Forward dynamics didnot match inverse dynamics

[Bo Ding]

Hi all,

I use the musculoskeletal model to calculate knee joint moment and use inverse dynamics to examine it. However, the moment obtained from forward is about 100Nm, while the results obtained from inverse dynamics is only 6Nm.
I only take knee joint into account and the equation of inverse dynamics that i applied can be found in figure.
Is there any problmes in the equation?
Thanks


Bo Ding

[Michael Sherman]

Hi, Bo Ding. Looks like a sign error in your equation unless I'm misunderstanding it. Since f=ma you have f_unknown + f_applied = ma => f_unknown = ma - f_applied.

Regards,
Sherm

[Bo Ding]

Hello Sherm,

Thanks for your reply. But I still feel confused. What does the f_unknown mean? Do you mean that the joint moment generated by muscles is comprised by the f_unknow and the ma ? If this is true, the observed human motion was resulted from only a small part of muscle forces.

Regards,

Bo Ding

[Michael Sherman]

Hi, Bo Ding. By f_unknown I meant the moment you called "M". Possibly I misunderstood your intent, but I'm assuming you want to know how much additional torque it would take at the joint in order to explain the angular acceleration (theta double dot) you observed, after removing the known effect of gravity.

Sherm

[Bo Ding]

Hi, Sherm. You are right. I assume that the joint torque produced by muscles would equal to the torque obtained form inverse dynamics. For example, if the knee angle is a constant, f_unknown will equal to the amount of torque generated by gravity.


Bo Ding

[die ting]

What does the f_unknown mean? Do you mean that the joint moment generated by muscles is comprised by the f_unknow and the ma ? If this is true, the observed human motion was resulted from only a small part of muscle forces.???