Hello Sarah,

I think I have to take this bit by bit. Please find my answers (and

some further questions below):

> I have created a simple arm model, similar to the tutorial

> where a forearm curls a dumbbell (Biceps Long and Short,

> Triceps Long and Short, Brachialis, Brachioradialis,

> and Deltodeus A and B). In my model, the upper arm hangs

> down vertically, and the forearm holds the dumbbell steady

> 90 degrees from the upper arm (static representation).

> When I ran the model with all 8 muscles, both triceps and

> the deltodeus B had no effect on the model so I neglected

> them in further analyses. Running the model

> with any one of the remaining muscles produces the error

> “Musclerecruitment analysis failed, simplex solver reports

> that solution does not satisfy all constraints” for both

> a simple muscle model (F0=300) and a 3E Hill-type muscle

> model. Is this because there is no need to do inverse

> dynamics because there are 3 equations, 3 unknowns and

> the systems needs no iterative solver to do this?

> When I add two muscles to the model, it

> works sometimes but not always.

There is a mathematical/numerical problem that arises in some cases

where the model is in perfect symmetry and needs no forces in either

direction. It sounds like you may have run into this problem, perhaps

due to the vertical upper arm. The problem is more prevalent for some

solvers than for others (AnyBody has several you can choose from).

Please check the study section in your model for the specification of

RecruitmentSolver, and set it to

RecruitmentSolver = MinMaxNRSimplex;

This is the more robust of the available solvers, and it might solve

you problem. If not, please come back. (There are good reasons why we

have other solvers as well, but that is a topic for another

discussion)

> Also, interestingly I get different Fm and Elbow Reaction

> values for the two different muscle modeling scenarios.

> Why does the addition of these parameters

> (K1, K2, Fcfast, Jt, etc) affect an essentially static

> model? Is there a mathematical explanation

> behind why these parameters give different Fm and

> Reaction Force values than the simple F0=300 Nm model?

Yes, of course there is. But let me propose the following. If the

advice above solves your first problem, then please upload your

working model to the files section of the group (and post a message

here to let us know you have uloaded it). It is much easier to

answer your questions when we can download your model and try for

ourselves.

Please notice that we have recently published a new muscle modeling

tutorial here: http://www.anybodytech.com/58.0.html. It will not

answer the questions above yet, but a chapter on muscle models is

temporarily lacking and will be uploaded soon. The chapters already

available may help you in your further work.

> And lastly, I represented the shoulder joint exactly as I

> represented the elbow joint (as a revolute joint with a

> fixed position and velocity), and I am not getting any

> shoulder reaction forces. I am getting elbow reaction

> forces, like I said, but no shoulder ones. Any suggestions?

That sounds strange and like there is some problem with your model.

But I think we need to take a look at the model to give you a proper

answer.

> Sorry for the long email!

No problem at all. We are here to help.

Best regards,

AnyBody Support