Soft Tissues

The types of force-producing soft tissues available in LifeMOD/BodySIM™ are ligaments and muscles/tendons. Both soft-tissue elements transmit tension forces only. Ligaments are passive spring/dampers and muscles/tendons produce tension forces between bone attachments.

Just like the joints, LifeMOD/BodySIM has two types of muscle formulation. The first is trainable muscle, or "effective force" formulation, and the second is a Hill-type muscle element. For a technical description, see Muscle Formulation.

The trainable muscle elements consist of "training" muscles for inverse dynamics simulations and active (contractile) muscles for forward dynamics simulations. Muscles learn and record shortening/lengthening patterns while the model is being driven by the motion capture data in an inverse-dynamics simulation. They then repeat those patterns and serve as actuators for the forward-dynamics simulations. The muscle actuators are programmed not to exceed the physiological limits of the individual muscle.

The Hill-type muscle formulation is the traditional combination of a contractile element (CE) and a parallel elastic element (PE) describing the passive force. The contractile element contains an muscle activation state which controls the active muscle force capability for the muscle. Data from an EMG test may be used as activations for the contractile element.

Various muscle parameters may be adjusted. See Parameters to tune the model for simulation. Choosing Model Parameters offers more information on data sources and on how to select the parameters mentioned in this section.

Sections:

• Creating the Full Body Base Muscle Set
• Creating Trainable Muscle Elements the Model
• Creating Hill-Based Muscle Elements in the Model
• Adding Individual Muscles, Ligaments and Tendons
• Modeling Antagonistic Tissue Effects
• Moving Tissue Attachment Points.
• Reassigning Attachment Points
• Tissue Wrapping - Contact Based
• Tissue Wrapping - Slide-Point Based
• Deleting Soft Tissues

Full Body Muscle Set

A full-body set of 118 muscles is automatically generated and attached to the bones at anatomical landmarks. See the appendix for muscle names and attachments. The muscles and attachments are scalable with the skeletal geometry when creating the body segments.

Figure 1: Full body muscle set. Trainable muscle elements (left) to be trained in an inverse dynamics simulation, trained muscle elements (center) to be used to drive the model in a forward dynamics simulation and Hill-based muscle elements (right) to be used to drive the model in a forward dynamics simulation.

The set covers most major muscle groups in the body. Most large muscles are dispersed in several elements. There may be some cases where a muscle set must be expanded to provide better coverage than available with the standard muscle set. This is accomplished by adding new muscles to the model (see Adding Individual Muscles Section).

The muscles created for each muscle set do not wrap around geometric features such as other bones, however, each tissue may be set up to wrap around features using using either the slide point-based wrapping or the contact-based wrapping features.

Muscle attachment points may be relocated using the tissue relocation points.

Muscles are created on the body in separate sections; legs, arms and head/trunk. Each set is generated automatically and attached to the 19 base segments at predefined, scalable attachment points. Figure 2 displays several segments in an exploded view to illustrate the topology of the muscles. The base set of muscles connect the base set of body segments.

Figure 2: Exploded view of body segments to view muscle connections

Figure 3: Right Click on the muscle graphic to bring up the name

When the muscle set is created, each muscle is attached to a particular standard body segment (see appendix). However, the attachment point may be reassigned to another body segment or a non-standard body segments (see cervical spine example).

To identify a particular muscle, right click on the muscle to bring up the name of the outline element. The name of the muscle is contained in the text string (see Figure 3). For both muscle formulations the muscle sets are divided into the following groups:

• Head/Trunk
• Left Arm
• Right Arm
• Left Leg
• Right leg

To create the muscle set with either Hill-Type Elements or Trainable Muscle Elements, use the main modeling panel shown in Figure 4. When either Hill-type or Trainable muscle elements is selected, a sub-panel is displayed to collect global parameter information. Individual muscle parameters may be edited when selecting the Modify button.

Figure 4: The full body set muscle creation panel for both the trainable elements and the Hill-based elements.

The body may contain any combination of Hill-Type muscle elements and trainable muscle elements (Figure 5). This combination is frequently used to model co-contraction or antagonistic effects.

Figure 5: Model with a combination of Hill-based muscles (brown color) and trainable muscles (rust color) on the right leg.

