# muscle moment arms

Dear support team

I want to check the moment arms of some muscle: is there a way to
calculate the instantaneous muscle moment arm? The muscle length change
in dependence of the joint angle only represents the moment arm
variability, but if I want to now the effective moment arm, I need the
absolute distance at least for one time point?
Is it possible to present the muscle force as force vector: i.e. I want
to know the line of action of the muscle forces inserting at a specific
segment in the local coordinate system of this segment?
Christine Schaerer

Hi Christine

Yes it is possible to measure the effective moment arm. This is done
by setting the joint velocity to 1, if you then look at at the
muscle velocity LmtDot it will display the momentarm.

I have made a small sample which shows how to do this, please see
the file “MomentArmMeasure.any”. It contains a muscle which is
wrapped around a cylinder, and LmtDot ends up with the magnitude
equal to the size of the cylinder.

Your second question can not be solved yet, in an elegant way. It is
not possible to use the AnyForceMomentMeasure for this, since it can
not measure forces from muscles.

You will have to do the following :

1 create a dummy segment with zero mass
2 connect the node on the segment that the muscle inserts in with
the dummy segment using a AnyStdJoint, it will lock all six dof
3 move the muscle insertation point to the dummy segment
4 read out the forces in the AnyStdJoint they will be given in the
local segment coordinate system

Best regards
AnyBody support

— In anyscript@yahoogroups.com, “christine_schaerer”
<christine_schaerer@y…> wrote:
> Dear support team
>
> I want to check the moment arms of some muscle: is there a way to
> calculate the instantaneous muscle moment arm? The muscle length
change
> in dependence of the joint angle only represents the moment arm
> variability, but if I want to now the effective moment arm, I need
the
> absolute distance at least for one time point?
> Is it possible to present the muscle force as force vector: i.e. I
want
> to know the line of action of the muscle forces inserting at a
specific
> segment in the local coordinate system of this segment?