How to create muscles with correct xi directions for mechanics

How to create muscles with correct xi directions for mechanics

If you're developing muscles from scratch (i.e. digitising) then i suggest you do it this way as it will save you having to change your xi directions at a later date. This is especially important if you are going to be doing any mechanics analysis with your muscles. Xi 3 needs to run through the thickness of the muscle, i.e. from outside to inside, if you are going to apply any pressure in this direction. So correspondingly this means xi 1 goes bottom to top and xi 2 goes left to right.

Once you've digitised and selected your nodes from the 3D data cloud you need to use the element creator tool (on command window select >model >element creator) to connect the nodes to form linear elements. These need to be selected in a precise order as detailed below using a simple cube example:

1. Select the bottom left (front) node
2. Select the top left (front) node
3. Select the bottom right (front) node
4. Select the top right (front) node
5. Select the bottom left (back) node
6. Select the top left (back) node
7. Select the bottom right (back) node
8. Select the top right (back) node

If you were to have more cubes/elements, say 3 columns x 2 rows, then just repeat the same process working from left to right across the columns before you move bottom to top up the rows.

This image shows the node selection for the adductor brevis muscle. The numbering on the nodes of the standard element matches that detailed above.

Collapsed elements are selected in a similar manner, but have repeated nodes. In this case the element is collapsed in the xi3 direction. I tend to use collapsed nodes on the outer edge of all of my muscles, but they are particularly useful when the muscles are quite thin (in xi3) compared to their length in xi2.