Biomedical Engineering Reference
In-Depth Information
(so-called antagonistic muscles). In physics-based character animation, use of
muscle-based actuation models is uncommon, because of the increased number of
DOFs that require control and decreased simulation performance [
62
]. However,
examples of muscle based actuation do exist, and we witness an increased interest
in using more advanced muscle-based actuation models for controlling physics-
based characters [
63
-
65
]. This reflects the need for more accurate anatomical virtual
humans.
In one of the latest work to date, Wang et al. [
66
] propose a biologically-motivated
locomotion controller. Their lower-body model is actuated by sixteen Hill's type
MTUs (see Sect.
6.2.2.2
). To determine muscle excitation patterns, biologically-
motivated laws are used for muscle control, and stance and swing phases. The
parameters of these control laws are set by an optimization procedure that satis-
fies a number of locomotion task terms while minimizing a biological model of
metabolic energy expenditure. This work demonstrates the importance of modeling
constraints on torque generation due to muscle physiology, both in restricting the
space of possible torque trajectories and in providing a realistic model of effort.
6.3.2 Deformable Body Simulation
The human body consists of intricate deformable tissues. To achieve realistic anima-
tion and to study biomechanics of deformation in medical applications, the realistic
deformation of the human body system is required. Several approaches have been
proposed to model human body deformations. As the emphasis of this chapter is on
challenges in biomechanics and animation, we classified modeling approaches into
non-physically based methods and physically based methods.
6.3.2.1 Non-Physically Based Methods
Non-physically based methods are useful methods in many applications especially
when a high level of geometric control is needed. They usually use simplified physical
principles to achieve visually appealing results. The most important non-physically
based methods described here are based on surface data (parametric and polygonal
surface and implicit surface) and free-form deformation.
3D Surface
Parametric and polygonal surfaces can be used to model deformable bodies. One
method to model deformation is using
splines
. Splines are used as a tool to create
and interpolate curves and surfaces mainly in the field of computer aided geometric
design. This technique is based on the representation of both planar and 3D curves
and surfaces by a set of control points or landmarks. Bezier curves are widely used
to model smooth curves. The curve is completely contained in the convex hull of its
control points which can be graphically displayed and used to manipulate the curve
