HAL Id: hal-01844866
https://hal.archives-ouvertes.fr/hal-01844866
Submitted on 19 Jul 2018
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of
sci-entific research documents, whether they are
pub-lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
human models for assessing seat discomfort
Léo Savonnet, Sonia Duprey, Xuguang Wang
To cite this version:
Léo Savonnet, Sonia Duprey, Xuguang Wang. Coupling rigid multi-body and deformable finite element
human models for assessing seat discomfort. 4th International Digital Human Modeling conference,
Jun 2016, MONTREAL, Canada. 7 p. �hal-01844866�
Coupling rigid multi-body and deformable finite element
human models for assessing seat discomfort
Léo Savonnet1,2, Sonia Duprey1, Xuguang Wang1
1
Université de Lyon, F-69622, Lyon; IFSTTAR, LBMC, UMR_T9406, Bron; Université Lyon 1, Villeurbanne, France
2Zodiac Seats France, Issoudun, France
Abstract
According to literature, both rigid multi-body (MB) musculoskeletal and deformable finite element
(FE) human modeling approaches are used to assess discomfort related to sitting; a deformable FE
model can predict the occupant seat interaction while the MB model estimates muscular activity and
spinal joint loads. Combining both MB and FE human models could better help assess sitting
discomfort, because it would estimate all mechanical factors leading to discomfort. In this research
study, a FE model of the pelvis and thighs has been developed from MRI images of a 50
thheight male.
The full body musculoskeletal model available in AnyScript Model Repository public domains’ was
used and personalized according to the subject’s anthropometric dimensions. Two methods of
coupling MB and FE models were compared. In the first method, several simulations of the two
solvers (Anybody Modelling system and Radioss) were launched sequentially. At each phase, the rigid
body model outputs (forces and moments) were used as boundary conditions for the deformable
model; the deformable model gives new inputs (position of the bones) for the rigid body model. In the
second method, only the posture obtained from the MB model simulations was set as boundary
conditions in the FE model. The pelvis position was constrained in each direction and the seating
process was simulated by imposing the seat pan vertical displacement until the resultant contact force
reaches the level estimated by the MB model. This method did not require several simulation loops.
The first method may help predict a more accurate posture due to the consideration of realistic
occupant/seat interaction (soft tissue deformation) for body positioning. However, this method
requires a longer computation time because of the simulation loops.
Work is still in progress especially for personalizing and positioning the finite element model.
Experimental data will also be collected for validation.
Keywords: Comfort, Seat, Musculoskeletal model, FE model
4th International Digital Human Modeling conference http://dhm2016.etsmtl.net/home
2
1. Introduction
The average amount of time spent sitting on a daily basis is concerning for overall health in today’s society. North Americans spend on average 10 hours a day sitting (Holmes et al. 2015). Prolonged sitting leads to discomfort and musculoskeletal complaints (Vink and Hallbeck 2012), decrease seated workers efficiency (Hertzberg 1958), is a risk for Low back pain (LBP) (Lis et al. 2006) or even lead to pressure sores in the case of persons confined to a bed or wheelchair (Olesen et al. 2010). Previous literature predominantly aims at understanding static sitting discomfort (Zemp et al. 2015, Vink 2011, Harrison et al. 1999) and attempt to find objective discomfort criteria experimentally. The occupant/seat pressure is the most studied factor especially in the automotive industry. Since interface pressure is linked to soft tissues compression (Luboz et al. 2014), a poor distributed center of pressure distribution may lead to a deprivation of oxygen in the gluteal muscle and is generally considered as a factor causing seat discomfort. Maximum pressure thresholds were proposed by some investigators (Ciaccia and Sznelwar 2012,Conine et al. 1994) and discomfort criteria were proposed based on the whole pressure distribution on the seat pan (Hartung et al. 2006, Mergl et al. 2005). Seat pan shear force was also proposed as a discomfort factor. Several studies (Reichel 1958, Chow and Odell 1978,Goossens et al. 2000) showed that shear forces together with normal pressure might reduce blood flow and lead to a lack of oxygen in the tissue. A high spinal disc compression may lead to a lack of nucleus oxygenation during sitting and can trigger LBP ( Harrison et al. 2000, McBeth and Jones 2007). Disc compression is much higher in sitting than in standing or lying postures (Nachemson and Elfström 1970, Andersson et al. Nachemson 1977,, thus it should be minimized to reduce risk of LBP (Zenk et al. 2012). Finally a continuously constant muscular activity, even low (under 5% of maximum voluntary contraction) but maintained during a long time, might lead to muscular fatigue/discomfort, and should also be minimized (De Carvalho and Callaghan 2011, Rasmussen at al. 2007,Grujicic et al. 2010).