Creating Trainable Muscle Elements in the Model

The trainable muscle element is a two-phase muscle element. The first phase serves as the "training" phase, where the muscle contractions are recorded during manipulation of the model via motion agents. In the second phase, the individual muscle contractile histories are used to generate a muscular force that maintains the desired contraction pattern.

The muscles may be created in base sets or individual muscles.

Training the Muscles with an Inverse Dynamics Simulation

In the first phase, the training element records contractions when external forces are applied to the body manipulate it. This usually accomplished by using motion agents attached to the the model. The motion agents are directed to move using time-displacement curve data for each axis of motion.

Figure 6: Panel to create tissue set for the right leg. Global parameters in the sub-panel (top) consisting of muscle tone multiplier, resting load and max tissue stress are applied to the right leg muscle panel (below). Individual tissue properties may be adjusted in the muscle panel (below).

Soft Tissues -> Create Base Set
The panel in Figure 6 is used to create the muscle-tendon Base tissue sets for the right leg. By selecting APPLY in the panel, the trainable muscles (rust colored) displayed in Figure 7 are created on the model.

Figure 7: Motion agents guiding leg while learning elements record muscle contractions

After the muscles are generated the body and limbs may be manipulated using motion agents in an inverse dynamics simulation to train the muscles. Figure 7 displays a leg flexion activity using a single motion agent to guide the foot. During this process each muscle in the leg records the muscular contraction.

There are many ways to drive the model during the inverse dynamics simulation. The model may be manipulated with a single motion agent (see total knee replacement utorial) or by using a mechanical system to drive the model (see rehabilitation tutorial and bicycle tutorial), or by using motion data from a motion capture system (see golfing tutorial, dancing tutorial, hip replacement tutorial, gait analysis tutorial).

Using Trained Muscles to Drive the Model During Forward Dynamics Simulation

After the inverse dynamics simulation has been performed the is ready for a forward dynamics simulation using the muscle forces as drivers in the simulation.

Figure 8: Animation slices for the forward dynamics simulation. Note the color of the muscles is now deep red indicating the active elements. Also note the scaling muscle graphics based on muscle force.

Figure 9: Panel to create replace trainable elements with trained contractile elements

Soft Tissues ->Training
The panel in Figure 9 is used to replace each training element with a trained contractile element. Select the light bulb and enter the values for the servo controller proportional gain (P_gain) and derivative gain (D_gain). These values are used in the equation 1 listed in the appendix.

As an example of muscle force calculation, Figure 10 displays the forward dynamics simulation results for the vastus lateralis muscle. The blue curve is the muscle force for an F_filter value of 100%. Note how the force does not exceed the 850 Newton limit. In a second simulation, the F_filter for this muscle is reduced to 25% and is displayed as the red curve in Figure 10. It can be observed that the force output is decreased. By setting a F_filter to 0%, the muscle can be essentially "turned off," or it can be doubled by setting the F_filter to 200%. A wide variety of effects can be discovered and explored simply by altering the force capability of the muscle. See the Total Knee Replacement and the Muscle Relocation tutorials for examples of this effect.

Figure 10: Forward dynamics simulation results for the vastus lateralis muscle. The blue curve is the muscle force for a F_filter of 100%. Note how the force does not exceed 850 N the physiological maximum for this muscle. The red curve indicates the muscle force for the same muscle with a F_filter of 25%.

There is no physiological analogy to the P_gain and the D_gain. These values influence how the muscle will produce the displacement recorded in the proceeding inverse dynamics simulation. A higher value will decrease the error. The D_gain decreases the vibration in the tracking. To gain a feel for the effect of these values, simulations my be performed for altered values of the P_gain and D_gain using the parameters panel and viewing the muscle forces using the strip chart displays form the results panel.

Figure 11 displays the effect of changing the P_gain and D_gain from 1e6 and 1e4 (blue curve) respectively to 1e5 and 1e3 (red curve).

Figure 11: Example of the effect on the muscle force of changing the P_gain and D_gain. The forward dynamics simulation is repeated using P_gain and D_gainvalues of 1e6 and 1e4 (blue curve) and 1e5 and 1e3 (red curve).

