1. Forward dynamics simulations are sensitive to initial conditions, and it is good practice to double-check that the initial conditions are appropriate for the desired simulation.
  2. If the Forward Dynamics Tool fails gracefully (i.e., without crashing OpenSim) or the output of the Tool drifts too much (i.e., the model "goes crazy"), shorten the interval over which the Forward Dynamics Tool runs (i.e., make initial_time and final_time closer to each other in the Forward Dynamics Tool setup dialog box or setup file). Open-loop forward dynamics tends to drift over time due to the accumulation of numerical errors during integration.
  3. Forward dynamics will be slow and generate very large results files if the problem is stiff and the integrator has to take many small time steps. Some common cases that result in stiff problems:
    1. Bodies with small masses relative to other model bodies make the system stiff. Small masses combined with large forces can also make the problem stiff.
    2. If there's a contact model, check the settings. Try reducing the "transition_velocity" if forward dynamics is slow. Also having lots of damping in the contact model combined with a high stiffness value can also lead to problems. 
  4. Having many wrapping surfaces in your model can increase computation time. Cylindrical wrapping surfaces tend to have better performance that ellipsoidal wrapping surfaces.