In the recent years, both deformable finite element (FE) (Choi et al. 2007, Siefert et al. 2008) and rigid multibody (MB) musculoskeletal (Rasmussen and Zee 2008) human models have been developed for assessing seating discomfort. FE models can estimate pressure and shear forces at the occupant/seat interface as well as the internal strains and stress in the soft tissues, while MB musculoskeletal models can estimate muscular activity and spinal joint loads. Ideally, a hybrid modeling approach combining both MB and FE human models is required in order to obtain all
parameters necessary for sitting discomfort assessment.
A full body FE model could be used to calculate contact forces and position as inputs for a MB model. However, due to the difficulties in personalizing and positioning a full body FE model, such a simulation is generally limited to small numbers of human models and postures (Siefert et al. 2008, Choi et al. 2007,…). A solution could be to model the thighs and the buttock as deformable while the rest of the body would be modeled as rigid multi-bodies; the external pressure distribution and shear forces on this area are main factors affecting sitting discomfort.
In the present work, two methods of coupling MB-FE models were tested and compared, using a whole body model for MB simulations and a buttock-thigh model for FE simulations.
2. Material and Methods
2.1 Finite element model of the buttocks
MRI images of the buttock-thigh complex of healthy subjects (males, 24 years, 174 cm, 66 kg) were obtained from a lying position with an angle of 130° between L5-S1, the hip joint and the knee joint (T1 weighted, resolution of 0.853*1.1*1.0 mm). The geometry of bones and soft tissues were obtained by segmentation. Soft tissues were meshed using four-node tetrahedral solid elements of a minimum size of 5 mm after a preliminary study of mesh convergence. The skin and bones were meshed with triangular shell elements of 5 mm (Figure 1). The bones were assumed rigid and soft tissues were attached to the bones. A ball and socket joint was introduced between the femur and the pelvis at the center of the head of the femur.
Figure 1: Mesh of the FE model
The soft tissues were modelled with a Mooney Rivlin hyperelastic isotropic material law. The material properties were defined from the literature, with the first two coefficients of the strain energy function being A1=1.65 kPa and A2= 3.35 kPa
2011, Grujicic et al. 2009). The skin was modeled as elastic linear with the Young modulus E= 0.15 MPa and Poisson ratio ν = 0.49 (Verver et al. 2004, Grujicic et al. 2009). The active component of the muscles was not modelled.
2.2 MB model
A full body model developed by Michael Daamsgard and John Rasmussen (Damsgaard et al. 2006, Rasmussen et al. 2009) called AAUHuman can be found in the AnyScript model repository (http://forge.anyscript.org/gf/). Its geometry was scaled to the FE model. The pelvis and femur model present in the AnyScript model repository were replaced by the one of the FE model. The length of other segments was linearily scaled to the subject used for the FE model. The cross sectional directions of each segment were scaled according to their segment mass. Muscle strength was scaled in proportion with the fat ratio calculated using the Body Mass Index.
2.3 Coupling MB and FE models
Two methods of coupling MB and FE models can be proposed for simulating an occupant seating on a seat.
