The Franken Muscle object, like a common Muscle object, is made of two primary components: the muscle geometry and the muscle rig.
The muscle geometry can be derived from any geometry operator in Houdini. Optionally, multiple geometry sources can be incorporated into this single muscle. Any and all geometry surfaces included as sources will be first converted to a volume, merged, re-surfaced, and made solid to form the franken muscle geometry component.
The rig component is made up of a merged collection of external 
Muscle Rigs and 
Muscle Pins.
While the geometry components of a franken muscle are internalized and modified somewhat, the rig components remain as distinct entities. Interacting with handles, or tension parameters, or Jiggle qualities, etc. is all accomplished via the sourced muscle rigs and muscle pins directly. The franken muscle only serves to bring all the components together to produce a valid muscle within the muscle system.
Existing muscles can be used both as sources for geometry and as rigs. For example, you can merge together multiple existing muscles to create a single new muscle with this tool. A more typical scenario, however, is where you will provide a custom model and that you will want to control with muscle rigs and muscle pins.
Note
The geometry used to build your franken muscle should be derived from a non-animated source. The geometry and any of its attributes must be static and unchanging over time. If a time-dependent model is referenced, this will cause the franken muscle to rebuild itself every time it is cooked.
For viewport tool information see: 
Franken Muscle.
Parameters
Muscle ID:
Muscle ID
A tag used to identify this muscle asset within the muscle system. A muscle ID can be any string or numeral. The muscle ID is used by other muscle assets to group muscles that share a common muscle ID, or to keep muscle influence separate where muscle ID’s are unique. For example, the Muscle Deform SOP will treat muscles with a shared, common muscle ID as a single compound displacement tool. Also, Tissue Solver SOP will attach region constraints to muscles if the constraints and the muscles share common ID’s.
Geometry
These parameters affect the construction of the geometry component of the franken muscle. A franken muscle can be used in either FEM dynamics workflows, where a solid tetrahedral mesh is required, or in non-FEM deformer workflows where only surface geometry is required. Therefore, a franken muscle can produce both a tetrahedral solid and surface geometry.
Bypass Solid Embedding
This parameter bypasses the conversion of the muscle geometry into a solid tetrahedral mesh. In non-FEM workflows, where the 
Muscle Displace tool is used, only the muscle surface geometry is required. By enabling this toggle, the extra processing required to create a solid will be avoided.
If this franken muscle is used in an FEM workflow, the 
tissue solver will disregard the state of this toggle and fetch the tetrahedral solid regardless.
Cap Open Meshes
If polygonal surfaces are are not water-tight, this toggle can be used to ensure a Poly Cap step is performed prior to solidifying.
Remove Stray Pieces
In the conversion process of your surface geometry into a solid, situations may arise where some finer details of your geometry are not large enough to be fully resolved by the solidification parameters below. This can lead to the creation disconnected fragments of franken muscle geometry. If this occurs, you have the option to reduce the Initial Voxel Size or Iso Divisions to attempt to capture finer details in your source geometry. This, of course, may come with a greater expense in processing time and memory. Or, by use of this toggle, fragments with smaller surface area will be discarded.
Reverse Normals
In order for the Muscle Displace tool to function properly, surface normals on the sourced geometry must be oriented correctly (ie, outwards relative to the muscle center). Use this toggle in situations where the surface normals of your geometry sources are pointing inwards towards the muscle centers.
Tet Mesh
These parameters set the resolution and scaling options for the Solid Embedding step within this muscle asset.
For detailed help on these individual parameters, see the 
Solid Conform SOP.
Geometry Sources
These parameters specify the Houdini objects and SOPs that provide the franken muscle with its surface geometry.
Stashed Source Geometry
Use Stashed Geometry
Use the local internal copy of the source surface geometry if it was previously stashed (stored).
Stash Geometry
Activate this button to store a local internal copy of the referenced source geometry. Once a stashed copy has been created, the franken muscle can be made independent of the original source geometry.
Clear Stashed Geometry
The internal local copy of the surface geometry can be cleared using this button. Once cleared, the franken muscle asset will then resort to using the path specifications below to find its sources.
