Soft Tissues

Ligaments and muscles comprise the force-producing soft tissues available in LifeMOD™. True to the actual physiological limitations of muscles and ligaments, soft tissues in LifeMOD can only transmit tension forces.

Ligaments are modeled as passive spring/dampers and are not included in the generic full body tissue set. To use them, you must add them manually using the Adding Individual Muscles, Ligaments and Tendons section. Muscles are the primary soft tissues used in LifeMOD and produce tension forces between bone attachments.

All of the muscles contain both trainable and active elements that allow you to run either inverse or forward dynamics simulations on any given muscle group. The trainable elements learn and record shortening/lengthening patterns while motion capture data drives the model in an inverse dynamics simulation. The elements 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 Muscle Matrix

There are five muscle types in LifeMOD:

• Passive recording
• Simple Activated
• Hill activated
• Simple Controlled
• Hill controlled

The passive recording muscles are most commonly used for "crash dummy"-style tests, or they can function as the initial, trainable stage of the remaining four active muscle types. These four active muscle types form a matrix (shown in Figure 1), based on open and closed loops and simple/Hill-based muscles.

Open loop muscles fire via a user-defined activation curve over a certain period of time.

Closed loop muscles contain proportional-integral-differential (PID) controllers. The PID controller algorithm uses a target length/time curve to generate the muscle activation, and the muscles follow this curve.

Simple muscles fire with no constraints except for the physiological cross-sectional area (pCSA), which designates the maximum force a muscle can exert. As a result, graphs of simple muscle activation curves will generally peak at a flat force ceiling value.

Hill-based muscles operate on the traditional combination of an active contractile element (CE) and a passive parallel elastic element (PE). The contractile element contains an muscle activation state which controls the active muscle force capability, while the parallel elastic element exerts opposing forces that more accurately simulate the movement and force exertion of real muscles. The combination introduces the F-V and F-L muscle physics laws into the formula, thus refining the activation curves. the calculated At curve from the controlled Hill can be compared to EMG data for validation.

Figure 1: Muscle Matrix for the active muscle types

For more detailed information on the different muscle types, see Muscle Formulation.

You can adjust the various muscle parameters. 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 Passive Recording Muscle Elements in the Model
• Using Trained Drivers in a Forward Dynamics Simulation (Closed Loop)
• Using Trained Drivers in a Forward Dynamics Simulation (Open Loop)
• Adding Individual Muscles, Ligaments and Tendons
• Modeling Antagonistic Tissue Effects
• Moving Tissue Attachment Points.
• Reassigning Attachment Points
• Contact Based Tissue Wrapping
• Slide-Point Based Tissue Wrapping
• Muscle Recruitment
• Deleting Soft Tissues

Full Body Muscle Set

LifeMOD automatically generates a full-body set of 178 muscles and attaches them to the bones at anatomical landmarks. See the appendix for muscle names and attachments. You may scale the muscles and attachments with the skeletal geometry when creating the body segments.

Figure 2: Full body muscle set. L to R: Recording (coral), Open Simple (red), Open Hill (pink), Closed Simple (brick red) and Closed Hill (clay red/brown)

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 or bend around geometric features such as other bones, but you may set up each tissue to wrap around features using either the slide-point based wrapping or the contact-based wrapping features.

You can relocate muscle attachment points using the tissue relocation points.

LifeMOD creates muscles in separate sections for arms, legs, and head/trunk. It generates each set automatically and attaches them to the 19 base segments at predefined, scalable attachment points. Figure 3 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 3: Exploded view of body segments to view muscle connections

Figure 4: Right Click on the muscle graphic to bring up the name

When LifeMOD creates the muscle set, it attaches each muscle to a particular standard body segment (see appendix). You can change the attachment point by reassigning it to another body segment or a non-standard body segment (see the 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 4). 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 muscle groups, use the main modeling panel shown in Figure 5. When you select either Hill-type or Trainable muscle elements, LifeMOD displays a sub-panel with global parameter information.