For the two methods, the first step consists in finding the initial position of the MB model based on the seat geometry using inverse kinematics. Distances between marker points associated to body segments and the seat were specified. The position of the center of the ischial tuberosity was defined, the distance between the T2 vertebrae and the seat and distance between knee joint and the seat were set. Joint angles at the shoulder, knee and ankle were also imposed. This position (position of the lumbosacral joint, ischial tuberosity and knee joint) was then imposed to the FE model.
In the first method (Method 1), an inverse dynamic is conducted with Anybody to estimate the articular net forces and moments. The contact between the seat and the body was modeled with contact elements, which provides compressive reaction and friction force proportional to the reaction force in the element:
|Ff| ≤ μR
Ff is the friction force, R is the reaction force and µ
is the Coulomb friction coefficient. These elements are placed on multiple support points segment in contact. The reaction forces can be determined by considering them as unknown muscle forces. Force and moment calculated at the lumbosacral and knee joint centers were applied as boudary condition for the FE model. Once, a first FE simulation was
performed, the newly reached position of the pelvis and femur were imposed on the MB model. This loop was repeated until the the positions of pelvis and femur have converged.
In the second method (Method 2), the pelvis of the FE model was constrained in all directions and the femur was free to move. The FE simulation was performed by moving the seat pan vertically until the resultant contact force reaches the level estimated by the MB model. Since the pelvis position was fixed, MB-FE coupling loops were not necessary as required by Method 1.
3. A case study
In the present work, to test the two coupling methods, a simple case of a person sitting on a horizontal rigid plane was simulated with the feet being suspended.
The upper body position was defined by imposing an angle of 90° between the seat plane and the direction between the center of the ischial tuberosities and T2. The knee joint flexion was set to 90°. The knee joint center was positioned at 3 cm above the seat. The ischial tuberosities were maintained in contact with the seat surface. To fully locate the pelvis and spinal vertebrae, empirical relationships bewteen the spinal and pelvis joint angles were introduced in the Anybody seated man model. A ratio of 2:1 between hip flexion and pelvis-thorax flexion and a linear combination between the three rotation angles associated to each lumbar joint and the three angles associated to the T12-L1 joint was introduced (Rasmussen, Tørholm, and de Zee 2009). The arm was positionned along the vertical direction. Figures 3 and 4 show the imposed sitting posture for the MB and FE models respectively.
4 Figure 3: Initial position of the FE model. The net
force and moments applied on the sacrum and knee are also indicated
When using the first method, 10 loops were needed to reach a stable pelvis position (Figure 4) with a simulation of 50 ms in sitting for each loop, except for the first one with a sitting duration of 180 ms. The pelvis was re-positionned after the iteractive MB-FE simulations. A difference of 10 degrees in pelvic tilt was observed (Table 1). This led to slight different pressure distribution (Figure 5, Table 2) and muscle activity estimation (Table 3) between the two coupling methods.
Figure 4: Variation in the vertical position in meter of knee and lumbosacral joints during the 10 first
MB-FE iterative simulations
Table 1: Angle bewteen L5S1-Hip-Knee joints projected in the sagittal plane and ischial tuberositie to seat distance L5S1-Hip-Knee angle (degrees) Ischial-seat distance (mm) Method 1 87.4 21.5 Method 2 97.8 27,6
Figure 5: Pressure distributions by two MB-FE coupling methods (left: method 1, right: method 2)
Table 2: Occupant/seat pressure
Contact Area (cm²) Peak pressure (kPa) Average pressure (kPa) Method 1 332 40 1.99 Method 2 305 42 2.16
Table 3: Muscle Activity simulated by Anybody model in % of maximum voluntary contraction
Highest value in the
entire body Abdo muscles Back muscles Method
1 0.099 0.072 0.100
Method
2 0.140 0.099 0.122
The muscle activity is the muscular force divided by its strength. The strength is defined for each muscle as the force when the muscle is loaded to its maximum activity.
