AnyForceMomentMeasure2 includegravity=on?

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1

Hi, I don’t understand that when using AnyForceMomentMeasure2 to calculate the joint moments of the human body, why the IncludeGravity set as off? And if I switch it to on, the joint moment reduced. What happened here? I would really appreciate if someone can explain this

2

Hi Longbin

I can see how that could be confusing, but it is natural enough if you think about it. The purpose of AnyForceMOmentMeasure2 is to sum up forces/moments and express them at a certain point of interest.

Joint moments are the result of gravity, joint reaction forces, inertial forces and external forces. The only thing that can balance the joint moments are the muscles acting around the joint. So to calculate the joint moments we just sum all the muscles at the joint using AnyForceMOmentMeasure2.

The moment we get from that calculation must be equal to the joint moments. Now you can maybe understand why the moments can decrease if we include gravity in the sum. You would basically have gravity on both sides of the equilibrium. In fact, if you sum up all forces with AnyForceMOmentMeasure2 on some system you should get zero.

Here is an excerpt from the reference manual:

Notice that including all forces on a segment or a system of segments should ideally give zero equivalent force and moment due to equilibrium, but this must indeed include all forces including reactions, forces of gravity, and inertia forces.

Best wishes

3

Hi Morten

Thank you for your quick reply, I misunderstood before. Now this becomes more clear to me. By the way, What’s the difference between ‘KneeNetMomentMuscle’ and ‘KneeNetMoment’. I saw in ‘KneeNetMoment’

AnyReacForce &jnt1=..Jnt.Knee.Constraints.Reaction;
AnyReacForce &jnt2=..Jnt.Ankle.Constraints.Reaction;

were added. What’s the difference?

Best
longbin

4

Oh yes. For some joint, the joint reactions moments also plays a role in the total joint moments. In the case of the knee, the muscles have to provide all flexion/extension moment. But that is not the case for abduction- and internal rotation moments, since a revolute joint restricts provides reaction forces for those DOFs.

KneeNetMomentMuscle will give you the total muscles contribution to the joint moment. For a spherical joint that is the same as the total (net) joint moment.

KneeNetMoment: This the total (net) moment around the knee. This moment is the sum of all inertial, gravitational, and external forces, and it is the moment which has to be balanced if the knee had been a spherical joint.

Hope this makes sense.

5

Hi Morten

     Thank you. This helps a lot.

Best
Longbin