Editing Attachment Points

The paths of the muscles can be modified by selecting attachment points and moving them to new locations in the reference frames of the bodies to which they are attached. You can select multiple attachment points on multiple muscles and move them at the same time. Fixed points and via points can be selected and moved interactively in the model window. Moving muscle points cannot be selected, but the functions defining their movement can be modified with the Function Editor. Wrap points cannot be selected or moved, because their positions are calculated by the muscle wrapping algorithms.  The topics covered in this section include:

Selecting Attachment Points

There are two methods to select muscle attachment points. The first method operates in the 3D View window. Press the ctrl key to enter the selection mode. You will see the cursor change into a small crosshair to indicate that you are in selection mode. Keep the ctrl key pressed and left click on an attachment point to select it. If you also hold down the shift key, you can select multiple attachment points. To unselect all points, press ctrl and left click on a point away from the model.

The second method of selecting attachment points is to click on the Sel checkbox in the Attachments panel. Choose the muscle whose points you want to select (Selecting Models and Muscles), and then click on the Attachments tab. Click on the Sel box of the desired point(s). You can select points on additional muscles by switching to each of those muscles in turn, and selecting their points in the Attachments panel. This method of selecting attachment points is useful if there are several coincident points in the model (e.g., the insertion point of soleus, lateral gastrocnemius, and medial gastrocnemius on the calcaneous), because selecting coincident points in the 3D model view is problematic.

When an attachment point is selected, it is displayed in yellow on the 3D model, and its name is shown in the text bar at the bottom of the OpenSim window.

Moving Attachment Points

There are two methods for moving muscle attachment points. The first method is to move points interactively in the 3D View window. Select the points you want to move visit Selecting Attachment Points for information on how to select attachment points. Move the cursor over any of the selected points, and press the left mouse button. While holding the button down, you can drag the attachment points within the plane of the screen. All of the selected points will move the same amount, so it does not matter which one you click on. To move the points in a different plane, release the left mouse button, rotate the model view as desired, and then press the left mouse button on any selected point to resume dragging.

The second method of moving attachment points is to type their exact XYZ offsets into the number fields in the Attachments panel. For this method, the muscle points do not need to be selected, but you can only modify the attachments of the current muscle. Click on the appropriate X, Y, or Z number field, and type in the desired value. These offsets are expressed in the reference frame of the body to which the point is attached, which is shown just to the right of the Z field. For the moving muscle points, the number fields are replaced by Edit buttons. When you click on an Edit button, the function for that offset will be displayed in the Function Editor so you can modify it.

Adding and Deleting Attachment Points

The Attachments panel contains buttons for adding and deleting muscle attachment points. These are located below the list of current attachments.

When you click the left mouse button on the Add button, a drop down menu is displayed, giving you a choice of where to add the new point. If you add the new point between two existing points, it will be added halfway between the two. If you add the point before the first or after the last point, the new point will be added a short distance away from the appropriate end point. You can then select the point and move it to the desired location, or type its exact XYZ coordinates into the number fields in the Attachments panel.

To delete an attachment point, click on the Delete button and choose the index of the point you want to delete. For both the add and delete functions, the indices shown in the drop down menus are the same as the indices of the muscle points shown in the Attachments panel. They do not depend on whether or not via points are active, and do not take into account any wrap points which may be currently included in the path.