4. Discussion and conclusion
In the present work, two methods of coupling MB and FE models were compared for simulating occupant/seat interaction. Method 2 was used in most of the previous investigations by fixing the pelvis (Mergl et al. 2004, Lin et al. 2004, Luboz et
al. 2014). As no iteration was required, computation time was advantageously low, requiring the computation of 1h30 in a PC (HP 8 processors i7 CPU 2.90 GHz RAM 32 Go) instead of 6h45 for Method 1. In the present study, Method 1 was tested allowing a better consideration of the physical interaction between buttock and seat thus, correcting pelvis orientation initially estimated from geometric constraints. It is expected that the position of the pelvis corrected by Method 1 (once the simulation loops converged) to be more realistic. The corrected position should be checked by experimental observation. Furthermore, it should be noted that this corrected pelvic position was dependent on its initial position. The whole body was initially positioned by imposing some geometric constraints using the MB model. The initial boundaries conditions put into the FE model are strongly linked to this initial posture. The relationship between seat characteristics and body posture should be investigated in the near future.
It should also be noted that the actions of the upper body on the buttock-thigh FE model in Method 1 were only transmitted by the force and moments applied on the sacrum. Thus, the distribution of the forces applied at the section between upper body and pelvis was not considered. This may affect the tissue deformation and thus the contact pressure.
Moreover, the muscles were modeled as passive in the FE model, thus active muscle action was not considered in the stiffness between the pelvis and the femur. Only passive stiffness was taken into account due to the tissues surrounding the hip joint. The effect of muscle activation on the hip joint stiffness needs to be investigated.
Acknowledgement
This study was partially support by ZODIAC Seat France.
References
Andersson, G. B., R. Ortengren, and A. Nachemson. 1977. “Intradiskal Pressure, Intra-Abdominal Pressure and Myoelectric Back Muscle Activity Related to Posture and Loading.” Clinical Orthopaedics and
Related Research, no. 129 (December):
156–64.
Cheng, Zhiqing, Jeanne A. Smith, Joseph A. Pellettiere, and Scott M. Fleming. 2007. “Considerations and Experiences in Developing an FE Buttock Model for Seating Comfort Analysis.” SAE Technical Paper 2007-01-2458.
Warrendale, PA: SAE International. http://papers.sae.org/2007-01-2458/. Choi, Hyung Yun, Kyung Min Kim, Jiwon Han,
Sungjin Sah, Seok-Hwan Kim, Su-Hwan Hwang, Kwang No Lee, et al. 2007. “Human Body Modeling for Riding Comfort Simulation.” In Digital Human
Modeling, edited by Vincent G. Duffy,
813–23. Springer Berlin Heidelberg. http://link.springer.com/chapter/10.1007/9 78-3-540-73321-8_92.
Chow, W. W., and E. I. Odell. 1978. “Deformations and Stresses in Soft Body Tissues of a Sitting Person.” Journal of Biomechanical
Engineering 100 (2): 79–87. doi:10.1115/1.3426196.
Ciaccia, Flavia Renata Dantas Alves Silva, and Laerte Idal Sznelwar. 2012. “An Approach to Aircraft Seat Comfort Using Interface Pressure Mapping.” Work: A Journal of
Prevention, Assessment and Rehabilitation
41 (0): 240–45. doi:10.3233/WOR-2012-0163-240.
Conine, T. A., C. Hershler, D. Daechsel, C. Peel, and A. Pearson. 1994. “Pressure Ulcer Prophylaxis in Elderly Patients Using Polyurethane Foam or Jay Wheelchair Cushions.” International Journal of
Rehabilitation Research. Internationale Zeitschrift Für Rehabilitationsforschung. Revue Internationale De Recherches De Réadaptation 17 (2): 123–37.
Damsgaard, Michael, John Rasmussen, Søren Tørholm Christensen, Egidijus Surma, and Mark de Zee. 2006. “Analysis of Musculoskeletal Systems in the AnyBody Modeling System.” Simulation Modelling
Practice and Theory 14 (8): 1100–1111.
doi:10.1016/j.simpat.2006.09.001.
De Carvalho, Diana E., and Jack P. Callaghan. 2011. “Passive Stiffness Changes in the Lumbar Spine and Effect of Gender during Prolonged Simulated Driving.”
