How can I see the joint moment?

[Phuong bui]

Hello,

I'm new with Opensim. I'm studying about biomechanics and I want to use Opensim to learn about human motion. I tried to use inverse dynamics tool to get the joint moment. But in plot tool, I can not see the joint moment result. How can I receive the joint moment result. And what about muscle excitation. The muscle excitation result I got is zero. can anybody tell me about that?

[Anne Schmitz]

Your muscle excitation is 0 because inverse dynamics only computes joint forces and moments. You need to run CMC to get muscle excitations.

[Phuong bui]

How can I see the joint forces and moment? Do I use plot tool in Opensim?

[Pablo Ortega]

Hi Phuong:

Yes, you can use the plot tool to see them, all the moments should be in the inverse dynamics results file (created after run the inverse dynamic solution), you can also see it in matlab or similars.

Regards
Pablo

[Phuong bui]

It is a moment curve of a specific muscle relative to a generalized coordinate against a generalized coordinate. It is not net muscle joint moment. How can I see the net joint moment?

[daniel jaramillo]

Hi,

Would you happen to know what is the difference between joint reaction moments (Joint Reaction Analysis) and joint moments (Inverse Dynamics)?

Thanks,

Daniel

[Anantharaman Gopalakrishnan]

Imagine a planar mechanism consisting of just two links connected by a single pin joint. To change the angle between the two links, you would need to connect a motor at the joint which applies equal and opposite moments on the links.... this is analogous to joint moment.

Now instead of changing the joint angle, lets try to use force to pull one of the links out of the plane to which the mechanism is constrained. The pin joint is now being subjected to a bending moment whose equal and opposite reaction will manifest as a moment on the links. This internal self generated reaction moment arising due to geometry is analogous to the joint reaction moment.

I hope this helps.... and please do correct me in case I have also misunderstood

[daniel jaramillo]

Thanks Ananth I appreciate it

[Matt DeMers]

This is exactly right. To put it a little more concretely:

The joint moment calculated by inverse dynamics is a generalized force (for example the knee moment) actuating a generalized coordinate (for this example, the knee angle).

The forces and moments calculated by JointReactions are exactly what they sound like. They are they passive, reactionary loads caused by the structure of the joint. They do not act along the generalized coordinates of the relevant joint.

Anantharman's example is a perfect one. The moment about the pin axis is a generalized force acting along a generalized coordinate, thus it would be reported by inverse dynamics. However, motions along the other two rotations and three translations are not allowed by the joint and are not generalized coordinates. Thus reaction forces and moments can be produced along these constrained directions. These would be reported by JointReaction.

Matt

[Anantharaman Gopalakrishnan]

Are you referring to the 'joint moment' (Moment required to actuate the joint) or the 'joint reaction moment' (Moment of forces at joint contact, about the joint center)???

For joint reaction moment, you would have to use the 'JointReaction' analysis from the analyze toolbox.