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View Full Version : False ankle angles with VICON 460



Yves Blanc
01-07-2003, 02:54 AM
Dear all,
I wonder if the following observation is relevant to the discussion
triggered, before Christmas, by Dr Chris Kirtley "Calculation of 2D Angles ('CAST' problem)".

I use a VICON 460 with PlugInGait. Recently, from an hemiplegic patient with
a fixed equinus of 40░ and a dynamic equinus of about 60-65░ (combined
plantar flexion of ankle and mid foot joints), the range of computed plantar
flexion was 45░ to 60░ when the tibial torsion was estimated by the
software.
These values were compatible with the video recordings.

BUT when the clinically measured tibial torsion (25░ external) was forced in the
computation of the static trial, (entered as -25 + zeroing the "shank offset value" computed in the "subject measurement", the computed ankle angle on the equinus side became a DORSI FLEXION ranging
from 115░ to 130░....... However, on the normal side the ankle angle remains within a
normal range. These strange results and unrealistic angle values appear
every time a significant equinus is present. Is it an
effect of the gimbals lock phenomenon? Or just a bug in the software?

As usual all the answers will be compiled and posted.

yves

Yves Blanc Ph.D.
Laboratoire de cinÚsiologie
H˘pital Cantonal Universitaire
1211 GENEVE 14. CH
tÚl; XX 41 22/3727 827
fax: XX 41 22/37 27 799

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