One advantage of the Muscle Creator window is that you can use it to help sculpt your muscle shapes by growing them. That is, you can select a muscle and have its points “grow” outwards to the actual model surface. This can speed up your muscle modeling time since you no longer need to manually edit the shapes of the muscles.
For reference, the window needs to know what joint or bones run down the closest to the length of the muscle.
For example for a biceps muscle, you want to select the UpperArm and Elbow joints respectively for the Main and Tip bones. The window attempts to select the Main Bone Tip object automatically. If it is unsuccessful, or if it does not find the correct bone, select the correct tip bone and press the other button to load that bone as the tip.
By default, the growing occurs outwards from the centerline defined by the start/Main and end/Tip bones you selected. All points within the Angle range specified in the Grow settings are grown.
Depending on the values you have set in the %Rest/Sq/St fields, the muscle cross sections grow out to the surface you selected as the Grow To Surface. If you have set the %Rest/Sq/St values higher than 100%, the muscle grows based on the shape of the selected Grow To Surface mesh, but can grow past the selected mesh.
It is important to note that growing muscles is effectively moving the cross section points for the muscle. Since the muscle is a NURBS surface and the points of the cross section do not lie exactly on the surface, there may be offsets from the surface mesh even if the fat value is 0 and the percentage is set to 100.