Confusing velocity and joint load correlation

Main Forum
1

Hello everybody,

I did some simulations with the BergmannGH model and different scaled bodies.

First:
What I find confusing is how the GHReaction changes, when the body parameters stay constant but the duration of the motion changes.

Example:
1)
tEnd = 2.7;
Freq = 0.18519; //From AnyKinFourierDriver

Max GHReactionInHumerus= 509.2N

tEnd = 1.5;
Freq = 0.3333; //From AnyKinFourierDriver

Max GHReactionInHumerus= 392.95N

tEnd = 6.0;
Freq = 0.08333; //From AnyKinFourierDriver

Max GHReactionInHumerus= 550.36N

I cannot explain why with a longer motion the max load also increases and vice versa with a shorter motion.

Shouldn’t it be the exact other was around due to inertia and so on?

Second:
When changing a body’s height with keeping its weight constan in “AnyManUniform” why is the GHReaction decreasing?

Example:

AnyManUniform:

AnyFolder AnthroData = {
AnyVar Body_Mass = 85;
AnyVar body_height = 161/100;

Max GHReactionInHumerus= [b]509.2N

[/b]
AnyFolder AnthroData = {
AnyVar Body_Mass = 85;
AnyVar body_height = 194/100;

Max GHReactionInHumerus= 496.2N

I hope some body is able to clarify those things

Thanks for help,

cheers,

Marc

2

Hi Marc,

This is a good question

I tried to run the kinematics of the three models you listed, i looked at the acceleration of the hand segment

Roughly the values was like this when the arm is fully abducted

[ol]
[li] 0.5
[/li][li] 1.5
[/li][li] 0.8
[/li][/ol]

I think the reactions force are related to this, there exist a frequency which will be fitting with the speed of a pendulum which is swinging, if the arm follows that speed there will be a small amount of torque needed and low muscles forces and reactions as a consequence.

What you see is that case 2 has the lowest reaction force, this i think is because the acceleration when the arm turns around is the highest in this configuration, this acc is opposite the gravity so it helps unload the muscles.

So if you did the task very very slow you would get a higher force, not a lower one.

Concerning the second question this relates to scaling of the model, the higher subject may have bigger moment arms thus the reaction force may drop, this will also depend on which scaling law is being used.

Best regards
Søren