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For the following exercise, it is recommended that you have a program capable of recognizing XML tags and folding code (e.g., Notepad++ on Windows, TextMate on Mac, Sublime Text on either platform).

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The necessary files are located in the "Models/Tug_of_War" folder of the OpenSim Resources directory (by default, C:/Users/<username>/Documents/OpenSim/4.14).  Locate Locate the model file 'Tug_of_War_Millard.osim', the controls file 'Tug_of_War_Millard_controls.xml' , and the initial states file 'Tug_of_War_Millard_initial_states.sto' and copy them into your working directory. Copy the model and save it with the filename Tug_of_War_Millard_Iso.osim.

An OpenSim model is created using a series of XML elements. An XML element consists of a start tag, a value, and an end tag. The value between the start and end tags can be a string, a number, or another XML element.

Open the model file 'Tug_of_War_Millard_Iso.osim' in your favorite text editor. At the top of the file, you will find a block that indicates the presence of XML content, then an XML element called OpenSimDocument. Within the OpenSimDocument element is a Model element consisting of several model components. You may recognize the model components as categories shown in the Navigator Pane of the OpenSim GUI.

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An OpenSim model is created using a series of XML elements. An XML element consists of a start tag, a value, and an end tag. The value between the start and end tags can be a string, a number, or another XML element.

Open the model file 'Tug_of_War_Millard_Iso.osim' in your favorite text editor. At the top of the file, you will find a block that indicates the presence of XML content, then an XML element called OpenSimDocument. Within the OpenSimDocument element is a Model element consisting of several model components. You may recognize the model components as categories shown in the Navigator Pane of the OpenSim GUI.

<?xml version="1.0" encoding="UTF-8" ?>
<OpenSimDocument Version="30000">
    <Model name="Tug_of_War_Millard">
		...
		<BodySet> ... </BodySet>
		<ConstraintSet> ... </ConstraintSet>
		<ForceSet> ... </ForceSet>
		<MarkerSet> ... </MarkerSet>
		<ContactGeometrySet> ... </ContactGeometrySet>
		<ControllerSet> ... </ControllerSet>
		<ComponentSet> ... </ComponentSet>
		<ProbeSet> ... </ProbeSet>
    </Model>
</OpenSimDocument>

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You win the competition by getting the center of the block onto your side of the arena at the end of a 1.0-second simulation. For this competition, start with a copy of the original Tug_of_War_Millard.osim model file. 

The design variables that you can modify to create your competitor are:

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1.0-second simulation. For this competition, start with a copy of the original Tug_of_War_Millard.osim model file. 

The design variables that you can modify to create your competitor are:

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  body F_{\mathrm{o}}^{\mathrm{M}}

   – Maximum isometric muscle fiber force

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  body \ell_{\mathrm{o}}^{\mathrm{M}}

   – Optimal muscle fiber length

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  body \ell_{\mathrm{s}}^{\mathrm{T}}

    – Tendon slack length

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  body \alpha_{\mathrm{o}}^

    – Muscle fiber pennation angle at optimal fiber length

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  body u(t)

    – Muscle excitation time history

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  body \ell^{\mathrm{MMT}}

   – Maximum isometric muscle fiber forceTotal length of muscle–tendon actuator

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  body \elltau_{\mathrm{oact}}^{\mathrm{M}}

   – Optimal muscle fiber lengthActivation time constant 

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  body \elltau_{\mathrm{sdeact}}^{\mathrm{T}}

    – Tendon slack length – Deactivation time constant

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  body \alpha_{\mathrm{o}}

    – Muscle fiber pennation angle at

  \tilde{v}_{\mathrm{max}}^{\,\mathrm{M}}

   – Maximum contraction velocity normalized by optimal fiber length

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  body u(t)

    – Muscle excitation time history

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  body \ell^V^{\mathrm{MTM}}

   – Total length of muscle–tendon actuatorMuscle volume

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  body \tautext{Z}_{\mathrm{actorigin}}

   – Activation time constant The Z-coordinate of the muscle's attachment point on the ground body

The muscle must obey the following physical relationships:

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  body \tausigma_{\mathrm{deacto}}

   – Deactivation time constant

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  body \tilde{v}^{\mathrm{M}} = \frac{F_{\mathrm{maxo}}^{\,\mathrm{M}}}

   – Maximum contraction velocity normalized by optimal fiber length

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  body V^{A^{\mathrm{M}}}

   – Muscle volume

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  body = 35\,\text{ZN}_{/\mathrm{origin}}

   – The Z-coordinate of the muscle's attachment point on the ground body

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• text{cm}^2

   – The specific tension of the muscle at its maximum isometric force (from Zajac, 1989).

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  body \sigma_V^{\mathrm{oM}} ^= A^{\mathrm{M}} = \frac{Fell_{\mathrm{o}}^{\mathrm{M}} }{A^{\mathrm{M}}} = 35\,\text{N}/\text{cm}^2

   – The specific tension of the muscle at its maximum isometric force (from Zajac, 1989).

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  body V^{\mathrm{M}} = A^{\mathrm{M}} \ell_{\mathrm{o}}^{\mathrm{M}}

     , where

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  body --uriencoded--A%5e%7B\mathrm%7BM%7D%7D

  is the cross-sectional area of the muscle,

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  body --uriencoded--\ell_%7B\mathrm%7Bo%7D%7D%5e%7B\mathrm%7BM%7D%7D

  is the optimal fiber length of the muscle, and

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  body --uriencoded--V%5e%7B\mathrm%7BM%7D%7D

  is the muscle volume.

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  body \ell_{\mathrm{initial}}^{\mathrm{MT}} = \ell_{\mathrm{s}}^{\mathrm{T}} + \ell_{\mathrm{o}}^{\mathrm{M}}

  , where 

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  body \ell_{\mathrm{initial}}^{\mathrm{MT}}

   is the distance between the origin of the muscle (on the ground) and its attachment point (on the block) at the beginning of the simulation. Note that 

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  body \text{Z}_{\mathrm{origin}}

   must be set so this constraint is satisfied.

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