Real-Time Musculoskeletal Model for Injury Simulation on 3D Human Characters MIG2012.pdf

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Real-Time Musculoskeletal Model for
Injury Simulation on 3D Human Characters
Francis Lacl´ and Nicolas Pronost
e
Utrecht University, Information and Computing Sciences,
Princetonplein 5, 3584 CC Utrecht, The Netherlands
francislacle@gmail.com
nicolas.pronost@uu.nl
http://www.uu.nl
Abstract.
We describe a real-time musculoskeletal model created for
the purpose of simulating localized injuries on animated 3D human char-
acters. The baseline experiment will include projectile injuries that are
compared and validated against experimental data from the science of
ballistics. The research will also include an injury assessment model suit-
able for motion editing purposes that looks at the total combined injury
from separate biological layers such as bone, muscle, and fat tissues.
Keywords:
musculoskeletal model, injury simulation, human character,
OpenSim
1
Introduction
Human character animation affected by musculoskeletal injuries in real-time
might be considered a next step for interactive media such as video games. It adds
one extra layer of realism to the expected behavior of the human character while
interacting with its virtual environment. For instance, assessment of an injury
would influence the amount of DOFs that a character is able to perform. Because
our injury assessment model focuses mostly on the geometrical component, it
could be added on top of biologically-based locomotion controllers such as [1]
thereby increasing the realism of the dynamic computations to better reflect the
character’s current physical circumstance.
The first goal is to design a technique that can be used to simulate a mus-
culoskeletal model that is prone to damage by external forces and how such
interactions can be accomplished in real-time. The methods used in this tech-
nique will be validated through comparison with experimental data from other
areas including ballistics. The second goal would be to create a combined injury
assessment model that could be used for motion editing purposes in the future.
For instance, looking at the amount of injuries sustained at the muscle layer
would determine the amount of torque that can be generated at relevant joint
angles within the newly injured configuration.
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Francis Lacl´, Nicolas Pronost
e
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The Musculoskeletal Model
As input for the musculoskeletal model, biomechanical data was chosen from the
pivotal work of [2] that includes physical properties such as optimal fiber lengths
for each muscle and muscle origin and insertion points relative to parent joints.
For the rigid skeletal meshes, these were borrowed from [3] that were created for
the OpenSim software and includes models for both upper and lower extremities
of the musculoskeletal system. However, only the lower extremity is currently
used as a first tryout. The soft muscle meshes are currently constructed from
simple cylinders during run-time, similar to the work of [4]. Currently, each
muscle consists of up to four cross sections, each section is divided into six
patches of two triangles each. The area specified on each cross section can be
varied resulting in the fusiform shapes seen in Figure 1. Therefore, modifying
the vertices at each cross section gives the possibility to modify the volume in
real-time.
Fig. 1.
Screenshot of preliminary musculoskeletal model including data from [3].
3
Muscle Shape Dynamics and Injury Simulation
Within the active musculoskeletal system, muscles would have to deform in real-
time with physical constraints such as volume preservation. Model constraints
defined by via points that are present in [3] would also have to be taken into ac-
count. Furthermore, penetration with the skeleton, other muscles, and character
mesh should be avoided to maintain a realistic anatomical configuration fit for
injury simulation.
Except for the skeletal system, the human muscular system comprises of
mostly elastic material. This elastic property requires the use of techniques that
Real-Time Musculoskeletal Model for Injury Simulation
3
can simulate soft-body dynamics. The most popular method, the finite element
method suffers from high computational costs due to the computational com-
plexity of its solver [5]. Several advances have been made on this regard, such as
[6], [7], and [8]. However, other methods such as the boundary element method or
a more simpler spring-mass model such as the one given in [9] could be adapted
for this model.
With respect to injury simulation, as of this writing the baseline experiment
will consist of projectile based injuries i.e. bullet wounds. This is due to experi-
mental data that is available from the ballistic research that allows for validation
of the technique through comparison with experiments such as the ones carried
out on calibrated gelatin models. To give an example, ballistic research from [10]
explores the distinctive features of bullet wounds such as temporary and perma-
nent cavities that are dependent on the design of the bullet, seen in Figure 2.
This dependency will help in the validation of the technique by comparing the
results of the simulation with several types of bullet designs. Furthermore, be-
cause each bullet type leaves behind a specific damage signature, a boundary can
be obtained from experimental data that limits the volume that the simulation
has to solve thereby increasing real-time performance.
Fig. 2.
Wound profile taken from [10] depicts the effects of a 7.62 NATO cartridge that
is loaded with a soft-point hunting bullet. The profile illustrates among other things
that the permanent cavity expands more than twice the diameter of the original bullet
and that the bullet itself loses about one third of its mass due to fragmentation. A
process where small pieces of the bullet gets separated due to deformation of the bullet
upon impact.
Finally, an injury assessment method will be developed that looks at the
combined inflicted damage on three separate layers, namely the fat, muscle,
and skeletal layers. For the skeletal layer, the amount of bone tissue disruption
could give an indication on the amount of force that cannot be transferred to
other connected bone segments. Regarding the muscle layer, instead of trying
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Francis Lacl´, Nicolas Pronost
e
to simulate bundles of muscle fibers in real-time, an approximation can be used
which takes the angle between two vectors; the vector that defines the general
direction of muscle fibers within a fiber bundle and another vector that defines
the general direction of the passed projectile. This angle consequently indicates
the amount of muscle fibers that have been damaged proportionally and could be
used to account for the amount of force that cannot be exerted from the injured
muscle. This angle is referred to in biomechanical literature as the
pennation
angle.
With respect to fat tissue, more research is required from biomechanics
to see whether a method could be created that links damage of fat tissue with a
decrease in human movement performance. If no suitable link can be found, fat
tissue will only serve as an added representation layer within the inner anatomy
of the human character.
Acknowledgement.
This work is supported by the Dutch research project
COMMIT - Virtual Worlds for Well-Being.
References
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