Problem using cluster marker set using Vicon ProCalc

[Ashley Richardson]

Dear All,

I am having some issues with creating model kinematics on Vicon ProCalc using a modified plug-in-gait lower body marker set (additional cluster markers on thigh and tibia segment). I have no experience of writing scripts on MATLAB, Python or BodyBuilder so am ideally hoping to solve this using ProCalc.

I have tried two methods of calculating knee angles using a cluster marker set these are as follows;

* adding markers to a standard plug-in-gait trial on Nexus and saving as a new .vst

* opening this on ProCalc to create segments for the thigh and tibia using the 4 marker cluster on each segment respectively

* using the created segments to formulate an Euler angle sequence for the knee (replicated the Euler sequence from standard PlugInGait calculation)

The issue with this first method is the Euler angle displays through the standard plug-in-gait thigh marker and there seems to be in inability to set a joint origin when using segments created from a 4 point cluster (which would be the knee joint centre), whereby the Euler angle would be from this point.

The second method attempted was:

* Creating a cluster point (1 virtual marker from the 4 physical cluster points) for both the tibia and femur cluster plates.

* Set the main and secondary axis vectors, from which a virtual knee joint centre could be calculated

* From this segments were created using the virtual knee joint centre and the Euler angle sequence

This displays a very similar (but different) angle through the joint centre, which is what we want. However, I don't feel this is correct as it will depend on the location of the cluster (which cannot easily be accurately placed in the same anatomical location). My understanding of the benefits of cluster marker sets is they use markers not placed in a specific anatomical location but normalised to one known location (lateral knee marker/ASIS) from which the segment position is normalised to that. Currently my understanding of our calculations would not be able to do this through creating 1 point of the four marker locations?

Many Thanks,
Ashley

[Lasse Roren]

Re: Problem using cluster marker set using Vicon ProCalc

Hi Ashley,

Since I developed proCalc for Vicon, I will also attempt to answer your question! I also wrote a tutorial called "proCalc 6DOF Tutorial" that addresses this exact topic. In case you don't have this, I can send it to you (it's unfortunately too big - 633KB - to be allowed as attachment to this post).

The general workflow for cluster modelling in proCalc is based on using anatomical markers in a static trial to define the anatomical coordinate systems (for example by placing the knee joint centre halfway between lateral and medial knee markers). Also in the static, the clusters are used to define technical coordinate systems. During static processing, the transformations from technical to anatomical are saved out as parameters. For dynamic processing, the anatomical (static) markers are removed, but the technical coordinate systems are still measured from the clusters - and we can then use the transformations that were stored as parameters to re-create the anatomical coordinate systems from the technical ones, from which we then derive the joint angles. How to do this in proCalc is described, with illustrations, in the tutorial.

If you have further questions, please let me know!

Hope this helps,
Lasse

[Ashley Richardson]

Re: Problem using cluster marker set using Vicon ProCalc

Hi Lasse,

That would be really helpful if you could send me that tutorial, I want to ideally solve it myself and this will help with that. I knew I wasn't quite right with my calculations but was unsure of the steps to fix it, although should be able to with guidance and illustrations. Although it may spring up more questions! If you could please email it to a.richardson@abertay.ac.uk I would greatly appreciate it.

Many Thanks,

Ashley

[Josh Isherwood]

Re: Problem using cluster marker set using Vicon ProCalc

Hi Lasse,

If it is not too much trouble would it be possible for me to also receive a copy of this tutorial? As I am struggling with the same thing. If you could forward it to my email joshua@xtep.com.cn that would be greatly appreciated.

Thank you

Josh

[Paulo Lucareli]

Re: Problem using cluster marker set using Vicon ProCalc

Dear Lasse,

Would you mind to send it to me also?
plucareli@hotmail.com

Best

[Allan Carman]

Re: Problem using cluster marker set using Vicon ProCalc

I do not know how ProCalc reconstructs or analyses segment axes location or implements a cluster design. Hopefully the following should be helpful in understanding the process.

The cluster marker approach uses 4+ markers on each segment and a least square method to reconstruct a body fixed segment axes. There are several articles describing the least squares method [1-3]. The method defines a 4x4 transformation matrix that best maps known local coordinates of segment markers onto the global coordinates of those markers while minimizing the difference in least squares sense. The transformation matrix obtained contains the segment axes origin and orthogonal unit vectors that describe the segment’s position and orientation. The least squares method is used to help account for variations in global marker positions relative to their local coordinates established a priori in a static or reference pose when the segment axes were originally defined. These variations or errors in global markers relative to the underlying segment axes can originate from errors in 3D marker reconstruction, post-processing of 3D marker coordinate data and skin movement artefact during movement.

There is however a lack of information on cluster marker design principles and implementation, as indicated by this thread.
The initial subject calibration procedure has two key objectives:

1. To define the location and orientation of each segment axes, usually based on markers placed on anatomical land marks.
2. Establish the local coordinates of each segment marker relative to the respective segment axes.

Subject calibration is a two-step process:

1. Static: Segment axes are initially defined from markers placed on anatomical landmarks with the subject in a known static position. The calibration marker locations generally do not coincide with segment anatomical axes that we wish to describe rotations about (such as epicondyles of the knee or malleoli of the ankle) and are subject to large inter session and inter tester variability. As such anatomical markers, at best, give a first approximation to segment axes orientation.
2. Dynamic: A known or controlled movement is analyzed to refine segment axes orientation based on expected or known joint angles. An optimization procedure may be used. An example is to analyze a squat movement and walking trial to assess and minimize non-linear error in knee abd/add rotations to refine thigh and shank axes orientation about their respective longitudinal axes.

