A Muscle pin is similar to a 
Muscle Rig, only it is much simpler and designed to provide a single region of control.
Just like muscle rigs, muscle pins can be incorporated into the capturing of non-FEM Franken Muscles, or they can provide constraint regions for FEM dynamics simulated Franken Muscles.
Use the input to attach a parent to the muscle pin. You can do this manually by wiring an object directly into the input, or you can use the 
Attach To Bones shelf tool in your scene view.
For tool information see: 
Muscle Pin.
Parameters
Flex
Tension
This parameter drives muscle flexing. A tension value of 0.0 will put the muscle in a fully relaxed state, while a value of 1.0 will be fully tensed.
Handles
Handle Scale
This is the scale value applied to the pin-shaped handle visible in the viewport. The handle scale is a visual aid only and does not affect any functional part of the muscle.
Keep Position When Parenting
When parenting relationships are changed, determine whether or not the world space position of the muscle pin is maintained.
Output Transform
If any object node is wired to the output of this muscle pin, this parameter designates one of the muscle pin’s internal nodes as the source for the output transformation. For example, if this parameter is set to Input Object, attaching a child node to the output of this muscle will transform the child along with the parent of the muscle pin, and this object will not add to the hierarchical transform.
Handle Transforms
Reset Handle Transforms
Activating this button will reset all transform values and return the handle to the default position.
Translate
Rotate
Scale
The local transformation applied to the muscle pin.
Note
In contrast to Muscle Rig handles, a Muscle Pin utilizes Scale here to control the "animated" or "live" scale of captured geometry. This scaling is analogous to the concentric radius scaling found on muscles and muscle rigs. This scale parameter will not affect the FEM anchor region scale. See the Physical Properties tab on this asset to scale the FEM anchor region.
Capture
These parameters affect the static capture state of a muscle pin for both non-FEM and FEM dynamics workflows. When following the non-FEM workflow, a Franken Muscle’s geometry is first constructed using its build parameters in a static state, and then deformed into the "animated" or "live" position dictated by the muscle rigs and muscle pins. All muscle rigs and muscle pins are collected by the Franken Muscle, and used as a point deformer on the muscle geometry. This deformer must first capture the muscle geometry in its rest state, or capture pose, and reshape the captured muscle according to the position and orientation in the "live" or "animated" state. When using muscle pins in a FEM dynamics muscle setup, the capture pose establishes the base position where the anchor region is applied to the Franken muscle’s geometry. The capture pose is subsequently an important aspect of a muscle pin’s configuration as the rest of the muscle system will rely on the existence of this pose as a base position for the initial orientation of muscle geometry.
Edit Capture Pose
When active, the muscle pin will be displayed in its Capture Pose. When muscle pins are displayed in the Capture Pose, they can easily be distinguished from a "live" condition by their appearance. The muscle pin handles will turn blue, and change from the being "round" to being "boxy".
The muscle pin handle can be manipulated in the viewport when this mode is active, and this will directly affect the capture transformation parameters for this pin.
Set Capture Pose
This button will record the muscle pin’s current "live" position into the Capture Pose.
Copy Capture Pose to Current Pose
This button does the inverse of the Set Capture Pose button. The stored capture state parameters are extracted from the Capture Pose. The "live" pose is set to match the capture pose.
Capture Geometry
This parameter allows you to change the capture geometry used in non-FEM captures.
Built-in Geometry
The points used to affect non-FEM deformations of Franken muscles will be supplied by the internal box operator.
External Geometry
Allows you to specify an external operator to supply the capture points.
Divisions
The number of edge divisions to apply to the internal box geometry (capture geometry).
Object
Group
The operator path (SOP) and optional primitive group to be used as the capture geometry. This parameter applies when the Capture Geometry is set to External.
Capture Pose
The initial world space transformation of the muscle pin handle is stored here to record the Capture Pose. These parameters are not generally meant to be modified manually. Use the Set Capture Pose button to record the current handle transformation into these parameters. The capture pose transformation establishes the initial spatial relationship between the muscle pin and the affected muscle geometry prior to the muscle pin being animated.
Dynamics
Anchor Region
Anchor regions are used to constrain a muscle and affix it to a parent transformation when simulated with dynamics.
In a conventional muscle setup, the end handles of a muscle rig each carry a constraint region that affects freedom of movement of the muscle geometry nearby. The scale of this constraint region will encapsulate more or less of the nearby muscle geometry. The geometry contained with the bounds of the anchor region will be constrained using the strength and damping parameters found on the containing asset of the muscle geometry.
A Muscle Pin asset will only supply a single constraint region with a corresponding radius.
Display Regions
Toggle this parameter to display the scale of the muscle anchor region. Once displayed, the region can be selected and scaled interactively in the viewport.
Head Radius
The anchor region radius. When constraining muscle geometry with an anchor region, all tetrahedrons captured with the radius of the anchor will have the constraint strength applied to them during the dynamics solve. The constraint strength parameter can be found on the either Muscle or Franken muscle assets.
Display
Display
Use this toggle to control the display visibility of this muscle pin asset in the viewport.
Handles
Display toggle for the muscle pin handle.
