Introduction to Sports Biomechanics: Analysing Human Movement ...

DATA PROCESSING
QUANTITATIVE ANALYSIS OF MOVEMENT
points. These should, therefore, be equally distributed within or around
the volume in which the sports movement takes place. Errors depend on
the distribution of the calibration points and increase if the performer moves
beyond the confines of the control volume. This is the most serious restriction
on the use of the DLT algorithm. It has led to the development of other methods
that require smaller calibration objects, or the ‘wand’ technique now used
by many automatic marker-tracking systems. All of these alternatives have
greater computational complexity than the relatively straightforward DLT
algorithm.
If ‘joint centre markers’ are used, even greater attention must be paid to their
movement relative to underlying bones than in a two-dimensional study. The
markers, whether points or bands (see above) are only a guide to the location of
the underlying joint centres of rotation. To minimise errors, you need an even more
thorough anatomical knowledge of the joints and the location of their axes of
rotation with respect to superficial landmarks throughout the range of segmental
orientations than for a two-dimensional study. This becomes even more essential if
markers are not used.
Panning cameras can be used to circumvent the problem of a small image size,
which would prevent identification of body landmarks if the control volume was
very large. Three-dimensional reconstruction techniques that allow two cameras
to rotate freely about their vertical axis (panning) and horizontal axis (tilting)
have been developed. In these techniques, the cameras must be in known positions.
These approaches have also been extended to allow for variation of the focal lengths
of the camera lenses during filming.
It is obviously necessary to check the validity of these methods. This can be done by
calculating the root mean square (RMS) error between the reconstructed and
known three-dimensional coordinates of points, preferably ones that have not been
used to determine the DLT parameters. Furthermore, the success with which these
methods reproduce three-dimensional movements can be checked, for example by
filming the three-dimensional motion of a body segment of known dimensions or a
rod thrown into the air. In addition, reliability and objectivity checks should be
carried out on the digitised data.
The data obtained from digitising, either before or after transformation to threedimensional
coordinates, are often referred to as ‘raw’ data. Many difficulties arise when
processing raw kinematic data and this can lead to large errors. As noted in the previous
section, some errors can be minimised by careful equipment selection and rigorous
attention to experimental procedures. However, the digitised coordinates will still
contain random errors (noise).
The importance of this noise removal can be seen from consideration of an
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