Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
limbs, when one of the limbs is closer to the camera than the other. It occurs for
movements away from the photographic plane. The term is also sometimes used to
refer to the error in recorded length for a limb or body segment that is at an angle
to the photographic plane and, therefore, appears to be shorter than it really is.
Associated with this error is that caused by viewing away from the optical axis such
that, across the plane of motion, the view is not always side-on, as at the positions
marked (*) in Figure 4.3. This is sometimes referred to as parallax error. The
combined result of these optical errors is that limbs nearer to the camera appear
bigger and appear to travel further than those further away. This causes errors in the
digitised coordinates.
Lens distortions may be a source of error, particularly at wide-angle settings on
inexpensive zoom lenses.
Locations of joint axes of rotation are only estimates, based on the positions of
superficial skin markers or identification of anatomical landmarks. Use of skin
markers can not only help but also hinder the movement analyst, as these markers
move with respect to the underlying bone and to one another. The digitising of such
markers, or estimating the positions of axes of rotation without their use, is probably
the major source of random error (or noise) in recorded joint coordinates. A study
that highlighted this point was considered at the end of Chapter 3 (pages 107–9).
Locating joint axes of rotation is particularly difficult when the joint is obscured by
other body parts or by clothing.
Other possible sources of error include blurring of the image, camera vibration,
digitising errors – related to coordinate resolution and human digitiser errors – and
computer round-off errors.
Figure 4.3 Errors from viewing movements away from the photographic plane and optical axis of the camera: (a) side view;
(b) as seen from above.
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