View Full Version : summary: FOB & pelvic motion

Stephen Sprigle
08-30-2000, 05:54 AM
thanks to the folks who replied to the post appended below:

>we're currently validating the use of FOB to monitor pelvic
movement as people with SCI reach while seated.
validation is by using radiographs, but our question
arose while doing repeatability
we've attached birds in 2 manners to measure pelvic tilt (pitch), both
with pros and cons:
1) birds affixed to the ASIS and PSIS
pro: sensors are on ilium, the target structure
con: many folks have substantial tissue over these prominences (location &
movement error)
2) single bird affixed to sacrum via sacral belt
pro: much, much easier to don/doff and much less tissue over bone
con: movement of SI joint potentially adds error

data shows excellent repeatability of both approaches, but we'd be
curious to hear
if anyone has tackled a similar issue or has opinions on SI mobility in the
seated posture of persons
with SCI.

We have used Ascension's FOB in our product, The MotionMonitor, for the
6 years. It has been our experience that placement on the sacrum is
superior to placement on other areas that involve more soft tissue
With a tight application to the sacrum, we then digitize joint centers and
other landmarks that move rigidly with the sacrum and pelvis. While we
not validated these with radiographs, we have used Leardini and Cappozzo's
femur rotation method to locate hip centers and find that the results
correspond nicely with his data which, if I recall correctly, did include
validation by radiographs.

Let me know if I can be of further assistance.
Lee E. Johnson
tel: 773-528-1935
fax: 773-528-2149
Innovative Sports Training, Inc.
....The Total Solution in Motion Capture.

Go with the sacrum (use two sensors however). The overwhelming evidence
the lack SI joint motion supports this choice among other reasons.
The limited SI motion that is possible, only occurs with extreme
end-of-range stress ( as in doing the splits), and SI motion in general, is
only clinically significant in pregnant women, young women ,and hypermobile
pathological conditions and trauma.

The limited SCI pelvic motion in a chair is highly unlikely to elicit any
significant stress on the SI .

If you really want to be sure, then do both and compare on a group of
subjects. - Digitize three points on each illium in several positions
recording from the sacrum sensor simultaneously. Reconstruct the euler
angles and compare the pitch.

Kevin J. McQuade PT,PHD
Rehabilitation Research Fellow
University of Maryland School of Medicine
Balitmore, Maryland 21201

if you just go with the sacrum. using two is just a way of increasing the
accuracy because the different placements will have different skin slippage
and with two you can take the centroid or use some other algorithm to
average out the error, or at least know what it is.

define your sensor orientation such that the largest motion you want is
azimuth - that is best for euler angles and will avoid any gimbal lock
associated with euler rotations.


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