Hi Mohammadhossein, I'm not sure if this helps but if you download OpenSim's source code, you can view and replicate the equilibrium equations that OpenSim uses by accessing the muscle model of interest and then looking for the compute functions. For example, the equilibrium equations for the Millar...

Hi Xiao, I also wanted to change the cost function in OpenSim but unfortunately, it is embedded in source code. A possible alternative method, which I have got to work in Matlab, is to use an optimal control approach such as direct collocation to run the optimisation outside of the OpenSim framework...

To my knowledge, I don't believe static optimisation include the passive force-length curve of the muscle fibre so what you're predicting using static optimisation is active fibre force only.

Hope that helps!

Oh perfect. Thanks Tom!

Adrian

Hi Tom, Thanks for the advice about the flags. I didn't know that the fiber damping value could affect the passive forces. After relaxing this value a bit, I was able to get rid of the passive forces (when the lengths were below the optimal fiber length) as well as negative passive force values (whi...

Hi Jim, Thanks for the reply. According to the outputs from muscle analysis using CMC (i.e. PassiveFiberForce and ActiveFiberForce), the muscle does develop active force (due to the constraints of the activation levels) but has passive forces as well so the accumulated force is shown in the ForceRep...

Hi all, I was just wondering if anyone has come across the problem of obtaining passive fiber forces without the normalised muscle fiber lengths exceeding optimal fiber length. The only way I feel this could happen would be that the normalised passive muscle fiber F-L curve has been skewed left such...

I just wanted to let people following this thread that I found the issue. Similar to the thread that Brad referred to, it was not the default activation that was set incorrectly but a minimum activation value that I didn't have in my Millard model. The default value for this variable is set to 0.01....

Hi Brad, For now, I'm applying excitations through the standard forward dynamics tool. I've set all the default activations for all the actuators to 0 but it unfortunately made no difference. Thanks for the heads up. I'm hoping to transfer my whole workflow into the matlab API in the future. Adrian

Hi Brad, Which muscles are these (Millard, Thelen, Schutte)? I'm using the Millard2012EquilibriumMuscle model. Just a thought, but have you looked at the force in the tendon (series elastic element)? If your active element is initially too short and the tendon is not near slack, it could still produ...