Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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analysis, this discrepancy must be corrected in the analysis software if data accuracy is
not to be compromised.
Although digital video cameras are convenient and inexpensive, they often do not
allow the interchange of lenses, which can be useful. The quality of lenses on cheaper
video cameras may result in image distortion, particularly at wide-angle settings on
zoom lenses; this can lead to increased errors in digitised coordinates. Modern, solidstate
digital video cameras detect the image using an array of light sensors precisely
etched into silicon and are claimed to have zero geometric distortion.
Another important limitation of videography is the vast amount of manual coordinate
digitisation that is often required for quantitative analysis. This is particularly
the case when a three-dimensional analysis is undertaken, as at least two images have
to be digitised for each field or frame to be analysed. To overcome this drawback,
motion analysis systems such as Vicon (Vicon, Oxford, UK; http://www.vicon.com)
and EVa RealTime (Motion Analysis Corporation, Santa Rosa, USA; http://
www.motionanalysis.com), have been developed that automatically track markers
attached to the body. Some video software analysis packages, such as SIMI (SIMI
Reality Motion Systems GmbH, Unterschleissheim, Germany; http://www.simi.com)
and APAS (Ariel Dynamics, Trabuco Canyon, USA; http://www.arielnet.com)
also have an automatic marker-tracking option, but this is currently restricted to twodimensional
movements.
BOX 4.2 TWO-DIMENSIONAL OR THREE-DIMENSIONAL ANALYSIS?
An early decision that must be made in any quantitative video analysis of sports movements is
whether a two- or three-dimensional analysis is required. Both have advantages and disadvantages,
as summarised by the following.
Two-dimensional recording and analysis:
Is simpler and cheaper as fewer cameras and other equipment are needed.
Requires movements to be in a pre-selected movement plane (the plane of motion or plane of
performance). It can yield acceptable results for essentially planar movements but it ignores
movements out of the chosen plane. This can be important even for an event that might
appear essentially two-dimensional, such as the long jump.
Is conceptually easier to relate to.
Requires less digitising time and has fewer methodological problems, such as the transformation
of coordinates from the video image to the ‘real world’ movement plane.
Three-dimensional recording and analysis:
Has more complex experimental procedures.
Can show the body’s true three-dimensional movements.