Joint Reactions in Standing Model

Hello,

I have some questions about the joint reaction force values I am
getting from the Standing Model. I have ran the model and I’ve got the
following values for leg joints in global y direction:

Right Ankle Fo-y= Left Ankle Fo-y= -492.7821
Right Knee Fo-y= Left Knee Fo-y= -620.8352
Right Hip Fo-y= Left Hip Fo-y= 715.9036

Since this model is a static one, I have been approaching the model
as if it were a beam system with connecting hinges. Since the mass of
the model is 75kg, I can’t understand how there can be a reaction
force larger than 375N in any of the leg joints. I also don’t
understand why the direction of the reaction is positive in the hip
joint while it is negative in the knee and ankle joints. I would also
expect the reaction force to be highest in the ankle instead of the
hip joint. Where am I doing the mistake?

Best wishes,
Ceren Saglamer

Hello Ceren,
I’ll try to answer your question.

Muscles create forces which act across joints. The reactions at the
joints are also a function of the muscle forces, therefore they can be
quite different than the external forces and the body weight. For
examply, during lifting, loads on the lower back joints can be as high
as 7000 N, or higher.

As for the directions, the lines of action of the muscles affects the
directions of the reactions.

Brent Ulrey

— In anyscript@yahoogroups.com, “cerensaglamer” <ceren.saglamer@…>
wrote:
>
>
> Hello,
>
> I have some questions about the joint reaction force values I am
> getting from the Standing Model. I have ran the model and I’ve got the
> following values for leg joints in global y direction:
>
> Right Ankle Fo-y= Left Ankle Fo-y= -492.7821
> Right Knee Fo-y= Left Knee Fo-y= -620.8352
> Right Hip Fo-y= Left Hip Fo-y= 715.9036
>
> Since this model is a static one, I have been approaching the model
> as if it were a beam system with connecting hinges. Since the mass of
> the model is 75kg, I can’t understand how there can be a reaction
> force larger than 375N in any of the leg joints. I also don’t
> understand why the direction of the reaction is positive in the hip
> joint while it is negative in the knee and ankle joints. I would also
> expect the reaction force to be highest in the ankle instead of the
> hip joint. Where am I doing the mistake?
>
> Best wishes,
> Ceren Saglamer
>