**5.2 Generating a default human body model**

174 Haptics Rendering and Applications

the similar deformation of spine in the offline FEA simulator for greater detail. By this means, the user can obtain desired spine deformations conveniently in the haptic simulator and then observe every detail of stress/strain of vertebrae in the offline FEA simulator.

A straightforward way to reproduce the same deformation in the offline FEA simulator is to record the position and orientation of each vertebra along the spine and then apply the positional constraints to every vertebra of the offline spine model. However, this greatly increases the risk of finite element computation error because of too many constraints involved. In practice, we found this method almost always makes the FEA solver fail to converge. Another plausible way is to record the force vector and the vertebrae under the load in the haptic simulator and reproduce the same load condition in the offline FEA simulator. However, the deformation of spine is not solely determined by the load due to the high non-linearity of the spine. Plus, during surgery, the forces are often not directly applied to a vertebra; instead, surgeons often manipulate instruments, which are linked to

Our solution is firstly to record the position and orientation of vertebrae which are directly under the external forces or are constrained during the haptic simulation, then apply the position and orientation constraints only to these vertebrae in the offline FEA simulator. The position and orientation of a vertebra can be represented with the positions of point A ~ E of the vertebrae's beam elements model (shown in Figure 8). The procedure of reproducing the

b. Build the transformation matrix by comparing the original position and the current

Recently, many software applications have been developed for impact simulation, ergonomics, comfort study, biomechanical analysis and surgical planning. Such software enables users to perform human body modeling and interaction with the environment where the human motion and muscle forces can be simulated. These tools are very useful for simulating the human-machine behavior simultaneously. LifeMOD from Biomechanics

The LifeMOD Biomechanics Modeler is a plug-in module to the ADAMS (Automatic Dynamic Analysis of Mechanical Systems) physics engine, produced by MSC Software Corporation to perform multi-body analysis. It provides a default multi-body model of the skeletal system that can be modified by changing anthropometric sizes such as gender, age, height, weight etc. The created human body may be combined with any type of physical environment or system for full dynamic interaction. The results of the simulation are the human motion, internal forces exerted by soft tissues (muscles, ligaments, joints) and contact forces at the desired location of the human body. Full information on the LifeMOD

Research Group is a leading simulation tool that has been designed for this purpose.

a. Locate the vertebrae under external forces in the haptic real-time simulation.

c. Transform the vertebrae and set the constraints in the offline FEA simulator.

one or multiple vertebrae. Therefore, this method is not feasible.

deformed spine in the offline FEA simulator is as follows:

position of point A ~ E of the vertebrae.

**5. Human spine modeling in LifeMOD** 

**5.1 Introduction to LifeMOD simulation software** 

Biomechanics Modeler can be found online (LifeMOD).

d. Simulate the deformation.

The usual procedure of generating a human model is to create a set of body segments followed by redefining the fidelity of the individual segments. The body segments of a complete standard skeletal model are first generated by LifeMOD depending on the user's anthropometric input. The model used in this study was a median model with a height of 1.78 m and a weight of 70 kg created from the internal GeBod anthropometric database. By default, LifeMOD generates 19 body segments represented by ellipsoids. Then, some kinematic joints and muscles are generated for the human model. Figure 10 shows the base model in this study.

Fig. 10. Default human body model
