ARTICLE IN PRESS JID: JJBE [m5G;June 27, 2017;14:49] Medical Engineering and Physics 000 (2017) 1–9 Contents lists available at ScienceDirect Medical Engineering and Physics journal homepage: www.elsevier.com/locate/medengphy Influence of spinal disc translational stiffness on the lumbar spinal loads, ligament forces and trunk muscle forces during upper body inclination Rizwan Arshad, Thomas Zander, Maxim Bashkuev, Hendrik Schmidt * Julius Wolff Institute, Charité – Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany a r t i c l e i n f o Article history: Received 10 April 2016 Revised 4 May 2017 Accepted 27 May 2017 Available online xxx Keywords: Inverse statics Musculoskeletal modeling Spinal loads Spinal disc stiffness Muscle forces a b s t r a c t Inverse dynamic musculoskeletal human body models are commonly used to predict the spinal loads and trunk muscle forces. These models include rigid body segments, mechanical joints, active and passive structural components such as muscles, tendons and ligaments. Several studies used simple definition of lumbar spinal discs idealized as spherical joints with infinite translational stiffness. The aim of the current sensitivity study was to investigate the influence of disc translational stiffness (shear and compressive stiffness) on the joint kinematics and forces in intervertebral discs (L1-L5), trunk muscles and ligaments for an intermediately flexed position (55°). Based on in vitro data, a range of disc shear stiffness (100-200 N/mm) and compressive stiffness (1900-2700 N/mm) was considered in the model using the technique of force dependent kinematics (FDK). Range of variation in spinal loads, trunk muscle forces and ligaments forces were calculated (with & without load in hands) and compared with the results of reference model (RM) having infinite trans- lational stiffness. The discs’ centers of rotation (CoR) were computed for L3-L4 and L4-L5 motion seg- ments. Between RM and FDK models, maximum differences in compressive forces were 7% (L1-L2 & L2-L3), 8% (L3-L4) and 6% (L4-L5) whereas in shear forces 35% (L1-L2), 47% (L2-L3), 45% (L3-L4) and more than 100% in L4-L5. Maximum differences in the sum of global and local muscle forces were approx- imately 10%, whereas in ligament forces were 27% (supraspinous), 40% (interspinous), 56% (intertrans- verse), 58% (lig. flavum) and 100% (lig. posterior). The CoRs were predicted posteriorly, below (L3-L4) and in the disc (L4-L5). FDK model predicted lower spinal loads, ligament forces and varied distribution of global and lo- cal muscle forces. Consideration of translational stiffnesses influenced the model results and showed in- creased differences with lower stiffness values. © 2017 IPEM. Published by Elsevier Ltd. All rights reserved. 1. Introduction Inverse dynamic musculoskeletal models are frequently used to predict the joints reactions, bending moments and trunk muscle forces for different body movements and postures [1,2]. These find- ings are helpful in many areas such as work place safety design, ergonomics, injury prevention, performance enhancement, implant design and rehabilitation management. Due to large scale of mus- culoskeletal multibody models, involving several rigid body seg- ments connected by mechanical joints, including muscle tendon complexes and ligaments, many of the modeling aspects are sim- * Corresponding author. Julius Wolff Institute, Charité – Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany E-mail address: hendrik.schmidt@charite.de (H. Schmidt). plified. One such simplification is the idealization of the interverte- bral discs as spherical joints having 3 rotational degrees of freedom (DoF) and infinite translational stiffness (zero translations). In real- ity, intervertebral discs have complex structure [3] which makes it flexible in all directions with a high load bearing capacity and pro- vide ability to deform against the internal and external loads [4]. During large movements such as upper body flexion, lumbar spinal discs deform and allow relative translations of the verte- bral bodies such as in anterior – posterior direction ranging from 1 to 4 mm and axial direction up to 2 mm [5]. Recently, Ghezelbash et al. showed that ignoring translational flexibilities influences the prediction of spinal loads and muscle forces up to moderate level for larger inclinations [6]. The study was based on the comparison between a specific nonlinear finite element model with a single set of model properties and other two models (stiffened finite element http://dx.doi.org/10.1016/j.medengphy.2017.05.006 1350-4533/© 2017 IPEM. Published by Elsevier Ltd. All rights reserved. Please cite this article as: R. Arshad et al., Influence of spinal disc translational stiffness on the lumbar spinal loads, ligament forces and trunk muscle forces during upper body inclination, Medical Engineering and Physics (2017), http://dx.doi.org/10.1016/j.medengphy.2017.05.006