Figure 5: The full body set muscle creation panel for both the trainable elements and the Hill-based elements.

Only the trainable (passive recording) and Hill-based (Simple Hill) are active options at this time, however, by selecting "Table Editor" in the top left pull-down menu, you may change the muscle group as illustrated in Figure 6.

Figure 6A: Select the top left pull-down menu after creating a base muscle set.

Figure 6B: Table Editor window that allows you to change the type of muscles used on the model.

The body may contain any combination of muscle elements (Figure 7). This combination is frequently used to model co-contraction or antagonistic effects.

Figure 7: Model with a combination of Closed Hill muscles (brown color) and Open Hill muscles (pink color) on the right leg.

Creating Passive Recording Muscle Elements in the Model

The recording muscle element is usually the first step in a two-phase simulation. It serves as the "training" phase, where LifeMOD records the muscle contractions during manipulation of the model via motion agents. Simulations involving the closed loop muscle groups will nearly always require the use of the recording muscle. In the second phase, the individual muscle contractile histories are used to generate a muscular force that maintains the desired contraction pattern.

You may create the muscles 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 to manipulate it. Typically this is done 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 8: Panel to create tissue set for the lower body.

Soft Tissues -> Create Base Set
The panel in Figure 8 is used to create the muscle-tendon Base tissue sets for the legs. LifeMOD allows you to specify either the Updated 45 muscle set for the legs, which is the default, or select the Legacy 16 Muscle Set. Select "Apply" to create the trainable muscles. They are shown as the rust-colored muscles in Figure 9.

Figure 9: Motion agents guiding leg while learning elements record muscle contractions

After generating the muscles, you may manipulate the body and limbs using motion agents in an inverse dynamics simulation to train the muscles and record reaction information. Figure 9 displays a leg flexion activity using a single motion agent to guide the foot. During this simulation, LifeMOD records records the muscular contraction for each muscle in the leg.

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 tutorial) 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 Drivers in a Forward Dynamics Simulation (Closed Loop)

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

Figure 10: Animation slices for the forward dynamics simulation. Note the deep red color of the muscles indicating the active elements. Also note the scaling muscle graphics based on muscle force.

Soft Tissues ->Training
Use the Soft Tissues > Training panel to replace each training element with a trained contractile element. Select the light bulb icon, select either Closed-loop Simple or Closed-loop Hill, and enter the values for the servo controller proportional gain (P_gain) and derivative gain (D_gain). These values are user-defined constants that contribute to the muscle formulations listed in the appendix. You may specify any saved analysis to use. The default is the most recently run analysis. You may also list specific muscles to convert from training to trained in the Select Muscles field. The default is all muscles, which you will get if you leave the Select Muscles field blank.

Figure 11: Panel to replace recording elements with trained controlled elements

Figure 12 shows an example of muscle force calculation, 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 12 where the force output is significantly decreased. The muscle can be essentially "turned off" by setting a F_filter to 0%, 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 these effects.

Figure 12: Forward dynamics simulation results for the open-loop 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 which is 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, run simulations after altering values of the P_gain and D_gain on the parameters panel and view the muscle forces using the strip chart displays from the results panel.

Figure 13 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 13: Example of the effect on the muscle force of changing the P_gain and D_gain. The forward dynamics simulation was run using P_gain and D_gainvalues of 1e6 and 1e4 (blue curve) and and again with values of 1e5 and 1e3 (red curve).

Retraining the Muscles (optional)

You may retrain the muscles at any time by selecting the check mark in Figure 14. This replaces the active elements with trainable elements again. Once the elements are replaced, you may rerun the inverse dynamics simulation using new motion data, muscles, ground reaction force parameters, etc. You 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 14: Panel used to replace contractile elements with learning elements.

Using Trained Drivers in a Forward Dynamics Simulation (Open Loop)

Unlike the Controlled Muscle elements, the Activated muscle groups do not require inverse dynamics training and you may use them immediately in a forward dynamics simulation. You must supply an activation curve.

You can create the muscles in base sets or as individual muscles.

