scaling issue

[XIAOHAN XU]

1.If I want to add a set of new markers (.xml) on a model, does it have to fit with the experimental marker set (eg. 36 or54 markers)? Where to get the experimental marker-fitted marker set (.xml)?

2. Even though I have read the instruction, I am still confused about the difference function between "scale model " and "adjust model markers" in the setting pane. Under what condition that two files of .trc will be different? I check the box of preview the static pose, but always nothing appears on the visulizer.

3. If I use the Measurement-based scaling, how can I know what the marker pairs that the original model used to scale the original bone.

4.If I use the manual scaling, should I test the actual length of the human structure? But how can I know the length of original model, so how to know the ratio of two.

Thanks in advance! I have watched all of the videos on the Youtube about these, I think I need more videos

[Dimitar Stanev]

Hi Xiaohan,

1. You can chose to use less model markers than the ones that are reported on your experiment. However, it will be a pity not to use all the available information to better scale and track the recorded movement. Unfortunately, if the marker set that you use is not available in OpenSim or in any project on simtk.org (there might be projects that use the same marker set) then you will have to manually define the markers on the model.

2. The scaling tool can do two things: 1) scale the model based on the distance between pairs of markers, and 2) adjusts the position of the model markers so that they reduce the RMSE of the experimental data. By checking "adjust model markers" you permit the scaling tool to change the position of the markers. While this is a good practice, you should carefully assign higher weights on markers that reside on anatomical landmarks (e.g., RSIS, and knee epicondyles) and less weight on markers with high uncertainty in the placement (e.g., thigh). You should always inspect the outcome and refine the scaling parameters.

3/4. When you perform scaling (run), the distances of all marker pairs are reported in the message window. Therefor, you can perform a dummy scaling and take note of the absolute distance before scaling in order to calculate the scaling factors.