Procedia IUTAM Procedia IUTAM 00 (2011) 1–9 2011 Symposium on Human Body Dynamics Musculoskeletal Morphing from Human to Mouse Yoshihiko Nakamuraa,∗, Yosuke Ikegamia, Akihiro Yoshimatsua, Ko Ayusawaa, Hirotaka Imagawaa, and Satoshi Ootab aDepartment of Mechano-Informatics, Graduate School of Information and Science and Technology, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, Japan bBioresource Center, Riken, 3-1-1 Takanodai, Tsukuba-shi, Ibaragi, Japan Abstract The analysis of movement provides various insights of human body such as biomechanical property of muscles, function of neural systems, physiology of sensory-motor system, skills of athletic movements, and more. Biomechan- ical modeling and robotics computation have been integrated to extend the applications of musculoskeletal analysis of human movements. The analysis would also provide valuable means for the other mammalian animals. One of current approaches of post-genomic research focuses to find connections between the phenotype and the genotype. The former means the visible morphological or behavioral expression of an animal, while the latter implies its genetic expression. Knockout mice allows to study the developmental pathway from the genetic disorders to the behavioral disorders. Would musculoskeletal analysis of mice also offer scientific means for such study? This paper reports our recent technological development to build the musculoskeletal model of a laboratory mouse. We propose mapping the musculoskeletal model of human to a laboratory mouse based on the morphological similarity between the two mammals. Although the model will need fine adjustment based on the CT data or else, we can still use the mapped musculoskeletal model as an approximate model of the mouse’s musculoskeletal system. The preliminary results of muscle length analysis is shown for the motion captured data of a hairless mouse. Keywords: Musculoskeletal Model; Phenotype Analysis; Homology Mapping; Laboratory Mouse; Morphing Mammalian Bones 1. Introduction Mice are useful laboratory animals for biomedical studies. Mice are classified to early mammalia in taxology and have close biological relationship with human. Bifurcation on the evolutional tree from mice toward human started 75 million years ago[1]. Homology of the two species is the reason of importance of mice as laboratory animals. The completion of decoding the whole genome was declared for mouse in 2002 and for human in 2003. The two species are reported 90% of genome in common and show 70% homology in their phenotypes[1]. The whole database including the mouse genome is made available in the WEB as the Mouse Genome Informatics[2]. Recent development of the genetically modified mouse and low cost of breeding and managements in laboratories have increased the importance ∗Corresponding author. Tel: +81-3-5841-6379; fax: +81-3-5841-7916 Email address: [email protected] (Yoshihiko Nakamura) Yoshihiko Nakamura, Yosuke Ikegami, Akihiro Yoshimatsu, Ko Ayusawa, Hirotaka Imagawa, and Satoshi Oota / Procedia IUTAM 00 (2011) 1–9 2 of laboratory mouse as a model animal for studies in medical and pharmaceutical fields [3]. In genetic disorders, it is considered a useful approach to transfer the knowledge from mouse experiments to human. Observation of phenotype is commonly done for cell development and for social behaviors of individual mice. More quantitative observation is to be of critical importance for biomechanical evaluation of the musculoskeletal sys- tem. Technological developments are most demanded for such qualitative biomechanical observation of the laboratory mouse. A model of musculoskeletal system is necessary for analysis of musculoskeletal phenomena. The model should include the skeletal model for kinematic analysis and the musculotendinous model for analysis of motor function. While there are some literatures on the skeletal system of mouse [4], there are not many works done and published on modeling of musculotendinous system of mouse or even of the other mammalian animals. As it is done for human, MRI and CT imaging is the main source of anatomical information. Since resolution of MRI imaging depends on the relative scale of animals and excitation coils, MRI imaging has technical difficulties for small animals. X-ray CT scanning is more useful for small laboratory animals due to the additional fact that the problem of radiation exposure is not too critical for such animals. The ANR project conducts musculoskeletal analysis for laboratory rat based on the X-ray CT scanning [5]. More recently, Oota et al.