Figure 15: Panel to create the Hill Activated tissue set for the right leg. Global parameters in the sub-panel consist of Hill passive and contractile element properties and the activation data spline.

Soft Tissues -> Create Base Tissue Set
The panel in Figure 15 is used to create the muscle-tendon base tissue sets for the right leg. By selecting "Apply" in the panel, the Hill Activated muscles (pink colored) displayed in Figure 16 are created on the model. To generate the Simple Activated muscle group, simply repeat the process illustrated in Figure 6.

Figure 16: Full body muscle set consisting of open-loop Hill-based muscles.

Adding Individual Muscles, Ligaments and Tendons

Non-standard muscle forces are individual muscles, ligaments, and tendons that are not included in the base set of soft-tissues provided by LifeMOD. For more complex tissue mapping, or for areas of the body that are not included in the base set, (e.g., hands and feet -- as shown in the Grasping tutorial), you may add individual muscles and ligaments to the model. You may set up the muscles and ligaments to wrap around tissues using either the slide-point based wrapping or the contact-based wrapping features. You can relocate muscle attachment locations using the tissue relocation tools.

Muscles

You may create individual muscles on the model to supplement an existing base set of muscles or just add new ones. For certain activities, the 178 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 17 displays a model with a base-set of muscles and an augmented base-set of muscles. Figure 18 displays a model consisting exclusively of individual muscles (see Grasping Tutorial).

Figure 17: 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 18: Hand model consisting exclusively of individually created muscles.

Soft Tissues -> Create Individual Tissue -> Trainable Muscles

Figure 19 shows the panel used to create individual trainable muscles. The trainable muscle created using this panel spans part_1 and part_2 at the locations selected on the screen with the cursor.

Figure 19: 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 20 shows 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 20: Panel used to create Hill-Type muscles between part_1 and part_2 at the attachment locations displayed.

Ligaments/Tendons

Ligaments are spring forces used to stabilize joints (see the Total Knee Replacement tutorial for a detailed example). Use the panel in Figure 21 to create ligaments between part_1 and part_2 at the locations selected on the model with the cursor.

Figure 21: Panel to create an individual ligament/tendon

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

To create a ligament/tendon, enter the two body segments the tissue would span and the tissue attachment locations (either manually or by selecting the locations on the screen). Next, enter strain stiffness, damping and preload (see the panel in Figure 21). Figure 22 displays the passive tissues (ligaments/tendons) generated in the Total Knee Replacement tutorial.

Figure 22: 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

You may model co-contraction, or antagonistic muscular effects, using either of the Hill-based muscle sets or a combination of trainable and Hill-based muscles. Figure 23 displays a right arm model with both trainable muscle elements (in the trained state) in red and Hill-based muscle elements in pink. When you specify an activation history to control the triceps muscle, they produce a counter reaction to the biceps which flexes the elbow joint. This methodology may be extended throughout the body for both the base set muscles and the individual muscles. See the Antagonistic Muscles Tutorial for modeling co-contraction in greater depth.

Figure 23: Right arm modeled with both the trained Simple Controlled elements (brick red color) and open-loop Hill Activated elements (pink 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 flexion.

Moving Tissue Attachment Points

When LifeMOD generates muscles, it attaches them to the bones based on geometric landmarks on the bone graphics. The location of the attachment affects its force magnitude during a particular activity (see the Muscle Relocation example). You may move any attachment point as appropriate for your model.

Soft Tissues -> Edit Properties

LifeMOD displays each muscle as a cylindrical tube with two rounded ends that indicate the attachment points -- orange for the origin and indigo for the insertion. To change the attachment locations, bring up the Soft_Tissues->Edit Properties panel and use the tools to move the attachment. You may enter a new location or adjust the current position with the arrow buttons.

Figure 24: Moving the muscle attachment point by selecting the attachment ellipsoid

Figure 25: Panel for changing muscle attachment

Reassigning Attachment Points

If you refine the body segments for greater number or detail, you may have to reassign muscle attachments on the standard body segments 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 27, and enter a new global location or use the increment tools.