International Journal of Industrial Ergonomics 41 (6): 617–24. doi:10.1016/j.ergon.2011.08.002.
Goossens, Richard H. M., Martijn A. Pas, Robin Teeuw, and Chris J. Snijders. 2000. “Decubitus Risk: Is Shear More Important than Pressure?” Proceedings of the Human
Factors and Ergonomics Society Annual
Meeting 44 (28): 700–703.
doi:10.1177/154193120004402881. Grujicic, M., B. Pandurangan, G. Arakere, W. C.
Bell, T. He, and X. Xie. 2009. “Seat-Cushion and Soft-Tissue Material Modeling and a Finite Element Investigation of the Seating Comfort for Passenger-Vehicle Occupants.” Materials
6
& Design 30 (10): 4273–85. doi:10.1016/j.matdes.2009.04.028.
Grujicic, M., B. Pandurangan, X. Xie, A. K. Gramopadhye, D. Wagner, and M. Ozen. 2010. “Musculoskeletal Computational Analysis of the Influence of Car-Seat Design/adjustments on Long-Distance Driving Fatigue.” International Journal of
Industrial Ergonomics 40 (3): 345–55.
doi:10.1016/j.ergon.2010.01.002.
Harrison, D. D., S. O. Harrison, A. C. Croft, D. E. Harrison, and S. J. Troyanovich. 1999. “Sitting Biomechanics Part I: Review of the Literature.” Journal of Manipulative
and Physiological Therapeutics 22 (9):
594–609.
Harrison, Donald D., Sanghak O. Harrison, Arthur C. Croft, Deed E. Harrison, and Stephan J. Troyanovich. 2000. “Sitting Biomechanics, Part II: Optimal Car Driver’s Seat and Optimal Driver’s Spinal Model.” Journal of Manipulative and
Physiological Therapeutics 23 (1): 37–47.
doi:10.1016/S0161-4754(00)90112-X. Holmes, Michael W. R., Diana E. De Carvalho,
Thomas Karakolis, and Jack P. Callaghan. 2015. “Evaluating Abdominal and Lower-Back Muscle Activity While Performing Core Exercises on a Stability Ball and a Dynamic Office Chair.” Human Factors:
The Journal of the Human Factors and Ergonomics Society 57 (7): 1149–61.
doi:10.1177/0018720815593184.
Lin, F., B. Moran, J. Bankard, R. Hendrix, and M. Makhsous. 2004. “FEM Model for Evaluating Buttock Tissue Response under Sitting Load.” Conference Proceedings : ...
Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual Conference 7: 5088–91.
Lis, Angela Maria, Katia M. Black, Hayley Korn, and Margareta Nordin. 2006. “Association between Sitting and Occupational LBP.”
European Spine Journal 16 (2): 283–98.
doi:10.1007/s00586-006-0143-7.
Luboz, Vincent, Marion Petrizelli, Marek Bucki, Bruno Diot, Nicolas Vuillerme, and Yohan Payan. 2014a. “Biomechanical Modeling to Prevent Ischial Pressure Ulcers.”
Journal of Biomechanics 47 (10): 2231–
36. doi:10.1016/j.jbiomech.2014.05.004. ———. 2014b. “Biomechanical Modeling to
Prevent Ischial Pressure Ulcers.” Journal
of Biomechanics 47 (10): 2231–36.
doi:10.1016/j.jbiomech.2014.05.004. McBeth, John, and Kelly Jones. 2007.