Retraining the Muscles (optional)

The muscles may be retrained at any time by selecting the check mark in Figure 12. This replaces the active elements with trainable elements again. When the elements are replaced, the inverse dynamics simulation may be rerun using new motion data, muscles, ground reaction force parameters, etc. The user can switch between the inverse dynamics and the forward dynamics simulations as often as desired.

This is typically done for parametric simulations like the Muscle Relocation Tutorial and the Cervical Spine Tutorial

Figure 12: Panel used to replace contractile elements with learning elements.

Creating Hill-Based Muscle Elements in the Model

The muscles may be created in base sets or individual muscles.

Figure 13: Panel to create Hill-based tissue set for the right leg. Global parameters in the sub-panel (top) consisting of Hill passive and contractile element properties and activation data spline. Individual tissue properties may be adjusted in the muscle panel (below).

Soft Tissues -> Create Base Tissue Set
The panel in Figure 13 is used to create the muscle-tendon base tissue sets for the right leg. By selecting APPLY in the panel, the Hill-basedhem muscles (brown colored) displayed in Figure 14 are created on the model.

Figure 14: Full body muscle set consisting of Hill-based muscles.

Adding Individual Muscles, Ligaments and Tendons

Non-standard muscle forces are individual muscles/ligaments/tendons that are not included in the base set of soft-tissues provided by LifeMOD/BodySIM. For more complex tissue mapping, or for areas of the body that are not included in the base set (ie: hands and feet -- see Grasping tutorial), individual muscles and ligaments may be added to the model. The muscles and ligaments may be set up to wrap around tissues using either the slide point-based wrapping or the contact-based wrapping features. Muscle attachment locations may be relocated using the tissue relocation tools.

Muscles

Individual muscles may be created on the model by supplementing an existing base set of muscles or just adding new muscles. For certain activities, the 118 muscles included in the base set must be supplemented, either by substituting multiple strands for a muscle group or by adding additional muscles to the muscle set for better muscle coverage in the model. When the individual muscles are added to the model they become part of the total body muscle set during training and functioning. Figure 15 displays a model with a base-set of muscles and an augmented base-set of muscles. Figure 16 displays a model consisting exclusively of individual muscles (see Grasping Tutorial).

Figure 15: Original base-set of muscles for the gluteus maximus and the gluteus medius muscles (left), individual muscles added to the set to provide better coverage (right).

Figure 16: Hand model consisting exclusively of individual muscles.

Soft Tissues -> Create Individual Tissue -> Trainable Muscles

The panel used to create individual trainable muscles is displayed in Figure 17. The trainable muscle created using this panel spans part_1 and part_2 at the locations selected on the screen with the cursor.

Figure 17: Panel used to create trainable muscles between part_1 and part_2 at the attachment locations displayed.

Soft Tissues -> Create Individual Tissue -> Hill-Type Muscles

Figure 18 displays the panel used to create Hill-Type muscles. The muscle will span part_1 and part_2 at the locations selected on the screen with the cursor.

Figure 18: Panel used to create Hill-Type muscles between part_1 and part_2 at the attachment locations displayed.

Ligaments/Tendons

Ligaments are spring forces and are used to stabilize joints (see the Total Knee Replacement tutorial for a detailed example). The panel in Figure 19 creates ligaments between part_1 and part_2 at the locations selected on the model with the cursor.

Figure 19: Panel to create an individual ligament/tendon

Soft Tissues -> Create Individual Tissue -> Ligament/Tendon Tissues

To create the soft-tissue, enter the two body segments the tissue would span and the tissue attachment locations. Next, enter strain stiffness, damping and preload (see the panel in Figure 19). In future versions, the user will also be able to specify the free length of the tissue by checking the "Specify Free Length" box next to the three property values. As yet, the feature is unimplemented, though intended. Figure 20 displays the passive tissues (ligaments/tendons) generated in the Total Knee Replacement tutorial.

Figure 20: Left leg mode with a total knee replacement illustrating the patellar tendon, the medial and lateral collateral ligaments. Note the blue color used for the ligament/tendon tissues.

