Page 154 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOMECHANICS OF HUMAN MOVEMENT 131

if the subject (and therefore the sensors attached to the subject) stops moving within the magnetic
field, we might think the induced voltage in each sensor would remain constant. However, revisiting
Eq. (6.1) shows that this is not the case, because the magnetic flux must change with respect to time
or the EMF goes to zero. There are two things that can be controlled to ensure a changing flux:
(1) make sure the subject never stops moving or (2) change the strength and direction of the mag-
netic field. Electromagnetic tracking systems use the latter strategy to ensure the magnetic flux
changes. The transmitter not only emits a magnetic field, but also serves as a fixed reference about
which position and orientation of each sensor is reported.
Each receiving sensor contains three orthogonal coils used to detect the magnetic field emitted
by the transmitter using the principles of magnetic induction. The receiving coils are contained within
3
a 1-in plastic housing for protection and provide a convenient method for attaching the sensor to
the subject. The sensors are generally secured using double-sided tape and wrapped with an elasti-
cized band. Proprietary signal processing takes place in real time, compensating for the strength of
the earth’s magnetic field. Individual coil signals can be used to determine the orientation of the
sensor relative to the antenna generating the magnetic field. Each coil within the sensor detects three
signals from the transmitter (i.e., one for each antenna of the transmitter) for a total of nine signals.
These nine signals suffice to locate the position and orientation of the sensor relative to the trans-
mitter. For example, the receiving coil most parallel to the currently active transmitting antenna will
experience the largest EMF, while the more orthogonal the coil, the smaller the induced voltage.
Because each coil within a sensor is the same distance from the transmitter, it is possible to deter-
mine the distance and orientation of the sensor relative to the currently active antenna by comparing
the strength of the induced EMF in each coil to the strength of the emitted magnetic field.
There are two types of electromagnetic tracking systems that are used for the study of human
movement. The biggest difference between these systems is that one (e.g., Polhemus Incorporated)
uses an AC magnetic field, while the other type (e.g., Ascension Technology Corporation) uses a
pulsed DC magnetic field. The precision and accuracy of electromagnetic tracking systems is affected
by metallic objects, low-frequency electronic noise, and also by the distance of the sensor from the
transmitting antennae. The radius within which precise and accurate data are sampled depends on
the particular system and strength of the transmitter. However, when used in an ideal environment,
the precision and accuracy of both systems is more than adequate for studying human movement.
With proper setup, static accuracy of less than 2 mm RMS and 0.5° RMS for both systems is possible.
Real-time data can be sampled at a rate of up to 144 Hz depending on the system, the number of
sensors tracked and the type of data communication interface with the computer. These systems do
not suffer from line of sight problems typical of optoelectronic systems and are, therefore, ideal for
capturing complex movements.

6.3.3 Optical Methods: Camera-Based Systems
The most common method of recording human movement involves “filming” the motion of interest.
Historically, images were stored on conventional media such as 16 mm film or on videotape. Today’s
standard is based on digital technology, bypassing physical media per se, sending the data directly
to the computer. Data are commonly sampled between 50 and 240 frames per second, depending on
the movement of interest. For example, natural cadence walking is often sampled at a rate of 60 Hz,
while running and arm movements tend to be sampled at 100 Hz or faster. There are two types of
high-speed video-based systems that are used for studying human movement. The fundamental
difference between designs is related to their use of active or passive tracking targets.
Active tracking target systems use infrared light-emitting diodes to indicate the position of the
target in space. The diodes are pulsed in order so that only one target is illuminated (i.e., active) at a
time. Thus, if a target is not detected by a camera and then suddenly reappears in the camera’s field
of view, it will automatically be identified based on its order in the pulse sequence. Active target
systems are subject to “line of sight” problems common to all optical-based tracking systems. That is,
the target must be seen by a camera to be detected. Active targets emit a restricted angle of light that
may not be detected by the camera if the target rotates relative to the segment to which it is attached.