Biomedical Engineering Reference
In-Depth Information
clavicular, scapular and arm joints has three DOFs for rotation, and the scapula is
bound to the thorax bymeans of a fiveDOFs joint (three rotations and two translations
constrained to the surface of the thorax).
6.2.2.2 The Muscles, Tendons and Ligaments
Muscles are the active tissues that generate forces and cause motion. There are three
different types of muscle in the body: cardiac, smooth and skeletal muscles.
Skeletal
muscles
play a major role in motion. Their contraction is controlled through the
somatic nervous system and this contraction produces active muscle forces that lead
to postural stability and human bodymovement. Skeletal muscles are alsomajor body
components in size and mass, and partly characterize the body shape. Hence they
constitute a critical part of biomechanics analysis and animation modeling. Skeletal
muscles are attached to the bones by
tendons
while
ligaments
connect bones to bones.
Tendons transmit the contractile force produced by muscle contraction to the bones
and improve stability.
Skeletal muscles are composed of
muscle fiber bundles
called fascicles which
themselves consist of internal fibers. Different internal arrangements of the muscle
fascicles make different
muscle architectures
. In the simplest architecture, fascicles
are all parallel to the length of the muscle. But in most of human muscles there is
an angle between the fascicle's tendinous attachment direction and the longitudinal
axis of the muscle. This angle is called
pennation angle
.
The earliest musculoskeletal mathematical model of skeletal muscle was sug-
gested by Gasser and Hill [
15
]. That model was a one-dimensional representation of
the muscle, called
action line
, and captured the global muscle mechanical properties.
This model allowed later to describe the macroscopic relationships between muscle
actuation, force-length and velocity along a muscle path, known as the
Hill's type
model
[
16
]. In this model, the muscle is modeled by three components including the
series element (SE), the parallel element (PE), and the contractile element (CE). A
skeletal muscle is considered as a large sarcomere that is the contractile element and
accounts for producing active muscle force which is dependent on the muscle length
and time-varying neural signal. The series element represents the additional passive
viscoelastic properties contributing for the tendon and aponeuroses. And the parallel
element represents the behavior of the connective tissues epimysium, perimysium
and endomysium. This model describes the contraction force of a muscle as the sum
of the three elements.
The Hill's type model was improved by Zajac [
17
] to a dimensionless aggre-
gate model which can be scaled to represent subject specific musculo-tendon units
(MTUs). The force components are modeled from the measurement of isolated
muscle fibers, which directly reflect the non-linear properties due to the sliding
filaments. The series elastic elements can then be grouped with the tendon and re-
moved from the model. Pennation effects are directly included into the model. For
an extended view on skeletal muscle modeling we refer the reader to Lee et al.s'
survey [
18
].
