simtk.org

Why are the arms not included on the walking gait models provided in OpenSim? Is this for simplicity reasons?

Yes. Arms are typically not needed when computing kinematics of the legs.

Why aren't they needed? I was asking because I wanted to determine if there was a difference in the kinematic output of gait models with arms vs. gait models without arms.

Why aren't they needed?

Well adding unnecessary complexity to model is undesirable. If you add arms to a model that you are using to research ankle kinematics, then what exactly are you adding? In fact, you are adding more sources of error that can increase your processing time and reduce your validity. Your level of modeling is entirely determined by your research question, but people who are doing research into leg kinematics tend not to need the arms, so they correctly don't add them.

I was asking because I wanted to determine if there was a difference in the kinematic output of gait models with arms vs. gait models without arms.

Go for it. You will see a difference in the kinematics. IK is a tracking algorithm, so if you add additional bodies to track, without adjusting any tracking weights, you are sure to see changes in the kinematics. You won't learn anything from this since it is entirely non-linear and relies on all types of accuracy concerns with scaling and marker registration—which I guarantee you won't be able to solve.