KINEMATICS - Biomechanics and Motor Control of Human Movement - page 64

Disadvantages

1. Most multiple-camera systems are expensive (cine, TV, optoelec-
tric), as are the digitizing and conversion systems for all imaging
sources.
2. For film, the turnaround time for development may be a problem, and
the labor to digitize film coordinates may also be a constraint. The
digitizing errors, however, are less than those from many commercial
imaging systems.
3. Encumbrance and time to fit wired light sources (e.g., IREDs) can be
prohibitive in certain movements, and the number of light sources is
limited.
4. Some imaging systems (e.g., IREDs) cannot be used outside in daylight.

3.3.7 Summary of Various Kinematic Systems
Each laboratory must define its special requirements before choosing a partic-
ular system. A clinical gait lab may settle on TV because of the encumbrance
of optoelectric systems and because of the need for a qualitative assessment,
rapid turnaround, and teaching. Ergonomic and athletic environments may
require instant or near-instant feedback to the subject or athlete, thus dic-
tating the need for an automated system. Basic researchers do not require
a rapid turnaround and may need a large number of coordinates; thus, they
may opt for movie cameras or an optoelectric system. And, finally, the cost
of hardware and software may be the single limiting factor that may force a
compromise as to the final decision.

3.4 PROCESSING OF RAW KINEMATIC DATA

3.4.1 Nature of Unprocessed Image Data
Film and television are sampling processes. They capture the movement event
for a short period of time, after which no further changes are recorded until
the next field or frame. Playing a movie film back slowly demonstrates this
phenomenon: the image jumps from one position to the next in a distinct step
rather than a continuous process. The only reason that film or television does
not appear to jump at normal projection speeds (24 per second for film, 60
per second for television) is because the eye can retain an image for a period
of about 1/15 s. The eye's short-term “memory” enables the human observer
to average or smooth out the jumping movement.
The converted coordinate data from film or television are called raw data.
This means that they contain additive noise from many sources: electronic
noise in optoelectric devices, spatial precision of the TV scan or film digitizing
system, or human error in film digitizing. All of these will result in random