Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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impact of the ball and racket is not the focus of the study; 100 Hz is often needed
for quantitative analysis of activities as fast as a golf swing – this is double the
maximum sampling rate of most standard digital video cameras. The Nyquist
sampling theorem requires that the sampling frequency is at least twice the
maximum frequency in the signal (not twice the maximum frequency of interest) to
avoid aliasing (Figure 4.6). Aliasing is a phenomenon seen in films when wheels on
cars and stagecoaches, for example, appear to revolve backwards. Furthermore, the
temporal resolution – the inverse of the sampling rate – improves the precision of
both the displacement data and their time derivatives. For accurate time measurements,
or to reduce errors in velocities and accelerations, higher frame rates may be
needed.
If several lighting conditions are available, then natural daylight is usually preferable.
If artificial lighting is used, floodlights mounted with one near the optical axis of the
camera and one to each side at 30° to the plane of motion give good illumination.
Careful attention must also be given to lighting when choosing the camera shutter
speed.
Whenever possible, information should be incorporated within the camera’s field
of view, identifying important features such as the name of the performer and
date. The ‘take number’ is especially important when videography is used in conjunction
with other data acquisition methods, such as force plates (Chapter 5) or
electromyography (Chapter 6).
The recording of the movement should be as unobtrusive as possible. The performer
may need to become accustomed to performing in front of a camera in an experimental
context. The number of experimenters should be kept to the bare minimum
in such studies.
In controlled studies, away from competition, as little clothing as possible should be
worn by the performers to minimise errors in locating body landmarks, providing
that this does not affect their performance.
In any videography study, written informed consent should be obtained from all
participants; in sport, the coach may be able to provide consent for his or her
athletes, but this varies from country to country and should always be checked.
Furthermore, approval from your Institutional Research Ethics Committee may be
required.
The above procedures impose some unnecessary restrictions on two-dimensional
videography, leading to severe practical limitations on camera placements, particularly
in sports competitions, because of the requirement that the optical axis of the camera
is perpendicular to the movement plane. A more flexible camera placement can be
obtained, for example by the use of a two-dimensional version of the direct linear
transformation (see below). This overcomes some of the camera location problems that
arise in competition because of spectators, officials and advertising hoardings. It
requires the use of a more complex transformation from image to movement plane
coordinates. Many of the above procedural steps then become redundant, but others,
similar to those used in three-dimensional analysis, are introduced. This more flexible