Figure 26: Example of reassigning attachment points (cervical spine tutorial)

Figure 27: Panel used to edit (like Figure 25) and reassign attachment points

Contact Based Tissue Wrapping

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

Figure 28 displays the panel you use to create the contact-based tissue wrapping and tissue-wrapped model itself.

Figure 28A: Panel to create the contact-based tissue wrapping. The numbers on the panel correspond to the knee model below. The medial quad tendon 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 28B: Knee model with the medial quad tendon set up to wrap around the patella component of a knee replacement

Slide-Point Based Tissue Wrapping

The second tissue-wrapping method is the slide-point based method, which you use to model tissues sliding or bending around geometric features (tendon wrapping) using slide points. Figure 29 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, or 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 29: The flexor digitorum profundus muscle group before slide points are introduced (left) and after (right).

LifeMOD allows for slide-point creation at the same time as the generation of individual soft tissues, by entering multiple locations in the origin attachment location field. Figure 30 displays the panels used to create the slide-point based wrapping and a model of the wrapping.

Figure 30A: Panel that allows for creation of individual tissues and slide points simultaneously. The numbers roughly correspond to the numbers on the model below. (1) designates the tissue, (2) designates the span of the tissue -- the anchor body segment(s) -- and (3) specifies the attachment locations. Be sure that the number of anchors in the Origin field matches the number of locations in the Attach Loc. field.

Figure 30B: 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 30C: Forearm model with the flexor digitorum profundus set up to slide with respect to the third metacarpal bone.

Muscle Recruitment

See the Muscle Recruitment tutorial for an example of using this feature.

In LifeMOD, muscle recruitment is the process that determines the activations or muscle forces used during a dynamic analysis by a group of closed loop muscles, when more than one muscle force pattern is capable of meeting the specified kinematic profile.  Each closed loop muscle uses a PID control algorithm that uses a target length/time curve to generate the muscle activation required to meet its target curve.  For many muscle systems there are redundant muscles such that the specified target length curves can be met with more than one pattern of muscle forces.  In these situations an additional recruitment criteria can be added, and an optimization performed to determine the specific muscle recruitment pattern that best meets the criteria, while preserving the original target curve profiles.

Muscle recruitment is an optimization that modifies the muscle PID algorithm by multiplying the PID control force by a dynamic gain representing the recruitment level.  The closed loop algorithm then becomes:

where  is the dynamic gain representing the recruitment level.

Figure 31: Muscle Recruitment Setup panel

By changing the relative gains between two or more muscles, it is possible to modify the pattern of muscle forces generated (if the muscles are redundant for the motion being analyzed).  This can be done dynamically at each time step of a solution to get accurate muscle recruitment patterns or can be done more coarsely if a less accurate solution is desired with proportionally smaller computation time required.  The dynamic gains, therefore, become the design variables that are modified by the muscle recruitment optimization.

The goal of the muscle recruitment optimization is to pick the dynamic gains, and therefore the muscle forces, that match what is expected for human motion.  A typical assumption for this is that the muscle recruitment that produces the minimum amount of muscle force, adjusted for muscle size and maximum stress levels, is the desired recruitment level.  This can be represented mathematically as follows:

        Minimize:                           

where  is the muscle force for the i^th muscle, and α is an exponent that is typically equal to 2 or greater.  Frequently in this type of formulation muscle force is normalized by pCSA.  In LifeMOD, the muscle force may be replaced by the muscle activation value to perform the same normalization.  The muscle recruitment function is further scaled so that the function being minimized is root mean function proportional to the force or activation.  The formulation then becomes:  

        Minimize:                          

An additional case where   can be handled as a MinMax formulation.

Figure 32: Muscle Recruitment Optimize panel

Deleting Soft Tissues

Soft Tissues -> Delete

The figure below displays the panel used to delete soft tissues; You may delete soft tissues individually or as sets.

Figure 33: Panel used to delete soft tissues. Tissues may be deleted individually or as entire sets.