A Model to Investigate Knee Contact Force During Walking on Ballast Hang Xu, Andrew Merryweather, Donald Bloswick Ergonomics and Safety Laboratory, University of Utah, Salt Lake City, UT, USA Introduction The effect of walking on ballast on worker biomechanics is still unclear clear, especially for knee contact force (KCF). Musculoskeletal models are widely used for simulating gait on hard level surfaces. Most models are utilize simplified knee joints and fail to account for multiple degrees of freedom (DOFs), ligaments and patella-tibia/patella-femur interactions. The aim of this study was to develop a musculoskeletal model with robust knee structures to investigate knee contact force for different ballast conditions. Methods Independent variables: Surface conditions: main ballast (MB), walking ballast (WB) and no ballast (NB); Surface configurations: level surface and a slanted surface (7° slope in transverse plane) Uphill and downhill limbs Trial Data: Eight male participants (IRB #18667) Five successful trials for each combining of surface condition and configuration for each participant Marker-based motion data and force plate data OpenSim model: 14 rigid segments, 29 DOFs and 56 muscles Patella and patella tendon Rotation of the patella relative to the tibia2 Three rotations and three translations of knee joint Knee proximodistal and anteroposterior translations were functional of knee flexion3 Knee cruciate and collateral ligaments (10 ligament bundles, Figure 1) Properties and parameters were defined for ligament bundles4-7 Biomechanics parameters: The KCF was calculated by the vector sum of the knee joint reaction force and the compressive forces from the muscles and ligaments crossing the knee joint. The timing of two peak KCFs was recorded and normalized using percent cycle. The co-contraction index (CCI) was calculated by the ratio of total muscle force of knee agonist muscles to knee antagonist muscles. Knee ligament force was determined by ligament length combined with the nonlinear, force-length curve. RANOVA and Post hoc tests were used for determining the effect of surface conditions. Paired t-test was used for comparison between two surface configurations and uphill or downhill limb. Significance was set at p < 0.05. Results Ligament function: The anterior (aACL and aPCL) bundles and posterior bundles (pACL and pPCL) of knee cruciate ligaments intersected during passive knee flexion. aACL and aMCL were recruited throughout the range of knee motion. LCL was recruited throughout the range of knee internal rotation and abduction/adduction. The posterior bundles of knee ligaments were recruited for knee internal rotation and adduction (partly shown in Figure 2). Knee biomechanics: Significant differences among NB, MB and WB for timing of first peak KCF and between NB and WB for timing of second peak KCF (Figure 3) Significant knee muscle CCI difference between WB and NB in two peak KCF, WB and MB in second peak KCF (Figure 3) Significant knee muscle CCI difference between uphill and downhill limbs in first peak KCF (Figure 4) Significant differences between uphill and downhill limbs including aACL, LCL, iMCL in first peak KCF, LCL and aMCL in second peak KCF Effect of surface configurations were significant for aACL and aMCL in first peak of KCF, LCL in the second peak KCF (Figure 5) Conclusion This study developed and verified a musculoskeletal model with robust knee structures in OpenSim, and investigated KCF when walking on different ballast conditions and configurations. Two main contributions of this model compared with the existing model were multiple DOFs for the knee joint and knee ligament along with geometrical and mechanical properties. The reasonableness of the ligament geometries was verified by simulating knee motions in three body planes compared to physical knee and other existing knee model parameters. The significant differences of surface condition were suggested in the timing of peak KCF and CCI. The effects of surface configuration were found in ligament force. The significant differences between uphill and downhill limbs were indicated in CCI and ligament force. Future work will refine the model to further study KCF during irregular surface gait and is sensitive to surface conditions and configurations. Limitations: KCF and absolute muscle forces are too high and ligament insertion points are difficult to scale between subject models Findings: Difference by surface condition in timing of peak KCF and CCI Ligament force change by surface configuration The CCI and ligament force are different between uphill and downhill limbs Acknowledgment The authors would like to thank the students in Ergonomics & Safety Laboratory at University of Utah (www.ergo.mech.utah.edu) for their help and assistance with this research. References [1] Wade, C., et al., Hum Factors, 2010. 52(5): p. 560-73. [2] S. L. Delp et al. IEEE Trans Biomed Eng, Aug 1990, vol. 37, pp. 757-67. [3] van Eijden, et al., Journal of Biomechanics, 1985. 18(10): p. 803-9, [4] Blankevoort, L., R. Huiskes, and A. de Lange, J Biomech Eng, 1991. 113(1): p. 94-103. [5] Herzog, W. and L.J. Read, J Anat, 1993. 182 (Pt 2): p. 213-30. [6] Pandy, M.G. et al., Comput Methods Biomech Biomed Engin, 1998. 1(2): p. 87-108. [7] Blankevoort, L. and R. Huiskes, J Biomech Eng, 1991. 113(3): p. 263-9. Figure 4: knee muscle CCI by limb. * indicated a difference from downhill limb Figure 2: Plots of relative length change vs. angle for selected ligament bundles and knee angles. Figure 5: ligament force by surface condition, * indicated a significant difference Figure 1: Geometry of knee ligament bundles. aACL and pACL, anterior and posterior bundle of ACL; aPCL and pPCL, anterior and posterior bundle of PCL; aMCL, iMCL and pMCL, anterior, inferior and posterior bundle of MCL; aDMCL and pDMCL, anterior and posterior bundle of deep MCL Figure 3: (left) Timing of peak KCF, * indicated a significant difference from no ballast (p<0.05). (right) knee muscle CCI by surface condition. * indicated difference from NB. ** indicated difference from other two Contact info: Name: Hang Xu Andrew Merryweather Donald Bloswick Email: hanghang500@gmail.com a.merryweather@utah.edu Bloswick@mech.utah.edu Telephone: +86-18641805683 (801)581-8118 (801)581-4163