The new forum looks great.
Hi,
I have being looking at the repository models, and am interested in applying the formulas for calculating segment radius and Jii values to another model I’m building (as below).
AnyVar Radius =
(Mass/(3.1416Length…StandardParameters.Shank.Density))^0.5;
I’m just enquiring about the density value of 1000, which is the same for all lower extremity segments. Is this the density of the actual segment or another feature such as the bone. Or, can this value be assumed for all body segments?
Thanks
Pete
Welcome to the new forum.
The density of 1000 represents the density of the human body in general. So when applied to a segment it should be seen as as the equivalent density of the mixed amount of bone, muscles, blood and skin.
Tehrefore this value is assumed to be the same for all body segment.
Hi Slyvain,
Thanks for the quick response.
I am experiencing a bit of a problem with segment interia properties and wondered if you could help. I am currently building a model of a pre-natal lower extremity. Due to the simple nature of the model (i.e. limited movement) I have started it from new, as there was no other model like it in the repository.
I successfully modelled the kinematics and am now studying the inverse dynamics. Gravity has been eliminated (as it is not experienced in utero), but since I want to examine the influence of segment mass on muscle activity, the segment moment of interia is required (hence why I was interested in how it was calculated in the repository models).
However, when I apply the known body parameters to calculate Jii of the femur for example (only the segment mass=0.2345kg and length=0.0599m are known), the calculations produce values of Jii = 0.000143, 0.000146, 0.000143. These values not only produce a segment which it far too big for the model, but the Iyy value is greater than Ixx & Izz, producing a segment not similar to those of the repository. For these calculations I am presuming the same density as in the repository.
Is there any other method which can determine the Jii of the segment for such a small model, with only the length and mass known?
I would be grateful of any help.
Thanks
Pete
I have reviewed the formulation of the interia of the Femur in the models and i think it is ok. I tried to do a small model replicating your numbers and i get the same numbers for the inertia as you do, please see the attached model.
I think the problem is that the mass is too big for the segment length. The inertia is calculated as a cylinder and part of this caluclation is the raddi of this cylinder and it is 0.03530m so the diameter is larger than the length of the cylinder. This is also what is reflected in Jii, so i think the either the mass, the length should be changed.