Biomedical Engineering Reference
In-Depth Information
weaknesses at specific local locations. The tests also impose tighter constraints
for the model testing and improvement. The validation of two PD tasks—normal
walking and box lifting—will be detailed in this chapter.
9.2
Motion determinants
The concept of motion determinants is characterized by a set of motion profiles
that are subtended by the human body and that are common to humans conduct-
ing the same motion.
In this work we will call the active DOF the determinants of the motion. Note
that these determinants are well established in gait analysis, but for all other tasks
we have introduced an analogous concept to ascertain the quantities that can be
measured and validated.
The motion determinants concept presents a very useful time history of the
trend of the major joints; it also generates large amounts of motion information.
Luckily, the determinants have distinctive signatures at certain locations in the
time history of the motion. For example, in normal walking, the knee flexion, as
one of the walking determinants, reaches a maximum value of 62
6 degrees
during the walking cycle (
Baliunas et al., 2002
). Such key frames should be
selected carefully and used as signatures for normal motion. Fortunately, critical
key frames are well defined in the literature for some tasks, such as normal walk-
ing; however, they need to be characterized for other tasks.
6
9.3
Motion capture systems
9.3.1
Overview
There are many motion capture systems on the market that can be used to acquire
the motion characteristics of a human with a relatively high degree of accuracy.
These include inertial, optical, and markerless motion capture systems. Each sys-
tem has its own advantages and disadvantages.
Inertial systems such as those by Xsens (
http://www.xsens.com/
)
and
Animazoo (
http://www.animazoo.com/
)
can acquire accurate local acceleration
data at different locations on the human body, but they suffer from motion drift-
ing because they lack a global coordinate system; also, they are sensitive to mag-
netic fields.
Optical motion capture systems can be categorized as passive (reflective mar-
kers), such as those by Vicon (
http://www.vicon.com/
)
and Motion Analysis (
http://
www.motionanalysis.com/
)
, or active (light emitting diodes [LED]), such as those by
Passive motion capture systems have high initial and running costs, but they are still
the most popular in international motion research labs because of their accuracy.
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