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Comment: typo in Study 4: should be "same setup from Study 3", not study 4)

Static Optimization is a method for estimating muscle activations and muscle forces that satisfy the positions, velocities, accelerations, and external forces (e.g., ground reaction forces) of a motion. The technique is called  "static" since calculations are performed at each time frame, without integrating the equations of motion between time steps. Because there is no integration, Static Optimization can be very fast and efficient, but it does ignore activation dynamics and tendon compliance. (See Hicks et al., (2015) for more details regarding this and similar modeling and simulation choices and their pros and cons.)  

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The first step is to add a coordinate spring to the model. You will do this by running a GUI script that loads the current model, adds a coordinate spring to the right ankle, and saves it as a new model. 

  • Close the Static Optimization tool pane if it's open.
  • Load the RRA adjusted model subject01.osim in the GUI
  • Click  Scripts > Run...
  • Select the Script file addAnkleSpring.py
A new model will appear in the GUI. In the navigator pane, notice the name of the model now has 'ankle_spring' appended.
Expand the model hierarchy to find the spring under Forces>Other Forces. From the Property Editor, you can observe and edit the spring's properties. For example, You will be able to see that the spring name is 'AnkleLimitSpringDamper'.

Also, observe that a new model has been saved to your RRA folder called 'subject01_spring.osim'.

Now that you have a new model with an ankle spring, you will re-run the Static Optimization analysis using the same setup from Study 4 3 (with normal reserve actuators). Rename the output folder to \ResultsSO_NormalActuators_RRA_Spring

Plot the AnkleLimitSpringDamper moment from the file Scaled_Model_StaticOptimization_force.sto

Plot the Gastrocnemius activation from the analyzes with and without the Spring (the results should be in the folder ResultsSO_NormalActuators from Study 4 and the latest folder, ResultsSO_Spring)

Questions:

  • Why did the Gastrocnemius activation decrease when the ankle spring was added?
  • Challenge: try increasing or decreasing the spring stiffness. How does this affect the Gastrocnemius activation? 
  • Challenge: are any other muscle affected by adding the ankle spring?

References

  1. Hicks J.L., Uchida T.K., Seth A, Delp S.L. (2015).  Is My Model Good Enough? Best Practices for Verification and Validation of Musculoskeletal Models and Simulations of Movement. Journal of Biomechanical Engineering, 137(2)