Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
is paramount, as it determines the maximum accuracy of other measurements.
In filming activities such as kayaking, ski jumping, skiing, or javelin flight just
after release, a group of vertical calibration poles, on which markers have been
carefully positioned, may be easier to use than a calibration frame. The coordinates
of the markers on the poles must be accurately measured; this often
requires the use of surveying equipment. The use of calibration poles is generally
more flexible and allows for a larger calibration volume than does a calibration
frame.
� The more calibration points used, the stronger and more reliable is the
reconstruction. It is often convenient to define the reference axes to coincide
with directions of interest for the sports movement being investigated. The usual
convention is for the x-axis to correspond with the main direction of horizontal
motion.
� All the calibration points must be visible to each camera and their image
coordinates must be clearly and unambiguously distinguishable, as in Figure 4.7.
Calibration poles or limbs of calibration frames should not, therefore, overlap or
nearly overlap, when viewed from any camera.
� Although an angle of 90° between the optical axes of the cameras might be
considered ideal, deviations from this can be tolerated if kept within a range of
about 60–120°. The cameras should also be placed so as to give the best views
of the performer.
� Accurate coordinate reconstruction can only be guaranteed within the space –
the calibration or control volume – defined by the calibration (or control)
Figure 4.7 Three-dimensional DLT camera set-up – note that the rays from the calibration spheres are unambiguous for both
cameras – for clarity only the rays from all the upper or lower spheres are traced to one or other camera.
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