The Muscle Editor gives you access to all of the parameters of the muscles and other actuators in the model. The paths of the muscles can be altered by selecting and moving attachment points in the 3D View window, and the force-generating parameters can be viewed and modified in the Properties window.  The topics covered in this chapter include:

As described in the section on Property editing, you can select muscles for editing by either selecting the corresponding nodes in the Navigator window (you may need to expand the nodes for the top level model, Forces, Muscles, all) to see the nodes corresponding to individual muscles in the model as shown below. You can also select muscles in the 3D view window.

Open

Using 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.  Next, click on the plus (+) sign next to Forces. If the muscles in your model are organized into groups, then click on the plus (+) sign next to Muscles to display the list of muscle groups. Expand the group of the muscle you want to edit by clicking on the plus (+) sign next to the group name. If the muscles in your model are not organized into groups, you will see a list of the muscles instead of groups. Click on the name of the muscle and its properties will show in the "Properties" window. Using the 3D View Window Although muscle segments are not selectable in the 3D view, muscle points are selectable.  To select a muscle, you can one of its attachment or other points. Then muscle associated with that muscle point will be selected in the Navigator window. Its properties will be shown in the "Properties" window.  To pick any object in the 3D view it must belong to the current model, please consult the section Opening, Closing, and Using the Navigator Window for more information on how to make a model current.

Open

Using 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.  Next, click on the plus (+) sign next to Forces. If the muscles in your model are organized into groups, then click on the plus (+) sign next to Muscles to display the list of muscle groups. Expand the group of the muscle you want to edit by clicking on the plus (+) sign next to the group name. If the muscles in your model are not organized into groups, you will see a list of the muscles instead of groups. Click on the name of the muscle and its properties will show in the "Properties" window. Using the 3D View Window Although muscle segments are not selectable in the 3D view, muscle points are selectable.  To select a muscle, you can one of its attachment or other points. Then muscle associated with that muscle point will be selected in the Navigator window. Its properties will be shown in the "Properties" window.  To pick any object in the 3D view it must belong to the current model, please consult the section Opening, Closing, and Using the Navigator Window for more information on how to make a model current.

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.

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The Geometry Path of a muscle is a representation of the muscle's line of action. This is a "Property" of all the muscles in OpenSim, as such it shows as an entry in the Properties window as shown above. If you click the ... Button next to the GeometryPath in the Properties window, the dialog at left will appear. The Attachment Panel, displays the fixed, via, and moving muscle points that define the muscle path.  The Wrapping Panel displays the list of wrap objects that are currently associated with the muscle.

Open

The Geometry Path of a muscle is a representation of the muscle's line of action. This is a "Property" of all the muscles in OpenSim, as such it shows as an entry in the Properties window as shown above. If you click the ... Button next to the GeometryPath in the Properties window, the dialog at left will appear. The Attachment Panel, displays the fixed, via, and moving muscle points that define the muscle path.  The Wrapping Panel displays the list of wrap objects that are currently associated with the muscle.

Open	The Attachments panel gives you access to all of the fixed, via, and moving muscle points that define the path of the muscle. The table below has more information about each column of information in this panel.

Open	The Attachments panel gives you access to all of the fixed, via, and moving muscle points that define the path of the muscle. The table below has more information about each column of information in this panel.

1. 2. ....

<muscle point index> – the index of the attachment point, corresponding to the numbers in the Attachment panel shown at left. This index is used by the Add and Delete buttons, described below, as well as the start point and end point parameters for wrapping. These indices do not change when via points turn on or off or when wrapping points are added to the path. They change only when you add or delete fixed, via, or moving attachment points.

1. 2. ....	<muscle point index> – the index of the attachment point, corresponding to the numbers in the Attachment panel shown at left. This index is used by the Add and Delete buttons, described below, as well as the start point and end point parameters for wrapping. These indices do not change when via points turn on or off or when wrapping points are added to the path. They change only when you add or delete fixed, via, or moving attachment points.
Open	Sel – this checkbox indicates whether or not the muscle point is selected in the model window. Selected attachment points can be dragged to new locations using the mouse.
Open	Type – the type of the attachment point: fixed, via, or moving. Fixed points are points whose XYZ offsets are fixed in a body's reference frame. Via points are 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. Moving muscle points are points whose X, Y, and/or Z offsets in a body's reference frame are functions of coordinates, rather than simple constants.
Open	X – the X offset of the attachment point in the body's reference frame. For fixed and via points, this offset is a constant. You can change the value by clicking in the number field and entering a new value. For moving muscle points, the offset is a function of a coordinate, rather than a constant. To edit the function, click the Edit button to display it in the Function Editor window.
Open	Y – the Y offset of the attachment point in the body's reference frame. For fixed and via points, this offset is a constant. You can change the value by clicking in the number field and entering a new value. For moving muscle points, the offset is a function of a coordinate, rather than a constant. To edit the function, click the Edit button to display it in the Function Editor window.
Open	Z – the Z offset of the attachment point in the body's reference frame. For fixed and via points, this offset is a constant. You can change the value by clicking in the number field and entering a new value. For moving muscle points, the offset is a function of a coordinate, rather than a constant. To edit the function, click the Edit button to display it in the Function Editor window.
Open	Body – the body to which the muscle point is attached.
Open	Coordinate – For via points, this is the coordinate that controls whether or not the point is included in the muscle path (active). The point is active only when the coordinate is between the specified minimum (Min) and maximum (Max) values.
Open	Min – For via points, the lower bound of coordinate values for which the point is active. If the coordinate has a value that is below this minimum, the attachment point is not included in the path of the muscle.
Open	Max – For via points, the upper bound of coordinate values for which the point is active. If the coordinate has a value that is above this maximum, the attachment point is not included in the path of the muscle.