Upper limb modelling help

[Emma Pratt]

Hi,

I am involved in a project modelling the upper limb of children through a range of movements, some of which will involve the shoulder joint being elevated more than 120 degrees. Whilst I would ideally like to use IHA to define the shoulder joint centre, I cannot find a way of implementing these within Vicon (our 3D motion analysis software) using bodybuilder, and we do not have access to MATLAB at our hospital. Instead I have decided I will have to use Meskers adapted regression techniques to define the shoulder joint centre, but I am very confused to how this can be done practically and wondered if anyone could help? I have read many many papers, but just seem to be getting more confused.

Meskers defines the GHJ relative to scapular markers. Is this done in a single static trial? If so, in what position? I'm guessing this has big implications to where the GHJ is located? Or is the location calculated continuously throughout a movement, based on the scapular markers. However, having read how difficult it is to track the scapular, this would seem to also introduce errors. I have read papers that talk about static positions of 30, 60, 90 and 120 degrees elevation and 0 and 90 degrees in the transverse plane. I could calculate the different GHJ centres calculated at each of these positions, but cannot understand how this is implemented practically during dynamic movements - is an average position used?

If I could calculate the GHJ from static trial(s), would it then be appropriate to relate the position to a pod of markers on the upper arm, so I can capture dynamic movements of the arm relative to the thorax? I would obviously like to track the scapular but from everything i've read this is not going to be reliable above 120 degrees elevation, so I am thinking it may be best at this stage to simply output humerus angles relative to the thorax.

Any help would be gratefully appreciated.

Thanks in advance

Emma

Emma Pratt
Clinical Scientist
Sheffield Children's Hospital Gait Laboratory, UK

[Dustin Bruening]

Re: Upper limb modelling help

Hi Emma, we used to collect upper extremity motion of children with brachial plexus palsy as they performed a variety of daily living activities. I imagine this is similar to what you are doing? There are a number of ways to model and track the upper extremity, but it sounds like you are after something that is easy to implement, which we were also after. We ended up simply placing two markers on the anterior and posterior shoulder, so that the midpoint approximates the center of the humeral head. We then used a marker cluster to track the upper arm without constraining the shoulder joint (giving it 6 DOF), and then calculated joint angles between the upper arm and torso. We used the ISB recommended YXY rotation sequence and expressed it as a globe system (see http://www.sciencedirect.com/science...66636207001762), which the clinicians found acceptable. There are more complicated/sophisticated approaches out there, but this is very easy to implement and handles large ranges of motion (as long as you don't have high velocities). Hope it helps.

Dustin