Sitting Biomechanics Laboratory Part of the Sensory Motor Performance Program Room 1340, Rehabilitation Institute of Chicago

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Development of a Finite Element Model for Sitting Posture

	Obtain Model Geometry from MRI Scans in Simulated Sitting Posture Experimental setup for recording MRI images from buttock-thigh area in a simulated sitting posture with simulated sitting load applied. Upper row: MRI setup to measure buttock-thigh structure under two loading configuration (Left: Without Sitting Pressure; Right: With Sitting Pressure) for the simulated sitting posture. Two belts tied the cushion with the buttocks. An air bladder was placed in between of the 2 layers of the cushion. Inflation and deflation of the air bladder provided different loading levels. Lower row: Corresponding MRI images
	Reconstructed Model Geometry and Meshes Final FE model: Structural elements, i.e. bones (pelvis and femur), skin, fat, and muscle groups were created based on the 3D reconstruction of the buttock-thigh structure obtained from MRI images under the “Without Sitting Pressure” configuration. The five muscle groups were as follows: Group1: Minimus, Medius and Maximus Gluteus; Group2: Adductor Longus, Adductor Brevis, Adductor Magnus, and Pectineus; Group3: Biceps Femoris (long head), Semitendinosus, and Gracilis; Group4: Piriformis, Superior and inferior Gemellus and Obturator Internus; Group5: Vastus Lateralis, Vastus Intermedius, Vastus Medialis and Rectus Femoris.
	Boundary Conditions Boundary and loading conditions used in the FE model. A) Boundary conditions (BC). BC1: The femur and the pelvis were assumed as rigid bodies and constrained to obviate rigid body motion; BC2: The upper plane and the medial plane were constrained against the anterior-posterior and medial-lateral motions; BC3: The ends of tissues that connect to the rest of the body (distal end to thighs, proximal end to lumbar region) were constrained against longitudinal motions. B) General loading conditions (GLC) and the input load (Specific loading condition) for the model. GLC1: A “skin initial strain” was applied into the skin layer of the FE model; GLC2: A “muscle tone”, which was 1% of maximal muscle force estimated from Fmax = ?max × Physiological Cross-Sectional Area (PCSA), was applied along the line of action of each muscle. In the above equation, ?max is the maximal muscle tension [22]; Input load (specific loading condition): an interface pressure of 20.34kPa was applied to the buttock sitting surface of the model.
	Inclusion of Initial Skin Tension Measurement of the buttock skin initial tensile strain induced by a sitting posture relative to a standing posture. The measurement was performed on the subject in the same hip and knee joint configuration as that in the sitting posture used to record MRI images, i.e. 80º hip flexion and 90º knee flexion. The buttock skin initial strain was measured by images of an array of dots painted on the buttock skin. Green strain [21] was calculated for an area of 15×15 mm2.

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Last updated May 25, 2008 by Makhsous.
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