Evaluating a Musculoskeletal Model

Getting Started: Defining your Research Goals for the Model

• What results do I need to answer my research question?
• What elements of a model might these results be sensitive to?
• In what ranges will my study be conducted and in what ranges has the model been validated? If a model is not valid in the ranges of your study, how can you extend those ranges and re-validate the model?

General Questions

• What structures or systems are modeled?
• Is it a kinematic model or full dynamic model? Is it 3D?
• Is it a model of a new structure or structures? Does it build on an existing model? What models can it be compared to?
• What experimental data was used to build and test the model? If human or animal subjects were used, how many and what type of subjects (age, size, etc.)?
• What platform(s) is the model implemented in?
• Can the model be scaled or otherwise customized?
• Has the model been published or otherwise peer reviewed?
• Do other studies use/cite the model?
• Has the author of the model changed or updated the model since the original publication?  Do I have and understand the most recent version?

Kinematic Models

Musculoskeletal Geometry

• Bone Geometry
  • What bony segments are included in the model?
  • What experimental or literature data was used to define bone geometry?
• Joint Geometry
  • How many joints and how many degrees of freedom?
  • How are the joints modeled? Types? Joint axis definitions? Coordinates? Neutral positions?
  • Is there coupling of joint motions?
  • What is the joint range of motion? Are there other limits or other constraints?
  • What experimental or literature data was used to define the joint geometry?
• Muscle Geometry
  • How many muscles are included in the model?
  • How are muscle attachment points defined?
  • Are broad muscles represented by multiple segments?
  • Does the model have wrapping surfaces or via points? What features do they represent?
  • What experimental or literature data was used to define the muscle geometry?
• Other Structures
  • Does the model include ligaments?
  • Does the model include prosthetics, orthotics, or other devices?

Dynamic Models

Inertial Properties

• Does the model include segment inertial parameters?
• What experimental or literature data was used to define the intertial parameters?
• Are there segments without intertial parameters?  (This is a challenge for creating forward simulations with the model.)

Actuators and Other Force-Generating Elements

• Does the model include muscle actuators?
  • What type of model is used for muscle-tendon force generation?  
  • How is muscle architecture modeled? What parameters define muscle architecture? 
  • How is activation-contraction dynamics of muscles modeled? What parameters define?
  • Was there any fine-tuning/adjustment of muscle model parameters?
• Does the model have any other actuators (e.g. powered prosthetics, reserve actuators at the joints)?
• Does the model have any other passive force-generating elements (e.g. damping at joint extremes)?

Controllers and Sensors

• Does the model have any custom controllers?
• Does the model have any custom sensors?

Contact

• Is a model of foot/floor contact included?
• Does the model include any other representations of of contact with external objects?

Model Testing

• Were model moment arms compared to experimental data?
• Were maximum active joint moments compared to experimental data?
• Were passive joint moments compared to experimental data?
• Was a sensitivity analysis performed for any of the parameters of the model?
• Has the model been used to generate simulations?
  • How much time is required to generate simulations of standard motions (e.g. walking, reaching)?
  • How do the results compare to previously published data?
  • For example, if a lower extremity model, does the model accurately predict knee forces from the grand challenge data set (https://simtk.org/home/kneeloads)?

More Help

• The webinar “Musculoskeletal Models Deconstructed” provides some specific examples of how to evaluate and develop a model (watch | view slides).