Modeling Antagonistic Muscle Effects

Co-contraction, or antagonistic muscular effects, may be modeled using a Hill-based muscle set or a combination of trainable and Hill-based muscles. Figure 21 below displays a right arm model with both trainable muscle elements (in the trained state) in red and Hill-based muscle elements in gray. By specifying an activation history to control the triceps muscle, they will produce a counter reaction to the biceps when flexing the elbow joint. This methodology may be extended throughout the body for both the base set muscles and the individual muscles. Co-contraction is modeled in greater depth in the Antagonistic Muscles Tutorial.

Figure 21: Right arm modeled with both trainable elements (red color) and Hill-based elements (gray color). Since the Hill-based triceps is based on an activation, user-input activations create a resistance (antagonistic) force to the biceps driving the elbow flexure.

Moving Tissue Attachment Points

When generated, muscles are attached to the respective bones based on geometric landmarks on the bone graphics. Obviously, the location of the attachment will affect its force magnitude during a particular activity (see the Muscle Relocation example). If necessary, any attachment point may be moved by the user.

Soft Tissues -> Edit Properties

Graphically, each muscle is displayed as a line connecting two attachment ellipsoids. To change the attachment locations, bring up the Soft_Tissues->Edit Properties panel and use the tools to move the attachment. A new location may be entered or the current position adjusted using the arrow keys.

Figure 22: Moving the muscle attachment point by selecting the attachment ellipsoid

Figure 23: Panel for changing muscle attachment

Reassigning Attachment Points

If the body segments are refined for greater number and/or detail, muscle attachments on the standard body segments may have to be reassigned to be more specific to the new non-standard body segments (see Cervical Spine example). To reassign the attachment point to a new body segment, bring up the Soft_Tissues -> Edit Properties panel and use the tool displayed in Figure 25, by entering a new global location, or by using the increment tools.

Figure 24: Example of reassigning attachment points (cervical spine tutorial)

Figure 25: Panel used to edit (like Figure 23) and reassign attachment points

Tissue Wrapping - Contact Based

The muscles generated in LifeMOD/BodySIM are all straight and do not wrap around the bone segments; in order to permit proper lines of action for muscle forces, two convenient methods are available in LifeMOD/BodySIM. The first method is a contact-based method which allows soft tissue to "wrap," or bend around a hard tissue (bones). Surface contact forces between the idealized muscular geometry and any bone or structure in the model are automatically created. This wrapping feature is available for all tissues including trainable and Hill-based muscles, ligaments, and tendons. For an illustration of contact based tissue wrapping see the Knee Replacement Tutorial.

Figure 26 displays the panel used to create the contact-based tissue wrapping and tissue-wrapped model itself.

Figure 26A: Panel to create the contact-based tissue wrapping. The numbers on the panel correspond to the knee model below. The rectus femoris tissue (1), is segmented into wrapping elements starting at tissue attachment (2). It will wrap around geometry (3) starting at a distance of (4) from the tissue attachment. The tissue is segmented for a certain length (5) into a specified number of segments (6).

Figure 26B: Knee model with the rectus femorus set up to wrap around the distal head of the femur.

Tissue Wrapping - Slide Point Based

The second tissue-wrapping method is the slide-point tissue wrapping, which allows the user to model tissues sliding or bending around geometric features (tendon wrapping) using slide points. Figure 27 displays a model of the hand and forearm with the flexor digitorum profundus muscle group before slide points are introduced (left) and after (right). The slide points (green) are rigidly attached to a user-specified segment. The muscle/tendon/ligament will "slide" through this point to create a tension force and a force on the attached part via the slide point. For an illustration of contact based tissue wrapping see the Grasping Tutorial.

Figure 27: The flexor digitorum profundus muscle group before slide points are introduced (left) and after (right).

Figure 28 displays the panel used to create the slide-point based wrapping and a model of the wrapping.

Figure 28A: Panel to create the slide-point tissue wrapping. The numbers on the panel correspond to the forearm model below. The flexor digitorum profundus muscle (1), is specified to wrap at part (2) rigid location (3).

Figure 28B: Forearm model with the flexor digitorum profundus set up to slide with respect to the third metacarpal bone.

Deleting Soft Tissues

Soft Tissues -> Delete

The figure below displays the panel used to delete soft tissues; they may be deleted individually or by sets.

Figure 29: Panel used to delete soft tissues. Tissues may be deleted individually or as an entire set.