In the movement trial each segment axes is reconstructed from known local and global coordinates of the segment markers using a least square approach (Veldpaus, 1988). As mentioned, the local coordinates are established previously in the subject calibration procedure. The practice of defining a technical frame and transformation from the technical to anatomical is unnecessary.

A Cardan rotation sequence is then used to describe one segment axes orientation relative to an inertial segment, usually the distal relative to the proximal. If the order of axis rotations are chosen appropriately (Flex/ext, Abd/add, IntExt rotation) then the joint angles describe an ordered anatomical rotational sequence.

Markers clusters (4+ markers) may be real (physical marker placed on the skin) or virtual (joint center defined from an adjacent segment). Not all calibration markers are removed and most remain on the subject and double as segment markers (ASIS, PSIS, lateral knee, lateral ankle). Calibration markers may however be removed because they may interfere with movement, are obstructed from camera view (medial ankles and knees) or are subject to large skin movement artefact (Greater Trochanter).

There are several objectives when placing segment marker clusters on the skin:

• At least four markers per segment [1,4-5], these may be real or virtual joint centers
• Markers are well distribution along at least two axes [6].
• Markers are non-symmetrical about two axes [5].
• Markers avoid areas of high skin movement artefact (proximal 1/3 thigh, Greater trochanter [7,8], calf musculature [8], proximal 1/2 forearm, large muscle bellies).
• Placing markers so as to include varying types of error or artefact amongst the cluster. These may include placing markers on the anterior, lateral and posterior aspects of a segment, on the body of the segment as well as lateral to joints, and including virtual joint centers.
• Use a least squares method [4-5], even with three markers [4]. The most stable and preferred least square method is that of Veldpaus (1988) [5]. Direct methods and inertial (principal axes) methods that use only the global marker coordinates should not be used to define segment axes location [4-5].

There are several advantages of the cluster approach:

• Can accommodate a missing marker or two.
• Dispersion of markers along at least two axes and avoidance of marker symmetries about axes.
• Varying sources of error are present within the marker cluster. With multiple surface marker locations, including virtual joint centers, each subject to varying nature and magnitude of error throughout the movement*. The least squares method will help reduce the effect of these errors on the reconstructed segment axes.
• 6 Degrees of freedom between segments. The magnitude of 3D marker error and errors in reconstructed segment axes varies substantially in magnitude between segments*. Six degrees of freedom allow the rigid segment system to accommodate these large differences in error when reconstructing the different segments.
• RMS Error between global marker coordinates and those reconstructed from the segment axes and local marker coordinates can be used to screen validity of reconstructed axes as well as accuracy of individual global coordinates across the frames of a movement trial.

*The error in global marker locations and reconstructed segment axes is referring to differences relative to the underlying anatomical (or true) segment axes that they represent and which we are trying to indirectly measure. Even when defining the anatomical axes in the subject calibration procedure the anatomical axes location contain error. Therefore, knowledge of sources of error from subject calibration, 3D marker reconstruction, post processing, skin movement artefact, number and placement of markers and mathematical approach need to be carefully considered. As well as the effect these errors have on axes misalignment relative to the anatomical (true) axes and the resulting non-linear errors in derived joint rotations.

The statement that the benefit of clusters is that they are not placed on a specific anatomical location is not strictly correct, as critical thought is needed when deciding how many and where segment markers will be placed.

[1] Miller, N.R., Shapiro, R., McLauchlan, T.M. (1980), A technique for obtaining spatial kinematic parameters of segment of biomechanical systems from cinematographic data. JBiomechanics, 13, 535-547.
[2] Veldpaus, F.E., Woltring, H.J., Dortmans, L.j.M.G. (1988) A least squares algorithm for the equiform transformation from spatial marker co-ordinates. J. Biomechanics, 21, 45-54.
[3] Challis, J.H. (1995) A procedure for determining rigid body transformation parameters. J. Biomechanics, 28, 733-737.
[4] Cappozzo, A, Cappello, A, Della Croce, U, Pensalfini, F (1997) Surface marker cluster design criteria for 3-D bone movement reconstruction, IEEE Trans. Biom. Eng., 44, 1165-1174.
[5] Carman, AB, Milburn, PD (2006) Determining rigid body transformation parameters from ill-conditioned spatial marker coordinates. J. Biomechanics, 39, 1778-1786.
[6] Solderkvist, I, and Wedin, PA (1993) Determining the movement of the skeleton using well-configured markers, J. Biomechanics, 26, 1473-1477.
[7] Cappozzo, A, Catani, F, Leardini, A, Benedetti, MG, Della Croce, U (1996) Position and orientation in space of bones during movement: experimental artifacts, Clinical Biomechancis, 11, 90-100.
[8] Stagni, R, Fantozzi, S, Cappello, A, Leardini, A (2005) Quantification of soft tissue artifact in motion analysis by combining 3D fluoroscopy and sterophotogrammetry: a study on two subjects. Clinical Biomechanics, 20, 320-329.

[Lasse Roren]

Re: Problem using cluster marker set using Vicon ProCalc

Hello,

The 6DOF proCalc tutorial is now online:

https://docs.vicon.com/display/ProCalc12/Vicon+ProCalc+6DOF+Modeling+Tutorial

Hope this helps!

Best regards,
Lasse