Muscle saturation during unloaded flexion

Hi guys,

I’m actually working with the BergmannGH shoulder model of AnyBody 5.1, AMMR 1.3.1 and the 3E muscle model. When I perform an elevation in the sagittal plane (forward flexion) up to 150°, I obtain a muscle saturation at around 90°. When I do the same elevation in the frontal or scapular plane with the 3E muscle model, I get no saturation at all. Moreover, when i do a forward flexion with the 1E muscle model, I get no saturation. Thus, it seems that the calibration of the 3E muscle model is not optimal. When I change the 40° of abduction in the first calibration and put a higher value (70°), I get a better muscle behavior (saturation in less muscles).

So my question is related to the positions of the humerus for the calibration process. Are these positions comes from guesses or from published data?

Thank you.

Pierre

Hi Pierre,

The saturation is indeed due to some muscles loosing strength in this position. And the calibration has a direct effect on that.

The calibration positions are not from the literature, they have been estimated to be the position where the limb have the maximum strength for a certain movement.

So those position can always be subject to discussion. However when modifying the calibration position to improve a certain movement (here the flexion) it may have the opposite effect on another movement where the muscle is also used. That’s the challenge of it.

Best regards, Sylvain.

Hi Sylvain,

I totally agree with you, since I experimented it. In fact, before I changed the calibration positions, the shoulder was stronger in abduction. Now its stronger in flexion. There is definitely some improvement to do with the calibration. I will try to find literature data on my side.

Thank you.

Pierre

Hi Sylvain,

After a deep research, I conclude that there is no reliable method to determine the right position at which each muscle produces its optimal force. I found multiple ways of measuring the strength of muscles (EMG, muscle architecture, manual muscle testing, etc.). However, none of these methods seems to point out towards particular optimal positions.

I like the following sentence from the tutorial: However, it is not surprising that we seem to have been built in such a way that our muscles attain their optimum fiber lengths in the joint positions where they do most of their work, and if you are unable to find the information about optimum joint positions you need, then your best choice may be to calibrate the muscle in the joint position where it primarily works.

Is this the rationale that you used to build the calibration of the arm ? I can see that you refer only once to the paper of Garner and Pandy (2003) for the latissimus dorsi, but for the rest, the method used to determine the optimum joint positions remains unclear. Correct me if I’m wrong, but here is a list of some of your calibrations:

Calibration #0:

  • Muscles : All (including middle deltoid)
  • Thoraco-humeral angle (abduction, flexion, rotation): 40,0,0

Calibration #2:

  • Muscles : Anterior deltoid
  • Thoraco-humeral angle: 0,35,0

Calibration #4:

  • Muscles: Supraspinatus
  • Thoraco-humeral angle: 0,0,0

Now if I use the Simple muscle model and simulate an abduction, the supraspinatus and middle deltoid “primarily works” (maximum force) around 90°. Thus, their calibration should be at 90° instead of at 40° or 0°.

What do you think ?

Regards.

Pierre