The work here features a number of different OpenSim models of the lumbar spine developed to study lumbar kinematics and dynamics.

Briefly, the models consist of the following bodies:
# rigid pelvis and sacrum
# five lumbar vertebrae (separated by joints with three rotational degrees of freedom)
# torso (thoracic spine + ribcage)
The motion of the individual joints are defined using constraint functions specifying the motion of the lumbar vertebra as functions of the net lumbar motion (flexion-extension, lateral bending and axial rotation). Future models will incorporate joints with stiffness properties to more accurately mimic the action of the intervertebral joints.

The most complex of these models also feature the 238 muscle fascicles associated with the 8 main muscle groups of the lumbar spine necessary to study the contribution of the lumbar spinal musculature to spinal motion. Simpler models incorporating two and seven of the main muscle groups of the lumbar spine are provided as well for completeness.

Read more about the model in the paper, freely downloadable at http://link.springer.com/article/10.1007%2Fs10237-011-0290-6.

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September 2011 Addendum
Click on the "Downloads" link to the left for downloads related to more recent work.

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September 2012 Addendum
The Constrained Lumbar Spine Model does not require any of the files uploaded after the creation of the Constrained Lumbar Spine Model project. The .vtp files (and descriptions) are included here for the benefit of those of you who wish to create your own model that has origins shifted to the center of the bones since this typically saves a number of transformations. Many apologies for any confusion(!).

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March 2014 Addendum
(1)
This model was build with OpenSim 2+. Version 3+ will not allow you to use periods (.) in your variable names. Unfortunately, a bunch of the variables used (muscles mainly) have periods in the names so it will throw an error if you try and run it in OpenSim version 3+. To fix this, either use version 2+, OR, rename the variables appropriately.

(2)
We have all graduated and are no longer actively working on this project (we haven't been working on it since the end of 2011 actually). At this point, you probably know more than us about OpenSim so we apologize in advance if our support is subpar.

(3)
The complex mode is not meant to be run straight out of the box. It has almost 250 muscles after all and unless you have a super computer, running CMC, or FD on it is going to bring up the rainbow ball of death on your computer.
Rather, it's meant to be a reference for those of you who intend to build up your own model. My advice would be to start with the simple 4 fascicle model, get it to work, then incrementally build up from there using the parameters provided in our model as a starting point. Copy-Paste is your friend here. :)

(4)
If this is your very first OpenSim project, I strongly _strongly_ *strongly* suggest that you go through the examples provided with the OpenSim version you just downloaded and understand how they work. This will save you months of pain down the road.