Prescribed motion/activation - Huge force predictions

[Dan Mulla]

Hi everyone,

I'm just beginning to get into OpenSim 3.3 and went through the tutorials/examples, but have been running into some issues with estimating muscle forces while manually prescribing the model some input activations.

I am interested in estimating muscle forces (both active and passive) in a static position with a certain activation level of a muscle, and determine the overall joint reaction force (and maybe the predicted accelerations going forward - but not of concern right now). To keep things simple, I decided to start with the Tug_of_War model and created a .mot file that held the block in a static position (q=0.00), while inputting an activation of 0.05 for each of the two muscle (attached the .mot file with this post) over a 1 second time interval. However, when I ran it through the joint reaction analysis, I got huge force estimates (see below). In addition, the passive fiber force doesn't match what I would predict using the simple plotter function (the curve between passive fiber force and block_tz states no passive force is expected when the position = 0), nor do the active force match what's predicted using the muscle model.

Passive FiberForce = 1820300.76659679
Active FiberForce = -1820300.76659679

I've tried running through Static Optimization, but that didn't work either - provided me with the same results. I feel like I'm missing a simple step, but if anyone would be able to guide me that would be really appreciated!

Thank you!

Dan

[jimmy d]

Dan- can you post all of your input files as a zip and we can have a look. Hard to give advice since it may be a number of things.

Thanks,
-J

[Dan Mulla]

Hi James,

Sorry, thought I had attached the file with the last post. I can't seem to be able to attach any files/zips with the post, but I included below the contents of the input file I used as a code (this was the only file I used as an input - to prescribe the model both a motion and an activation).

Code: Select all

Results_states
version=1
nRows=25
nColumns=7
inDegrees=yes

# SIMM Motion File Header:
name Results_states
datacolumns 7
datarows 25
otherdata 1
range 0.000000 1.0
endheader
time	block_tz	block_tz_u	muscle1.activation	muscle1.fiber_length	muscle2.activation	muscle2.fiber_length
      0.00000000	      0.00000000	     0.00000000      0.05000000	      0.10000000	      0.05000000	      0.10000000
      0.10000000	      0.00000000	     0.00000000      0.05000000	      0.10000000	      0.05000000	      0.10000000
      0.20000000	      0.00000000	     0.00000000      0.05000000	      0.10000000	      0.05000000	      0.10000000
      0.30000000	      0.00000000	     0.00000000      0.05000000	      0.10000000	      0.05000000	      0.10000000
      0.40000000	      0.00000000	     0.00000000      0.05000000	      0.10000000	      0.05000000	      0.10000000
      0.50000000	      0.00000000	     0.00000000      0.05000000	      0.10000000	      0.05000000	      0.10000000
      0.60000000	      0.00000000	     0.00000000      0.05000000	      0.10000000	      0.05000000	      0.10000000
      0.70000000	      0.00000000	     0.00000000      0.05000000	      0.10000000	      0.05000000	      0.10000000
      0.80000000	      0.00000000	     0.00000000      0.05000000	      0.10000000	      0.05000000	      0.10000000
      0.90000000	      0.00000000	     0.00000000      0.05000000	      0.10000000	      0.05000000	      0.10000000
      1.00000000	      0.00000000	     0.00000000      0.05000000	      0.10000000	      0.05000000	      0.10000000

Thanks!

Dan