This is the minimum requirement for three-dimensional reconstruction of the marker position in three-dimensional space, although reconstruction accuracy may be improved when some redundancy is introduced and more cameras can see the markers. Furthermore, the inter-marker distances must be sufficient that the system can differentiate between individual markers during marker reconstruction. If markers are placed too close together, the system will be unable to identify where one marker begins and the other ends. In this case, only one marker will be reconstructed instead of the two.
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Additionally, the further apart a segment markers can be placed, the smaller the impact of marker position reconstruction error on the joint angles that are calculated from the raw coordinate data.
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Markers and the underlying bone
The aim of on-line motion analysis is to determine the movement of the underlying bone structures by recording the movement of markers mounted on the skin’s surface. The discrepancy between the movement of the marker and the actual movement of the skeleton is known as ‘skin movement artefact’. The gold standard for marker mounting is to attach markers directly to the bones of interest using bone pins. Although this has been done in a few studies, it is obviously impractical in most cases as it is invasive.
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However, comparison studies between bone pin and skin-mounted markers provide an indication of the magnitude and type of error associated with skin-marker measurement. There are two types of skin-marker movement error, absolute and relative. Errors due to skin-marker movement at the knee during walking may be up to 10 per cent of the flexion–extension range of motion, 50 per cent of the abduction–adduction range of motion and 100 per cent of the internal–external rotation range of motion. Minimising skin movement artefact must be the main criterion in marker set design
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