Live Source Geometry
The following parameters are promoted from the Object Merge SOP contained within the franken muscle. For additional detailed help, please see Object Merge.
Number of Objects
The number of source surface operators to include in this franken muscle.
Enable Merge
The parameter path directly below this toggle can be included or disregarded using this switch.
Geometry Path
Specify the path to a surface operator (SOP) to supply the franken muscle with a geometry source.
Group
The optional primitive group to extract from the Geometry Path specified above.
Expand Group
Adds neighboring primitives to the selected Group.
Rig Sources
These parameters specify the Muscle Rigs and Muscle Pins to include in this franken muscle. Control of the rig behavior remains with the referenced muscle rigs.
Display All Rig Sources
Convenience toggle that will turn on or turn off the display of all the referenced rig components.
Add or Remove Rigs
Use this button to enter the viewport into a selection state where you can include or exclude muscle rigs to form the reference list below.
Number of Rigs
The number of rig paths you would like to include.
Muscle Rig Path
The path to the Muscle Rig or Muscle Pin that you would like to include as a controller rig for this franken muscle. The path may be absolute or relative, but must point to the object level muscle asset. TIP
A Muscle object can also be referenced as a valid source for a Muscle Rig.
Capture/Deform
Whether a Franken Muscle is constructed as a solid or as a surface, the geometry undergoes a Capture and Deform series of steps to take the constructed muscle from it’s static pose into the animated or live position as dictated by the referenced muscle rigs. The capture pose is subsequently an important aspect of a franken muscle’s configuration as it establishes the relationship between the geometry and rig components. The capture pose is comprised of the geometry in it’s initial static referenced position, and each of the rig components in their respective capture pose.
Edit Capture Pose (All Rig Sources)
When active, the franken muscle will be displayed in its Capture Pose. All component geometry and muscle rigs will be displayed in their respective capture pose or setup positions. When franken muscle are displayed in the Capture Pose, they can easily be distinguished from their "live" state by their appearance. The muscle geometry will be displayed as its merged constituent parts, the coloring will turn blue, and the rig handles will change from the being "round" to being "boxy".
Handles can be manipulated in the viewport when this mode is active, and thus the Capture Pose can be modified.
Biharmonic Capture Mesh
Capture weighting applied to the geometry is computed via biharmonic capturing. For additional help on biharmonic weighting see Bone Capture Biharmonic and Bone Capture Lines.
Display Guide
Enabling this toggle will display a guide geometry that represents the resolution and shape of the tetrahedralized capture mesh.
Capture Triangle Size
The construction of the capture mesh is reliant on this size parameter. This size parameter is expressed as a relative scale proportional to the overall enclosing volume surrounding the franken muscle geometry. Use smaller values to preserve more capture weight detail when muscle rigs are situated closer to each other. Coarse capture triangle sizing can produce adequate results in most cases at a reduced computational expense.
Capture Smoothing
Specifies the smoothing strength applied to the capture weight data prior to deforming the muscle geometry.
Capture Display Color
The color applied to this franken muscle when displaying the capture pose.
Dynamics
Every muscle asset contains a local dynamics solver for the purpose of previewing the simulation effects of the physical properties.
For detailed help on the simulation parameters, see the 
Tissue Solver SOP.
Use Physical Simulation
Enables the "preview" dynamics simulation of this muscle. Activating this toggle will simulate the muscle using its physical properties. The muscle build type is automatically set to Built-in Tet Mesh when this toggle is activated. Dynamics simulation requires that the muscle be constructed as a tetrahedral mesh.
Note
This toggle will not affect how this muscle might be incorporated into an external Tissue Solver. However, for more efficient run time performance, it is advised to leave this toggle "off" to avoid simulating a muscle independently as a preview at the same time as simulating the muscle as part of the larger tissue solve.
Display
Display
Use this toggle to control the display visibility of this muscle asset in the viewport and/or renderings.
Color
The color ramp applied to the franken muscle geometry from end to end (ie mapped from muscle pin and end anchor positions along the lengths of the individual muscle rigs).
