penalize ligament stretch with muscle recruitement

Hello there,

I was thinking of a way to penalize ligament stretch in FDK models.

My rationale: if ligaments are stretched too much, this must be at the expense of the general cost function of the muscle recruitment problem, so that a different solution can be found, which does not overstretch the ligaments.

After some trial and errors I came to the implementation here attached. For each ligament I created a virtual muscle, the strength of which is linked to the actual stretch of that ligament. These virtual muscles are made to pull against a constant force applied somewhere in the void (see LigPenalStudy), so that their activities lets the overall cost function increase.

I compare this with a study in which there is no constant force (NormalStudy), so that the activity of the virtual muscles is zero.


[li]the solutions for the two studies are identical[/li][li]the ligament penalization study does not result in less ligament stretch, compared to the normal study[/li][/ul]
So, is what I’m attempting to do conceptually wrong? Can’t the increase in the cost function (caused by the virtual muscles activation) lead to a different muscle recruitment, and hence a different FDK solution (both force and kinematics)?

Thank you in advance for your help.

Marco Marra

Hi Marco,

I am not a 100% it is possible, but here is a suggestion:


[li]For each ligament make a muscle on top
[/li][li]Make the strength of the muscle equal to the force of the lig
[/li][li]You can not use the force directly as S it will need to be an equation based on Positions otherwise you will get an error
[/li]ake muscle activation constraints that locks the activity so that the muscle creates the force instead of the ligament
[li]Exclude the force created by the ligament… or avoid apply it in the first place
[/li][li]Now the force is made by a muscle not a ligament… and it should change the muscle recruitment and the thus the FDK reactions

Again i am not 100% sure it will work… by changing the strength and the activation constraint with the same factor you can possibly make it “feel” the ligament in the recruitment more or less.

Best regards

Hi Søren,

Thank you very much for your tip! Indeed, I had not thought about using the muscle activity constraints. I guess you actually mean the AnyMuscleActivityBound class, is that right? As the other one (AnyMuscleActivityConstraint) is meant to constrain the activities of multiple muscles with each other… whereas in my case I could set both lower and upper bounds to the desired activity level (equivalent of my ligament force).

I will give this a try and get back to you in case of troubles.

Best regards,


Hi Marco,

Yes exactly you will need to use the
AnyMuscleActivityBound class

Best regards

Hi Marco,

I am still in doubt if it will work…

In this solution the activation is constrained to follow the ligament force…

So for a given position the force/activation is effectively prescribed… and there is nothing the other muscles can do about this.

The activation can be high but it will be still fixed by the constraint, the other muscles can not make a different activation pattern that will change this.

Best regards

Hi Søren,

You are right, indeed, the “activation” of these ligaments become position-dependent, and muscle recruitment simply cannot change that, no matter what.

I guess the real problem with ligaments as virtual muscles is that there is no degree of freedom to actuate, simply speaking.

So, in other words, this doesn’t seem to be possible at the moment. This also means that contact pressure penalization cannot work, for similar reasons…

Any idea whether the AnyBody solvers could be expanded to allow for this? It might be interesting to study e.g. co-contraction, pain-avoidance etc.


Hi Marco,

I think there is no quick fix for this, so i can not judge if it will be possible, but it is an interesting problem, if we find a solution we let you know.

The only way right now to introduce pain would be for Non-FDK models to replace joint constraints with AnyGeneralMuscles of limited strengths.

Best regards

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