# PEFactor

Hi Soeren,
Geometry is no longer a problem, now I start to look for the muscles.
The question I have is about the definition of the parallel elastic
part of the Hill muscle modell: Does a PEFactor of 5 really mean that
I have to pull on a non contracted muscle until he is four times
longer than before to make him produce the same force as if he
contracts at his optimum muscle length? This seems to be very much. I
donÂ´t know a muscle you can pull so much without destroying him. Do I
understand the definition of PEFactor right or does it mean something
else?
With best regards, Maren

Hi Maren

When the PEFactor is five you can theoretically pull the muscle five
times its fiber length before the F0 force is obtained. This do not
mean that this will ever occur in the model. In a typical muscle
operation range the elongation would be maybee 50% of the fiber
length. If we had PEFactor in such a case of 1 this would mean that
the passive force would be 50% of F0, having PEFactor equal 2 would
reduce this to 25% of F0, and PEFactor equal 4 would reduce it to
12.5%. So for the arms and legs i think a PEFactor of 5 is resonable,
you can move through the normal range of motion without eccessive
passive forces.

It should also be mentioned that these passive forces are the ones
which appears in the muscles only, the forces in ligaments is
something else.

Here is an explanation from the muscle tutorial

http://www.anybodytech.com/507.0.html

you might already have seen this, but here it is anyway:

This factor is related to Jpe. Where Jpe controls the shape of the
nonlinearity, PEFactor controls the steepness of the force in the
parallel-elastic element as it is elongated. If we imagine a
completely inactive muscle and load the muscle with a force
corresponding to the active strength of the muscle, i.e. F0, then the
length of the elongated muscle fibers will be PEFactor x Lfbar. In
other words PEFactor is a dimensionless measure of the flexibility of
the parallel-elastic element of the muscle.

Plausible values for PEFactor would be between 1.5 and 5 where the
lower end of the interval requires a very careful tuning of the muscle
to the skeleton to avoid unreasonably large passive forces.

So when we use values of 5 this is to avoid too large passive forces,
this is a way of reducing the influence of passice forces. You may
argue that it should be lower for some muscles but this will require
very carefull tuning of the muscle to the skeleton to avoid very large
passive forces. So you can reduce the number, but keep an close eye on
the size of the passive force.

Best regards
SÃ¸ren, AnyBody Support

— In anyscript@yahoogroups.com, “Maren” <anystarter@…> wrote:
>
> Hi Soeren,
> Geometry is no longer a problem, now I start to look for the muscles.
> The question I have is about the definition of the parallel elastic
> part of the Hill muscle modell: Does a PEFactor of 5 really mean that
> I have to pull on a non contracted muscle until he is four times
> longer than before to make him produce the same force as if he
> contracts at his optimum muscle length? This seems to be very much. I
> donÂ´t know a muscle you can pull so much without destroying him. Do I
> understand the definition of PEFactor right or does it mean something
> else?
> With best regards, Maren
>