Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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Videography
some other way of identifying key factors in the movement (as in Chapter 2), or more
advanced movement modelling techniques, such as computer simulation modelling or
the use of artificial neural networks, which are beyond the scope of this book.
The main method currently used for recording and studying sports movements is
digital videography. Cinematography – using cine film cameras – is now rarely used,
and the same almost applies to the use of ‘analog’ video cameras; neither will be
considered in any detail in this chapter. Motion analysis systems that automatically
track skin markers are increasingly used in biomechanics research laboratories; these
systems are many times more expensive than video analysis systems, are technically far
more complicated, require far more expert operators and currently cannot be used
outdoors during daylight hours. For these reasons, which usually mean that students in
the earlier years of their study will not encounter such motion analysis systems, they
are not dealt with in this book (interested readers should consult Milner (2007); see
Further Reading, page 152).
A great strength of videography is that it enables the investigator to record sports
movements not only in a controlled laboratory setting, but also in competition. It also
minimises any possible interference with the performer. Indeed, performers can be
‘videoed’ without their knowledge, although this does raise ethical issues that will
normally be addressed by your Institutional Research Ethics Committee.
Quantitative analysis will often involve the biomechanist having to digitise a lot of
data. This process of ‘coordinate digitisation’ involves the identification of body landmarks
used to aid the estimation of joint axes of rotation. In videography, particularly in
three-dimensional studies, this will normally be done by the investigator manually
digitising the required points using a computer mouse or similar device. Some video
analysis systems can track markers in two dimensions, saving the investigator much
time. Automatic marker-tracking systems, as their name implies, track markers automatically,
and in three dimensions, although operator intervention may still be needed
if too few cameras can see the marker during some part of the movement. Whichever
way coordinate digitising is performed, the linear coordinates of each digitised point are
recorded and stored in computer memory.
What quantitative analysts measure
After digitising a movement sequence, linear and angular positions and displacements
can be calculated and presented as a function of time – a time series (for example,
Figure 3.8). Some additional data processing will normally be performed to obtain
displacements of the centre of mass of the performer’s whole body (Figure 3.5 and
Chapter 5). Velocities and accelerations will also probably be obtained from the
displacement data (for example, Figures 3.7 and 3.10). As well as these time-series