Introduction to Sports Biomechanics: Analysing Human Movement ...

Figure 4.2 Modern digital video camera.
QUANTITATIVE ANALYSIS OF MOVEMENT
Video recording is a sampling process; the movement is captured for a short time
and then no further changes in the movement are recorded until the next field or frame.
The number of such pictures taken per second is called the ‘sampling rate’ or ‘sampling
frequency’. For the recording stage of movement analysis, this will correspond to the
field rate or frame rate. The overall sampling rate for the analysis may be less if not every
field or frame is digitised.
The two main drawbacks of using commonly available digital video cameras for
analysing sports movements are the resolution of the image, which restricts digitising
accuracy when compared with high-resolution cine digitising tablets, and the sampling
rate of 50 fields per second (60 in North America), which makes them unsuitable for
the quantitative study of very fast sports movements. For fast movements and for very
high-speed events, such as bat–ball impacts, higher frame-rate cameras are needed; such
cameras are commercially available but are much more expensive than standard digital
video cameras. The ‘resolution’ of coordinates obtained from digital video images is
limited by the number of individual spots of light, or picture elements (pixels), that can
be displayed on the computer monitor; 1024 horizontally by 1024 vertically would be a
very good specification. This is an important limitation of digital videography, but
some video analysis systems use interpolation of the position of the digitising cursor
between pixels as a partial solution to this problem.
A further drawback of digital video cameras is that most cannot be ‘genlocked’ to
allow the shutter openings of cameras to be synchronised when using more than one
camera to record movement, as in three-dimensional studies. Without a genlock
capability, cameras can take pictures up to ½ of a field (or 0.01 s) apart; in quantitative
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