Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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the underlying processes that give rise to the observed movement patterns. The method
of ‘inverse dynamics’ is used to calculate net joint forces and moments from kinematic
data, usually in combination with external force measurements from, for example, a
force platform (see Chapter 5). The method of inverse dynamics is valuable in sports
biomechanics research to provide an insight into the musculoskeletal dynamics that
generate the observed characteristics of sports movements; we won’t consider this
advanced topic further in this book (readers interested in a simple introduction to the
topic should see Bartlett, 1999; Further Reading, page 152). Many calculations are
needed to determine net joint forces and moments, and an assessment of the measurement
and data processing errors involved is important (see Challis, 2007; Further
Reading, page 152).
RECORDING THE MOVEMENT
Digital videography
Improvements in video technology in recent years have included the increased availability
of electronically-shuttered video cameras. In these cameras, electronic signals are
applied to the light sensor to control the time over which the incoming light is detected;
this time is usually referred to, somewhat misleadingly, as the ‘shutter speed’. It is
essential to use electronically-shuttered cameras with a range of shutter speeds to obtain
good-quality, unblurred pictures. Although such a range of shutter speeds is fairly
normal for a digital video camera (Figure 4.2), the user should ensure that a fast enough
shutter speed is used to minimise blurring of the image for the activity being recorded.
A setting of 1/1000 s is normally adequate (see also Table 2.1). Be wary of the ‘sports’
setting on some cheaper digital video cameras, as the shutter speed to which this setting
corresponds is often not specified.
Other important developments have included high-quality slow-motion and ‘freezeframe’
playback devices that allow the two ‘fields’ that make up an interlaced video
‘frame’ to be displayed one after the other. Standard (50 fields or 25 frames per second)
video equipment has, therefore, become an attractive alternative to cinematography
because of its price, immediacy and accessibility – to such an extent that cinematography
has virtually disappeared from sports biomechanics usage. Furthermore, the
advent of high-quality digital video cameras has transformed videography. An important
reason for this is that digital video can download directly to a computer without
the need to record to an intermediate medium, such as a video cassette. Although
‘analog’ video cameras (which record to VHS – or other format – video cassettes) are
still available, they have been largely superseded by digital cameras and will not be
considered further, except for comparison with digital video. The development of
digital video cameras has meant that a separate video playback system to display images,
which was needed with analog video cameras, is no longer necessary as the recording is
downloaded onto the analysis computer. The images are stored in the computer and
displayed on the computer monitor for digitising.