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I want to animate a tendon transfer surgery in blender. I want the tendon to follow the surface of the bone while it is pulled out at the incision area.

It should look like a shoelace being pulled out of a shoe: The shoelace should be able to deform but follow the hole it is pulled through

Many thanks in advance!

Shoestring being pulled from out of the shoe

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  • $\begingroup$ Can you illustrate what you are trying to achieve wit reference images? It is difficult to visualize from description alone. blender.stackexchange.com/questions/72118/… $\endgroup$ – Duarte Farrajota Ramos Jan 4 at 0:56
  • $\begingroup$ It should look like a shoelace being pulled out of a shoe. The shoelace should be able to deform but follow the hole it is pulled through $\endgroup$ – Laura Jan 6 at 15:15
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    $\begingroup$ I'm not too familiar with this topic, but I know that this can be done using Curves. $\endgroup$ – stphnl329 Jan 6 at 15:43
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    $\begingroup$ See @RichSedman 's answer in this post: How to simulate a rope? $\endgroup$ – user1853 Jan 6 at 17:30
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You can use a cloth simulation with collisions to animate a realistic string being pulled through a hole or around any other kind of object.

  1. Set vertex group (these will be the vertices that will move the whole string, like the ends) by selecting some vertices and in the Object Data Properties panel, then clicking the + under Vertex Groups. Make sure you click "Assign" with your vertices selected. Make Vertex Group

  2. Go to the Physics Properties panel and click the "Cloth" button, then in the cloth settings, go to Shape and under Pin Group, select the group we created above. enter image description here

  3. Make an empty object (or any other object you want to control your string with), and make the string a child to that object (select string first, then shift select empty object, then ctrl + p). This will tell the pinned vertices to follow the parent object, the motion of which will be considered during the cloth simulation. enter image description here

  4. Make your hole object. Then, with it selected, go to the Physics Properties panel and click the "Collisions" button. Adjust these settings later as needed. enter image description here

  5. That should be it! Animate your parent object and the string will follow, animating like a simulated cloth through holes and around other collision objects. enter image description here

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While @Carlo shows the very precise version with a string following a path, i have a different suggestion that could be interesting.

Depending on how tight the tendon is supposed to follow the bone and how natural it is supposed to behave while and after, i think the cloth simulation could give you some interesting visual appeal.

  1. Create the collision objects along the path, a tube or rings, whatever suits the situation best.
  2. Make sure the collision objects have their Normals face the right way, as it will make or break the simulation.
  3. Create a curve for the tendon, convert it to a mesh and give it a width, like a band worm.
  4. Select the first vertices and make a vertex group to assign it to.
  5. With the vertices selected, press Ctrl+H and select "Hook to new Object", which will create an empty that you can later use to pull the tendon.
  6. Select the tendon object and add a "Cloth" Modifier, assign the vertex group to the shape.
  7. Adjust the length for the cloth simulation! Very important. (You may need to test a lot, using the "Update all to Frame" can help clear the simulation in that case)
  8. Press Space to start the simulation and grab the "Empty", press G and start having fun pulling tendons.

Here a little visual demonstration how it looks pulling a red string through some loops and how the cloth simulates it:

enter image description here

I have to admit that it's a lot less precise, but depending on the right usage of collision objects to lead the cloth it could very well be an option aside of the version @Carlo brought up.

It may just look a lot more realistic if worked out right.

To make it easier to figure out, here the file i used to show it.

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Curve modifier

I'll start by modeling the shoelace along the X axis. Be sure to place enough edgeloops for a correct deformation.

enter image description here

Add a curve and assign a Curve modifier to the lace. You should see it deforming accordly to the referenced curve. The deformation axis should be X. Now you simply have to animate the X location parameter of the lace like the following.

enter image description here

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  • $\begingroup$ Thanks! Would it be possible to start somewhere on the middle of the curve too? I have 2 seperate parts that need to be animated in one tendon. - The first part should start on the curve and follow it to where the tendon is pulled out. - While the next part of the tendon remains in place. It should look like if you pulled a shoe lace somewhere from the middle of the shoe. I made a start by hand (but the whole tendon should be pulled out), it should look somewhat like this: link $\endgroup$ – Laura Jan 6 at 20:37

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