Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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All the calibration points must be clearly visible on the images from both cameras;
they must also have three-dimensional coordinates that are accurately known.
Placements of cameras must relate to the algorithm chosen for reconstruction of
the movement–space coordinates. Deviations from these requirements will cause
errors.
In summary, digitised coordinate data will be contaminated with measurement
inaccuracies or errors. These will be random (noise), systematic, or both. All obvious
systematic errors, such as those caused by lens distortion and errors in calibration
objects, should be identified and removed, for example by calibration or software
corrections. Any remaining sources of systematic error will then be very small or of low
frequency and will, therefore, have little effect on velocities and accelerations. The
remaining random noise in the displacement data, expressed as relative errors, has been
estimated as within 1% for a point in the photographic plane for two-dimensional
videography and within 2% for a point in the calibration volume for three-dimensional
videography. Random errors must be minimised at source by good experimental procedures.
Any remaining noise should be removed, as far as possible, from the digitised
data before further data processing. These two aspects will be covered in the next two
sections. Also, consideration needs to be given to the estimation of errors in digitised
coordinate data and their effects on derived values.
EXPERIMENTAL PROCEDURES
Two-dimensional recording procedures
The following steps have often been considered necessary to minimise errors recorded
during two-dimensional videography, thereby improving the accuracy of all derived
data, and can still often prove useful, particularly for students who are unfamiliar with
videography. These procedures also allow a simple linear transformation from image
to movement-plane coordinates using simple scale information recorded in the field of
view.
The camera should be mounted on a stationary, rigid tripod pointing towards the
centre of the plane of motion and there should be no movement (panning or tilting)
of the camera.
The camera should be sited as far from the action as possible to reduce perspective
error. A telephoto zoom lens should be used to bring the performer’s image to the
required size.
The focal length of the lens should be carefully adjusted to focus the image. This is
best done, for zoom lenses, by zooming in on the performer, focusing and then
zooming out to the required field of view. The field of view should be adjusted to
coincide with the performance area that is to be recorded. This maximises the size of
the performer on the projected image and increases the accuracy of digitising.