“Epidemiology of Chronic Musculoskeletal Pain.” Best Practice &
Research Clinical Rheumatology, General
Musculoskeletal Conditions, 21 (3): 403– 25. doi:10.1016/j.berh.2007.03.003. Mergl, Christian, Tobias Anton, Ramon
Madrid-Dusik, Jürgen Hartung, Alessio Librandi, and Heiner Bubb. 2004. “Development of a 3D Finite Element Model of Thigh and Pelvis.” SAE Technical Paper 2004-01-2132. Warrendale, PA: SAE International. http://papers.sae.org/2004-01-2132/. Mergl, Christian, Margit Klendauer, Claude
Mangen, and Heiner Bubb. 2005. “Predicting Long Term Riding Comfort in Cars by Contact Forces Between Human and Seat.” SAE Technical Paper 2005-01-2690. Warrendale, PA: SAE International. http://papers.sae.org/2005-01-2690/. Nachemson, A., and G. Elfström. 1970. “Intravital
Dynamic Pressure Measurements in Lumbar Discs. A Study of Common Movements, Maneuvers and Exercises.”
Scandinavian Journal of Rehabilitation Medicine. Supplement 1: 1–40.
Olesen, Christian Gammelgaard, Mark de Zee, and John Rasmussen. 2010. “Missing Links in Pressure Ulcer research—An Interdisciplinary Overview.” Journal of
Applied Physiology 108 (6): 1458–64.
doi:10.1152/japplphysiol.01006.2009. Rasmussen, John, Søren Tørholm, and Mark de
Zee. 2009. “Computational Analysis of the Influence of Seat Pan Inclination and Friction on Muscle Activity and Spinal Joint Forces.” International Journal of
Industrial Ergonomics 39 (1): 52–57.
doi:10.1016/j.ergon.2008.07.008.
Rasmussen, John, and Mark de Zee. 2008. “Design Optimization of Airline Seats.” SAE Technical Paper 2008-01-1863. Warrendale, PA: SAE International. http://papers.sae.org/2008-01-1863/. Rasmussen, John, Mark de Zee, and Søren
Tørholm. 2007. “Muscle Relaxation and Shear Force Reduction May Be Conflicting: A Computational Model of Seating.” SAE Technical Paper 2007-01-2456. Warrendale, PA: SAE International. http://papers.sae.org/2007-01-2456/. Reichel SM. 1958. “SHearing Force as a Factor in
Decubitus Ulcers in Paraplegics.” Journal
of the American Medical Association 166
(7): 762–63.
doi:10.1001/jama.1958.62990070004010a. Siefert, A., S. Pankoke, and H. -P. Wölfel. 2008. “Virtual Optimisation of Car Passenger Seats: Simulation of Static and Dynamic Effects on Drivers’ Seating Comfort.”
International Journal of Industrial Ergonomics 38 (5–6): 410–24. doi:10.1016/j.ergon.2007.08.016.
Siefert, S., S. Pankoke, and H. P. Wölfel. 2011. “Volumetric Muscle Approach for Human-Model CASIMIR.” In
Proceedings of the 1st IEA-DHM Conference.
http://media.univ-lyon1.fr/iea-dhm2011/abstracts/2265.pdf.
Verver, M.M., J. van Hoof, C.W.J. Oomens, J.S.H.M. Wismans, and F.P.T. Baaijens. 2004. “A Finite Element Model of the Human Buttocks for Prediction of Seat Pressure Distributions.” Computer Methods in Biomechanics and Biomedical Engineering 7 (4): 193–203. doi:10.1080/10255840410001727832. Vink, Peter. 2011. Aircraft Interior Comfort and
Design. CRC Press.
Vink, P., and S. Hallbeck. 2012. “Editorial: Comfort and Discomfort Studies Demonstrate the Need for a New Model.”
Applied Ergonomics 43 (2): 271–76.
doi:10.1016/j.apergo.2011.06.001.
Zemp, Roland, William R. Taylor, and Silvio Lorenzetti. 2015. “Are Pressure Measurements Effective in the Assessment of Office Chair Comfort/discomfort? A Review.” Applied Ergonomics 48 (May): 273–82.
doi:10.1016/j.apergo.2014.12.010.
Zenk, R., M. Franz, H. Bubb, and P. Vink. 2012. “Technical Note: Spine Loading in Automotive Seating.” Applied Ergonomics 43 (2): 290–95. doi:10.1016/j.apergo.2011.06.004.
