Conditional contact elements and AnyGeneralMuscle

Hi, I have interfaced a backpack (mass=15kg) at three different points (right and left shoulder and thorax respectively) with the musculoskeletal model using the CreateAnyGeneralMuscle. I changed the strength parameter Fx,Fy,Fz,Mx,My,Mz values. For different values, there was a varying lumbosacral joint reaction force. The force_x_sum, force_y_sum and force_z_sum values were also varying, what does this signify? What does the variation in joint reaction force mean? Will there be a change in the muscle activity also? Thanks in advance.

Hi Prathiban,

Welcome to the AnyScript Forum!

The AnyGeneralMuscles you use to simulate the interaction force are recruited in the same optimization problem as the physiological muscles. So, if their strength is really low, their recruitment can also affect the recruitment of the physiological muscles. Please also note that you have created a closed loop between the backpack and the human model. Having multiple AnyGeneralMuscle recruited force can exploit these closed loops if the solver finds a way to reduce the muscle activity.

It's not unusual that when you change the strength of the muscle, you are influencing the recruitment of also the other muscles.

Hope this helps

Best regards,

Thanks for your valuable reply sir

What does these strength Fx,Fy,Fz values physically represent?Do they indicate on how the straps of a backpack should be tightened? The parameter MaxMuscleActivity after inverse dynamics simulation indicates which muscle of the human?

Hi Nissy,

It's hard to compare the strength of Fx, Fy, Fz to an equivalent physical quantity. Essentially, the strength represents the ease with which these contact elements can be used in the model. Low strength corresponds to a high cost of recruitment of the contact element. So, the model will avoid using the contact elements. Conversely, high strength corresponds to a low cost of recruitment of the contact element. With a large strength, the behavior should be similar to implementing a reaction force, which the solver can use without any cost. Another aspect is that there is no strict definition of low and high strength. It will depend on the load that the contact element is expected to carry. I hope this makes some sense.

The MaxMuscleActivity in the inverse dynamics simulation output shows the maximum activity at the current time step of the simulation considering all the muscles in the human body. So, it can be different muscles at different time-steps in the simulation.

Best regards,

Thank you for your reply sir

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