Hi Amur,

I think you should use the kinematic analysis method (viedo analysis = motion capture), but it's depends of your goal or which parameters (variables) you want to analyse.

A best method is a particular decision, So, What kinds of specific materials for this analysis you have in your lab?

If you want to apply, for example, the kinematic mathematic model, you should verify mainly the positon of the cameras, and the position of the body markers. Though this method, you will can to obtain others interesting analyses that it will can be explored (i.e, velocity, angle of joints, position, acceleration of lower limbs, moment of trunk). However another method is also possible though the use of  Wearabe Inertial Sensors. There is an interesting method, this method is to analyze the movement using two devices integrated (accelerometers and gyroscopes). You will can to obtain differents variables through these analyzes, indicated to posture or movement analyze. 

In any way, I send to you some material which I believe that can to clarify your ideas.

I recommend also this book:

Human Motion Understanding, Modelling, Capture, and Animation (2008)

Editors: Rosenhahn, Bodo, Klette, Reinhard, Metaxas, Dimitris (Eds.)

Best regards.

dear Amur Salim Al Yahmed i think the best way to track trunk by use Motion Analysis

and the following site will help you

best regard hamdy

http://www.egr.msu.edu/~jchoi/files/papers/ReevesChoiJB2014.pdf

http://www.udel.edu/PT/PT%20Clinical%20Services/journalclub/sojc/13-14/November/Teng%20JOSPT%20Sagittal%20Plane%20Trunk.pdf

http://www.asbweb.org/conferences/2015/abstracts/145AE--Tracking%20The%20Trunk%20Segment-%20Backpack%20Vs.%20Elastic%20Strap--(Willigenburg).pdf

http://aut.researchgateway.ac.nz/handle/10292/392

https://web.stanford.edu/~hyatt4/research/manuscripts/Human%20Tracking%20via%20MARG%20Sensors.pdf

http://www.jospt.org/doi/pdfplus/10.2519/jospt.2001.31.5.255

http://www.ncbi.nlm.nih.gov/pubmed/11352192

https://books.google.com.eg/books?id=fxRCbctfhuwC&pg=PR11&lpg=PR11&dq=analyze+trunk+motion+sagittal+plane&source=bl&ots=bDECMTXewR&sig=E3wrMxkC0oSsMw54m4vuMqt9jBI&hl=ar&sa=X&ved=0ahUKEwisjqWMlPjKAhUEDSwKHVOJBF4Q6AEIMzAC#v=onepage&q=analyze%20trunk%20motion%20sagittal%20plane&f=false

https://theses.lib.vt.edu/theses/available/etd-01202006-144843/unrestricted/ETDMinhyungLee.pdf

http://bjsm.bmj.com/content/43/6/417.short

Thanks Marcelo and hamdi I will go through the info provided and get back to yoiu in case I have further questions

best regards