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The activation dynamic model used by OpenSim is a first-order dynamic model based on the work of Thelen (2003) and Winters (1995). OpenSim's implementation was modified by Matthew Millard, Ajay Seth, and Peter Loan to avoid a numerical singularity that occurs when the activation is zero. The following OpenSim muscles use the first-order activation dynamic model:
In the OpenSim API, the dynamics are implemented in the class MuscleFirstOrderActivationDynamicModel.
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