Calculating Work Done By Muscles After Static Optimization

[Stephen Riutta]

Hello,

I want to compare the work done by all the muscles to the change in kinetic and potential energy of the model.

I'm calculating the work done in the following manner:
1) Run Static Optimization to get muscle forces.
2) Run "Muscle Analysis" to get muscle lengths.
3) Differentiate those lengths to get velocity.
4) Multiply those velocities by the respective forces to get power.
5) Integrate that power to get energy.

In a particular trial, I get ~700J of energy with this approach. Calculations via kinetic and potential energy yield 800J. Adding the work done by actuators yields 10J of energy.

I'm trying to figure out why there is a 90J difference between the two methods.

Notes:
1) I've tested the 800J value with several other methods, and they have confirmed it.
2) I've run this analysis over several trials, and the muscle work is consistently 100J less than the kinetic and potential energy.
3) I've tried the "Actuator" analysis, but I get energy values that are orders of magnitude too large.
4) I'm using OpenSim 2.4 instead of 3.2 because I have programs that are customized for 2.4

Thanks!

Stephen

[jimmy d]

Hi Stephen,

Static Optimization (SO) is a static inverse method. There is unaccounted forces that don't necessarily get addressed. Indeed, if you took the SO results and tried to use them in Forward Analysis, it would fail. So your findings confirm this.

For your analysis, you should rather look at using CMC as it is a Forward method. The muscle activation's and forces will be higher.

Hope that helps,
-james