hi
when i modeled lifting with anybody (using standing model) , with a force on the hand. the reaction forces of the ground is reasonble (almost 400 n in each foot ) when force is low (44 n in each hand), but when i increase the force (88 n in each hand) the ground reaction forces is again resonable but suddenly became too large at the end period of lifting (almost 12000 n in each foot).
this happan for forces such as 66 or 77 or 80 n.

Anyone know why this happen ? and in that situatin can we trust to other resualts of model such as moment or muscle activtiy?

The short answer is no. If the model has small accelerations, then the sum of ground reaction forces must be equal to the gravity on the body and whatever it is lifting. If that is not the case, then something is wrong.

But there is always a natural explanation for these things. I would check the following:

The accelerations. Sudden increase in forces can be caused by non-smooth accelerations.

The boundary conditions: Are you sure you are summing all of the GRFs with their respective signs? If the feet are glues to the ground in this model and the model is out of balance, then you can get large force couples under the feet to balance the model.

thanks john,
you are right, the sum of GRFs of two legs is became resonable , because on of them has a apoosite sign.
Is that means the boundry condition wrong?

Boundry condition: like standing model , toes and heels of two legs with simple driver has been stick to ground :

[SIZE=3]AnyKinEqSimpleDriver[/SIZE] RToeGroundConstraint ={
[SIZE=3]AnyKinLinear[/SIZE] ToePos = {
[SIZE=3]AnyFixedRefFrame[/SIZE] &Ground = Main.Model.EnvironmentModel.GlobalRef;
[SIZE=3]AnyRefNode[/SIZE] &Ball = Main.Model.HumanModel.Right.Leg.Seg.Foot.ToeJoint;
};
MeasureOrganizer = {1}; [SIZE=3]// Only the y coordinate
[/SIZE]DriverPos = {0.0};
DriverVel = {0};
Reaction.Type = {On}; [SIZE=3]// Provide ground reaction forces
[/SIZE]
};

if the model (maybe the boundry condition) is wrong , why it works for lower forces?

It just means that the model is not balanced like a real human would be. The modelâ€™s feet are glued to the ground, and this is why is remains standing where a normal human with pressure-only contact conditions would fall.