View Full Version : dynamic joint stiffness calculation

01-09-2003, 10:17 PM
I am currently involved in a research about the botulinum effect on cerebral
palsy children. In particular i am tryng to evaluate the dorsi plantar ankle
stiffness of pci children at the first of toe-ground contact during the
gait. So i have a litte question about that calculation.
Dynamic joint stiffness of an articulation is defined by some authors as the
ratio between moment and angle variations.
An example (the others in leterature are quite similar abou the calculation
procedure of stifness) may be found in Davis RB, De Luca PA. Gait
characterization via dynamic joint stiffness. Gait and Posture 1996 4:
224-231. Here the authors investigated the ankle joint stiffness (in
sagittal plane).
To calculate the stiffness they plotted the ankle moment versus in function
of the corresponding ankle angle and then the performed a polynomial
regression to calculate the slope, i.e. the stiffness, of the moment-angle
curve .
I think that this procedure may be perfectly valid only when no lag exists
between the moment and the angle "signals".
Moreover that procedure is correct only when the model of the ankle is a
perfect spring, with no dumping effect.
Would not be better a procedure which takes into account the damping too?
So, would not be better to correlate the moment and the ankle angles each
other trying to identify a first order model spring+damper?

Eng. Fabrizio PATANE'
University of Rome "LA SAPIENZA"
Tel 0644585585
Fax 064881759
Street EUDOSSIANA 18 ZIP 00184 ROME, Italy

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