View Full Version : Re: Quaternions vs. Euler angles (How much does it matter)

05-08-2001, 07:05 PM
Dear All:
I read (with some delay, admittedly) the message from Dr. Rassoulian, who
disputed the relevance of the discussion initiated by Chris Kirtley on
Quaternions Vs Euler angles, at least for the clinical applications.

To clarify what follow, I can ensure that the clinical relevance is central
to any research running in my unit. The head of my department is a
surgeon, and we all have very clear in mind what are the constraints of the
clinical practice.

This said, I think positions like the one Dr. Rassoulian represented are
potentially dangerous for biomechanics, and I personally see them with
great suspect any time I meet them (which happen frequently, unfortunately).

The problem is that starting for a legitimate point (let's keep focus on
the clinical relevance of our researches), this argument is frequently used
to adverse methodological improvements.

Now, there is no reason to use a method if it has been proved
inappropriate, and if a better method is available. Of course the new
method must be implemented properly so to be useable in the clinical
context. Thus, in the specific, I guess it would be appropriate to user
quaternions (or even better attitude vectors, since we are using the list
founded by Dr. Woltring) for all computations and then at the end to report
angles ALSO with respect to an anatomical reference.

Dr. Rassoulian stated that under many clinical circumstances, gait/ motion
information is nearly unhelpful. Eventually, this may be true because in
the way it is used in many clinical setting gait analysis is so inaccurate
that it can discern only the most obvious motion patterns, those that are
visible at naked eye.

I am sure Dr. Rassoulian intention was to stress the importance of the
clinical relevance in any biomechanical discussion and I fully agree with
him. However, he should be aware that too many times similar arguments are
used to justify unacceptably poor methods.

In my opinion scientists are supposed to do their homework, i.e. to learn
and implement any new method that significantly improves the experiments as
soon as it becomes available, no matter how difficult this process may
sometime be.


Marco Viceconti

Hamid Rassoulian wrote:
>Does it matter which angular representation is used?
>Under many clinical circumstances, gait/ motion information is nearly
>unhelpful, and such mathematical delicacies are ignored completely.
>I don't dispute the academic validity of this type of argument, and
>perhaps its mathematical fascination. Detailed discussion of these
>concepts would be important for the purposes of computer simulations and
>calculations of movements, say in VR applications or other computerised
>processes, where dealing with the errors is not only a calculation but a
>mathematical flirtation. However, when trying to understand the gait
>recovery of a 57 year old stroke patient and decide where to focus the
>physiotherapy efforts, all these seem like "star trek".
>I would like to encourage contributions from all colleagues who have the
>pleasure of having to deal with the pragmatic world of health care. At
>least >from a clinical point of view, it is important to stimulate
>discussions about gait data analysis that aims to enhance patient care, or
>at least make it clear that how the discussion could be potentially
>relevant to "shop floor".
>Dr. H Rassoulian BSc, MSc, PhD, MIPEM, SRCS
>Head of Clinical Bioengineering Group
>Dept. Medical Physics & Bioengineering
>Southampton University Hospitals NHS Trust
>Southampton SO16 6YD
>United Kingdom
>Tel: 023 80796945
>Fax: 023 80794117
>email1: Hamid.Rassoulian@suht.swest.nhs.uk
>email2: hamidR@soton.ac.uk

MARCO VICECONTI, PhD (viceconti@tecno.ior.it)
Laboratorio di Tecnologia Medica tel. 39-051-6366865
Istituti Ortopedici Rizzoli fax. 39-051-6366863
via di barbiano 1/10, 40136 - Bologna, Italy

Tiger! Tiger! Burning bright in the forest of the night,
what immortal hand or eye could frame thy fearful symmetry?
Opinions expressed here do not necessarly reflect those of my employer

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