[6],[7] developed the model of skeletal system of laboratory mouse based on the X-ray CT scanning, while modeling of the whole musculotendinous system is still a future problem. On the other hand, there are many publications on the modeling and analysis of human musculoskeletal system based on the rich accumulation of anatomical and physiological knowledges [8],[9]. Nakamura et al. developed the wholebody model of the human musculoskeletal system that is appropriate and consistent with the algorithms of robotic kinematic and dynamic computation[10],[11][12][13] and proposed an optimization algorithm to estimate muscle activities based on the information from a motion capture system and the other sensory systems. The developed software and measurement system have been applied for analysis of human motions. The paper describes on a systematic method to build a musculotendinous model of a mammalian animal, specif- ically of a laboratory mouse in this paper. The use of MRI/CT image is assumed for fine anatomical agreements. However, our main proposal is to use the human musculotendinous model, that is developed in precision, and transfer it to build the preliminary model of another mammalian animal before fine anatomical adjustments from the MRI/CT image data. This approach relies on the homology between mammalian species. The proposed method consists of two consecutive steps. The first step is to find a geometrical morphing map from one bone of the human skeletal system to the corresponding one of another mammalian species, namely laboratory mouse in this paper. The second step uses the geometrical morphing maps of bones to transfer the terminal points and via-points of the elements of human musculotendinous system to those of mouse musculotendinous system. This paper also shows the results of muscle length analysis from the motion captured data of a mutant nude mouse. More detailed analysis of muscle activities will need anatomical refinements of the musculotendinous system. Contact measurements and analysis will also need be solved in future studies. The related technologies would be useful such as identification of mass properties of the body segments[14][15][16][17][18], and estimation of neuro- muscular activities[19][20]. 2. Of Mice and Men 2.1. Model of the human musculoskeletal system The human musculoskeletal system developed by Nakamura et al. [13] consists of the main elements to actuate 150 DOF of the whole body skeletal system, which excludes the DOF in the hands, feet, and head. The components as the objects of biomechanical analysis are currently 1206 in total, consisting of 997 muscles, 50 tendons, 125 ligaments, and 34 cartilages. We adopted the human musculoskeletal system in Fig.1 as a model of human to be used for mapping between skeletal systems and transformation of musculotendinous system. The skeletal system is based on an adult Caucasian model [21]. The model includes about 200 bones in total, which are grouped to make 50 segments connected by kinematic spherical joints. A bone is geometrically modeled as a polygonal object. Each of muscles, tendons, ligaments, and cartilages is modeled by a wire with the point of origin, the via points, the point of insertion on links. A link is either a link of bone segment or a virtual link. A link of bone segment has mass, while a virtual link doesn’t. The virtual links are fictitious links introduced for representation of connection or bifurcation of wires. The virtual links are floating links. The mass of whole body is distributed among the bone segments. Yoshihiko Nakamura, Yosuke Ikegami, Akihiro Yoshimatsu, Ko Ayusawa, Hirotaka Imagawa, and Satoshi Oota / Procedia IUTAM 00 (2011) 1–9 3 Soleus Virtual link Achilles tendon Fig. 1: Overview of Human Full Body Muscu- Fig. 2: Conceputual Example of Virtual Fig. 3: Mouse Skeleton Model: this poly- loskeletal Model: this model is provided by Y. Link gon data is provided by S. Oota, et al.[7] Nakamura, et al.[11] An example of the complex of soleus and Achilles tendon is shown in Fig.2. Achilles tendon has the origin on calcaneal tuberosity and the insertion on a virtual link. Soleus is represented by a wire with the origin on the complex of tibia and fibula and the insertion on the same virtual link. 2.2. Model of the mouse skeletal system The polygon model of the mouse skeletal system developed by Oota et al. [7] was used, by courtesy of the researchers. The model was made through segmentation and polygon reduction from the volume data obtained by a high precision X-ray CT scanner in University of Texas at Austin. The skeletal mouse system is shown in Fig.3. 2.3. Distinction of skeletal systems of human and mouse The principle of mapping skeletal systems is based on the homology between mammalian species. Although mouse and human has a large difference in scale, taxonomical difference is not too far within mammals. According to Simpson[22] human and mouse are in the same branch of Euarchontoglires within the tree of life of Mammals[23]. Namely, • (Euarchontoglires (Superorder: Glires (Order: Rodentia (Family: Muridae (Mouse))))) • (Euarchontoglires (Superorder: Euarchonta (Order: Primates (Family: Hominidae (Homo sapiens))))) It is important to understand the difference in skeletal geometry between human and mouse. The main difference is in the number of bones in coccyx. The coccyx of human consists of 3-5 bones of accretion, while that of mouse consists of 25-27 separate bones. The structure of spine is also noteworthy.