Selecting Models and Muscles

The topics covered in this section include:

Needed: graphic that defines the different parts: muscles/muscle points

Muscle Paths and Muscle Points

The path of a muscle is defined by a series of attachment points. In the simplest case, each attachment point is fixed to a body, and the path of the muscle is the set of straight lines connecting each pair of adjacent points. These attachment points are called fixed points.

There are three other types of muscle points that can be used to define a muscle path. Via points are attachment points that are fixed to a body, but they are used in the muscle path only when a specified coordinate is in a certain range. These points can be used to implement simple cases of wrapping, such as the quadriceps wrapping over the distal femur when the knee flexes beyond a certain angle.

Another type of attachment point is called a moving muscle point. These are points whose X, Y, and/or Z offsets in a body's reference frame are functions of coordinates, rather than simple constants. This type of point is useful when you want the muscle path to move as a joint flexes, but wrap objects are not suitable for implementing the proper motion.

The last type of attachment point is a wrap point. Wrap points are attachment points whose XYZ offsets are calculated automatically by OpenSim in order to wrap a muscle over the surface of a wrap object. Wrap objects are geometric shapes (spheres, ellipsoids, cylinders, and torii) that you can use to constrain the paths of the muscles. When the straight-line path of a muscle intersects a wrap object, an algorithm calculates a new path between the two points that wraps smoothly over the object. To define the new path, two wrap points are introduced: one at the tangent point where the path initiates contact with the object, and one at the tangent point where the path breaks contact. The muscle path between these two wrap points is a curved path that follows the surface of the object.

Selecting a Model to Edit

Like all of the model editing tools, the Muscle Editor operates only on muscles in the current model. When you change the current model, the Muscle Editor automatically switches to operate on this model, and displays the first muscle in the model.

Selecting a Muscle

To select a muscle for editing, you must first make sure that the model the muscle is in is the current model visit Opening, Closing, and Using the Navigator Window for more information on how to make a model current.

Needed: graphic illustrating the steps below

There are two methods to select a muscle. The first uses the Navigator window:

Expand the tree for the model to access the muscle of interest. You can do this by clicking on the plus (+) sign next to the name of the current model to display the model components. Click on the plus (+) sign next to Forces, and then click on the plus (+) sign next to Muscles to display the list of muscle groups. If the muscles in your model are not organized into groups, you will see a list of the muscles instead of groups. Expand the group of the muscle you want to edit by clicking on the plus (+) sign next to the group name.
Right click on the name of the muscle.
Choose Edit… from the drop down menu, and the Muscle Editor will update to show the properties of that muscle.

The second method of selecting a muscle for editing is from within the Muscle Editor window.

	In the top-left corner of the window, just below the model name, there is a box containing a list, sorted alphabetically, of all of the muscles in the current model. The selected muscle is shown in the box. To switch to a different muscle, left click on the box and choose another muscle.

Next: Muscle Editor Panels

Home: Muscle Editor