Below is a copy of my original question posted 21/04/2010 and a summary of the responses received. Thanks very much to everyone who took the time to respond, your inputs will be very useful in reviewing and updating both our clinical and modelling protocols.

Responses ranged from using lateral ASIS placement to digitizing pointers for subject calibration, with a general consensus for markers placed on the posterior pelvis for tracking. There was also general agreement for patients to understand the need to collect quality clinical data, and the requirement to expose ASIS.

Best wishes


Emma Pratt, Clinical Scientist.
Sheffield Teaching Hospitals NHS Foundation Trust, Sheffield, UK.

Original question:

As part our clinical service we regularly conduct 3D kinematic and kinetic gait analysis on adults. A growing number of our patients are obese, making accurate pelvic marker placement very difficult. These patients are generally also more reluctant to allow us to adjust clothing to place ASIS markers directly on the skin.

We were wondering if anyone has any practical solutions to this problem that they would be willing to share? At present we make a belt of thick micropore tape wrapped around the ASIS/PSIS, to try and stabilise both skin and clothing movement.

Dear Emma,

If you are willing to use other than the Conventional Gait Model (CGM) for your work, you may find this suggestion to be a solution for your problem
The PSIS and ASIS markers are quite useful for helping to identify an anatomically relevant local reference frame in the pelvis. Soft tissue at the PSIS is rarely a problem, so these markers are well behaved. But, it is not unusual to have ample soft tissue at the ASIS, and this can adversely affect both the derivation of the pelvic local reference frame, and tracking of the pelvis, when using the CGM.

Years ago, the Vicon Clinical Manager software (VCM) resolved the local reference frame problem this way:
1. Measure the inter-ASIS distance clinically, during a physical examination.
2. During the subject calibration, with the patient already standing in the calibration area, visualize a line passing through the two ASIS, and place markers where this line would pass through the skin. (Usually lateral to the ASIS, rather than anterior.)
3. When deriving the pelvic local reference frame, find the midpoint of these skin markers. Then, move laterally one half the clinically measured inter-ASIS distance to find the predicted R and L ASIS deep to the soft tissue. Use these locations to derive the local reference frame.

Also during the subject calibration, apply at least three markers on the sacrum: L and R PSIS, and one other marker. Identify these as tracking markers for the pelvis, thus avoiding the ASIS soft tissue all together.

Best regards,

Frank L Buczek Jr, PhD.
Branch Chief, HELD/ECTB Coordinator, MSD Cross Sector Program
National Institute for Occupational Safety and Health (NIOSH), Morgantown, WV.

Dear Emma,

As physiotherapist I realize very well that therapists must respect, for ethical reasons, patient's integrity and decency ... but on the other hand patients must understand that some clinical activities request to be "invasive" (although is placing a marker on ASIS really "invasive"?) if one wishes to help the same patients by quality clinical measurements (with the emphasis on quality = accurate, reproductible, etc).

There is an increasing concern in many institutions (clinical, but also governmental through national social security systems) that are questioning the real usefulness of motion analysis because it is relatively poorly reproducible and comparable from one location to the others.

Accepting that a patient is allowed to be reluctant will increase your measurement errors exponentially ...

.. I do believe the first solution to adopt is to try convincing the patient that he/she 's finding him-/her-self in a clinical context with well-trained people who have no intention to judge people by their physical appearance.

Many other clinical examinations (e.g., genecology, prostate, rectum) are much more "invasive" for the decency of the patient and to my knowledge they raise very few problems because patients are convinced that such exams is required. Compared to that kind of exams a gait analysis session is "peanuts" (sorry for the expression), but should be performed in strict and optimal conditions.

We are probably lucky in Belgium and people are maybe not to shy to "show" their ASIS ... on the other hand in order to increase anatomical landmark location precision, we developed a new system that is currently tested in clinics and that is very promising (Salvia P, et al. (2009). Precision of shoulder anatomical landmark calibration by two approaches: A CAST-like protocol and a new anatomical palpator method. Gait & Posture, 29: 587-591)

Note that the paper described the method on the shoulder girdle, but we are using it for any limb segments.

