load distribution, load magnitude, and walking speed on biomechanical surrogates of MSI. Whole-body marker kinematics and ground reaction forces were collected during the protocol outlined in study two. Subsequently, a scaled anatomic model was used in OpenSim’s inverse kinematic and inverse dynamic procedures to determine whole-body joint angles and lower-limb net joint moments, respectively. It was found that lower-limb joint moments increased when participants carried greater load and/or walked faster, while combined effects of carried load and movement speed were mostly additive. Peak plantarflexion moment was reduced by 16% when wearing cARM1 compared to the standard issue TBAS system while carrying 30kg and walking fast. This suggests there are positive benefits of load sharing at higher demands. Importantly, shoulder-to-hip load transfer resulted in soldiers walking with a more upright posture, and did not negatively alter joint kinetics. A manuscript has been submitted as Lenton, G. K., Saxby, D. J., Lloyd, D. G., Billing, D., & Doyle, T. L. A., Carried load configuration, mass, and movement speed alter biomechanical risk factors for musculoskeletal injuries in soldiers, Medicine and Science in Sports and Exercise. The fourth study examined how lower-limb joint work and power magnitudes, surrogates of performance longevity, as well as relative contributions of lower-limb joints to total average positive power were modulated in different load configurations and movement speeds during gait. The joint angle and joint moment data from study three were used to determine hip, knee, and ankle joint work and powers using standard computational methods. While total average power was largely dictated by task demands (i.e., carried load and gait speed), different armour systems and carried loads resulted in different lower-limb joint powers. Carrying 30 kg with cARM1 armour caused significantly greater contributions of hip powers to total positive power, while knee and ankle powers decreased insignificantly compared to carrying 15 kg with cARM1. Additionally, as gait speed and carried load increased, hip powers increased, and knee and ankle powers decreased, possibly as a control strategy to balance load while maintaining forward progression. These results suggest physical training programs and assistive devices should be designed based upon physical requirements of a particular armour type and/or carried load to maximise soldier in-field sustainability. A manuscript has been submitted as Lenton, G. K., Doyle, T. L. A., Lloyd, D. G., Billing, D., & Saxby, D. J., Lower-limb joint work and power are modulated during load carriage based on load configuration and movement speed, Journal of Biomechanics. In conclusion, load sharing systems should improve soldier capability immediately by reducing shoulder pressure and associated discomfort compared to current-issued armour, and long-term by enhancing performance longevity and reducing lower-limb MSI risk. The hip-dominant joint power profiles suggest that physical training programs and device design should target hip extensor muscles and provide assistance to the hip respectively. However, design-specific recommendations should maximise benefits as different design elicited slightly different joint power profiles. An optimal load carriage design building upon best features of existing designs may provide greater benefit than any single design we tested.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School Allied Health Sciences
The author owns the copyright in this thesis, unless stated otherwise.
Kinetic analysis measurement
Load sharing systems