The following supplementary material is available for this article:
Appendix A1. Real time sonograph of standing sways.
The sonograph view is the same as in figure 1. This movie shows the data presented in figure 2.
When the muscle shortens, the 8 red markers on the central aponeurosis move upwards and the two sets of green markers move downwards. Gastrocnemius is on the left of the central aponeurosis and soleus on the right.
The angle of sway (ankle angle) is shown by the two large red markers connected by a line. The angular changes are magnified by a factor of three to aid viewing.
By focussing on tracking markers and the surrounding muscle you can assess the extent to which the markers faithfully follow the actual muscle movement. Each marker represents the average movement of an 11 by 11 pixel square centred on the marker. The box ticks around the edge show 20 pixel (7 mm) intervals.
By looking at the muscle and sway markers simultaneously you can assess how the muscle shortens as the person sways forwards and how the muscle lengthens as the person sways back to the vertical.
The movie plays in real time.
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