Of course, this will probably not solve your problems with shy patients.

Very best wishes,


Serge VAN SINT JAN, Ph.D., Prof.
Laboratory of Anatomy, Biomechanics and Organogenesis (LABO) [CP 619]
Faculty of Medicine - Université Libre de Bruxelles, Belgium.

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Hi Emma,

C-Motion offers the digitising pointer which can be useful to define
anatomical locations ( We
have used it at LJMU occasionally and it is pretty simple although we
prefer the offline rather than online method as implemented in Visual

I am happy to provide more information related to our experiences with
the pointer if necessary.

Best regards,

Mark Robinson - Teaching Assistant in Biomechanics
School of Sport and Exercise Sciences, John Moores University, Liverpool UK.
Hi Emma,

At our lab we have a 6 cam Qualisys Oqus system, and we are using Visual3D from C-Motion ( )to the modelling and analyses. C-Motion have a pointer-devise that you can use to set virtual markers (virtual ASIS markers) for pelvis definition. These markers are related to tracking marker that you can place on SIPS and e.g. sacrum, you may also fix them to a plate...

Best regards
Inge Ringheim - Phd-student
Vestfold Hospital Trust, Kysthospitalet Clinic Physical Medicine and Rehabilitation

Hi Emma,

I had the same problem a few years ago. I put a marker cluster on the back of the pelvis, and used a pointer to create virtual points at the ASIS. I used this in patients that were obese and in others, as I was doing sit to stand and I often lost the ASIS markers. Previous to that I also used a lycra body suit to minimise skin motion, and instead of using a normal belt I used a weightlifting lumbar support belt.

Hope this helps.


Hi Emma,
If the ASIS are just used for tracking, I recommend you add 2
additional markers on the more posterior portions of the pelvis. If
you do need to use the ASIS and are having trouble locating them, ask
your subjects to put their hands on their hips. The ASIS will usually
be located between the index and middle fingers. I've found this to
work remarkably well for minimizing poking and prodding in an obese
population. If you are using morbidly obese individuals, however, the
ASIS will almost certainly be obstructed by tissue and I recommend
moving the markers laterally.

As for the belt around the pelvis, I've tried wide (about 6 inches and
1/3 cm thick) neoprene that wraps around the pelvis and thin, wide
elastic bandages with non-slip inner linings. While they probably
will not slip down or rotate about the torso if secured tightly, they
do have a tendency to roll down in obese populations much more than if
you tried using them with healthy-weight subjects. As soon as it
begins to roll, the natural response is to pull it up, which usually
moves the markers. The thicker (but same width) neoprene works
slightly better than the elastic bandages. I would discourage using
them over clothing. I tie the subject's t-shirt with a rubber band in
the back so they can keep it down over most of their stomach but it is
still high on the sides and in back so it does not obstruct other

Our biggest improvement to subject self-consciousness during data
collection has been the use of longer spandex shorts. Obese subjects
normally didn't mind the tight-fitting shorts, but were very averse to
short shorts. If that doesn't help, try wearing a white lab coat.

Best of luck!

Elizabeth Russell

Doctoral Candidate
Department of Kinesiology, University of Massachusetts at Amherst

Hi Emma,

Look at Derek Curtis's presentation from ESMAC 2009. He's come up with using wand markers on the ASIS with 2 markers on (one at the tip and one about halfway down) so that you can then calculate a vector back to the true ASIS position (about 2 lines of BB code). The main reason for doing it is for those patients whose ASIS markers get obscured & it doesn't get past the problem of having markers directly on the skin but because you don't have to have the ASIS clearly visible it does help with not having to pull shorts down so much.

Jo Bates, Clinical Scientist
Oxford Gait Laboratory, Nuffield Orthopaedic Centre UK.


There was a paper that has just been published in either Gait and Posture
or Journal of Biomechanics that looks at pelvic marker placement. Don't
have the reference to hand, but it has been published in the last month
or two.

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