Author's personal copy Developing a musculoskeletal model of the primate skull: Predicting muscle activations, bite force, and joint reaction forces using multibody dynamics analysis and advanced optimisation methods Junfen Shi a , Neil Curtis b , Laura C Fitton c , Paul O’Higgins c , Michael J Fagan d,n a Department of Engineering, University of Hull, Hull, UK b Department of Engineering, University of Hull, Hull, UK c Centre for Anatomical and Human Sciences, Hull-York Medical School, The University of York, York, UK d Department of Engineering, University of Hull, Hull, UK HIGHLIGHTS c We describe a new musculoskeletal computer model of a primate (Macaque) skull. c It predicts bite force, TMJ forces and muscle recruitment patterns during biting. c The results compare very well with experimental in vivo EMG and bite force data. c They reveal the complexity of muscle recruitment even during simple biting. article info Article history: Received 17 January 2012 Received in revised form 1 June 2012 Accepted 5 June 2012 Available online 18 June 2012 Keywords: Skull Musculoskeletal model Multibody dynamics analysis Primate Force abstract An accurate, dynamic, functional model of the skull that can be used to predict muscle forces, bite forces, and joint reaction forces would have many uses across a broad range of disciplines. One major issue however with musculoskeletal analyses is that of muscle activation pattern indeterminacy. A very large number of possible muscle force combinations will satisfy a particular functional task. This makes predicting physiological muscle recruitment patterns difficult. Here we describe in detail the process of development of a complex multibody computer model of a primate skull (Macaca fascicularis), that aims to predict muscle recruitment patterns during biting. Using optimisation criteria based on minimisation of muscle stress we predict working to balancing side muscle force ratios, peak bite forces, and joint reaction forces during unilateral biting. Validation of such models is problematic; however we have shown comparable working to balancing muscle activity and TMJ reaction ratios during biting to those observed in vivo and that peak predicted bite forces compare well to published experimental data. To our knowledge the complexity of the musculoskeletal model is greater than any previously reported for a primate. This complexity, when compared to more simple representations provides more nuanced insights into the functioning of masticatory muscles. Thus, we have shown muscle activity to vary throughout individual muscle groups, which enables them to function optimally during specific masticatory tasks. This model will be utilised in future studies into the functioning of the masticatory apparatus. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Functional analysis of the primate skull is relevant to evolu- tionary, biomechanical, functional, and clinical sciences. However, to truly understand the mechanical function of the bony skull we must have a grasp of the soft tissue forces that act upon it. Computer modelling and simulation can more readily provide detailed quantitative data on musculoskeletal system functioning than experimental procedures alone, and can also be used to predict parameters that are difficult, if not impossible, to record in vivo (Curtis et al., 2009, 2010; Koolstra, 2002; Koolstra and van Eijden, 1997, 2001 Langenbach and Hannam, 1999; Sellers and Crompton, 2004; Shi et al., 2009). Furthermore, societal pressures Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/yjtbi Journal of Theoretical Biology 0022-5193/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jtbi.2012.06.006 n Corresponding author. Tel.: þ44 1482 465058; fax: þ44 1482 466664. E-mail addresses: j.shi@hull.ac.uk (J. Shi), n.curtis@hull.ac.uk (N. Curtis), laura.fitton@hyms.ac.uk (L. Fitton), paul.ohiggins@hyms.ac.uk (P. O’Higgins), m.j.fagan@hull.ac.uk (M. Fagan). Journal of Theoretical Biology 310 (2012) 21–30