Determining the right muscle parameters (wrist model)
Posted: Tue Dec 04, 2012 7:04 am
Hello,
i'm currently trying to update the wrist model that comes with OpenSim, so it can be used for inverse dynamics / static optimization calculations. I've already added the missing joints and mass/inertia parameters to the model but now i have troubles finding the right muscle parameters.
First of all, to check if all my parameters are set correctly, i want to perform a simple forward dynamics simulation without any muscle activation or external forces (besides gravity) acting on the model. I would expect the hand to take a natural pose, something like this:
However, this is not the case. The hand does this (i hope the gif animation works):
(The thumb and elbow were locked in the simulation.)
While i'm pretty satisfied with the resulting wrist angle, the movement and resulting angles of the fingers are not really correct. It seems the extensor of the little finger is too tight, while the ones of the other fingers are too loose.
I'm not quite sure which parameter(s) affect the resulting passive force generation, but i suppose it is mainly the tendon slack length. So i tried altering the value a bit but couldn't get a satisfying result. Setting the value to zero (the tendon has no slack) or too low would result in a neverending calculation or an exception. Setting the value near a value of the estimated muscle/tendon length would get me some results. So is it correct that the tendon slack length determines the total length of the muscle+tendon? Or am i interpreting the parameter incorrectly?
Furthermore it seems pretty hard to balance the force generation of the flexors/extensors of the fingers correctly to get the intended result and i'm not quite sure if my model has all the necessary forces/constraints.
Do you know if changing the tendon slack length to the correct values will result in a correct behaviour of the fingers or do i need to add additional forces like joint stiffness or friction?
And lastly i would like to ask, if my approach to check the behaviour of the hand during a forward simulation to see if it behaves in a natural way is necessary to get realistic results during a static optimization calculation. Or could i ignore the wrong behaviour of the hand and still get satisfying results for the static optimization?
Any help is really appreciated!
Best regards,
Yannik
i'm currently trying to update the wrist model that comes with OpenSim, so it can be used for inverse dynamics / static optimization calculations. I've already added the missing joints and mass/inertia parameters to the model but now i have troubles finding the right muscle parameters.
First of all, to check if all my parameters are set correctly, i want to perform a simple forward dynamics simulation without any muscle activation or external forces (besides gravity) acting on the model. I would expect the hand to take a natural pose, something like this:
However, this is not the case. The hand does this (i hope the gif animation works):
(The thumb and elbow were locked in the simulation.)
While i'm pretty satisfied with the resulting wrist angle, the movement and resulting angles of the fingers are not really correct. It seems the extensor of the little finger is too tight, while the ones of the other fingers are too loose.
I'm not quite sure which parameter(s) affect the resulting passive force generation, but i suppose it is mainly the tendon slack length. So i tried altering the value a bit but couldn't get a satisfying result. Setting the value to zero (the tendon has no slack) or too low would result in a neverending calculation or an exception. Setting the value near a value of the estimated muscle/tendon length would get me some results. So is it correct that the tendon slack length determines the total length of the muscle+tendon? Or am i interpreting the parameter incorrectly?
Furthermore it seems pretty hard to balance the force generation of the flexors/extensors of the fingers correctly to get the intended result and i'm not quite sure if my model has all the necessary forces/constraints.
Do you know if changing the tendon slack length to the correct values will result in a correct behaviour of the fingers or do i need to add additional forces like joint stiffness or friction?
And lastly i would like to ask, if my approach to check the behaviour of the hand during a forward simulation to see if it behaves in a natural way is necessary to get realistic results during a static optimization calculation. Or could i ignore the wrong behaviour of the hand and still get satisfying results for the static optimization?
Any help is really appreciated!
Best regards,
Yannik