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Kevin Sims
06-23-1998, 02:29 AM
Dear all my original posting was as follows:

I am currently involved in a project where we have to
model/understand the forces acting about the hip during the stance
phase of gait. Being of a long lost mathematical background I am
having difficulty grasping a few things. My major problem is
understanding the forces of the muscles.

Gottschalk et al (1989) describe the function of the gluteus medius
(in particular the posterior fibres) is to maintain the femoral head
in the acetabulum. This appears to be in contrast to the view of
Bombelli (1993) who describes the hip abductors, comprising Gluteus
medius, minimus and piriformis, as having a resulatant force which can
be split into a vertical vector and a horizontal vector. The
horizontal vector is laterally directed and so tends to push the
femoral head out of the acetabulum. I would appreciate it if anybody
could help explain this apparent contradiction (which may not be a
contradiction at all!).

Thanks to all who answered below is a summary of salient points
relating to vectors of the abductor muscles:



Perhaps, the most important biomechanical contribution of the hip
abductors occurs during the stance phase and doesn't involve hip
abduction. In normal gait, the weight of the head, arms, and trunk
(HAT) causes a gravitional moment on the hip joint of the stance side.
In order for the HAT to "balance", the hip abductors function to
counteract the moment caused by the HAT's weight by pulling the HAT
in line with the joint resulting in a decrease in the gravitional
moment. The joint kinetics involve horizontal and vertical vectors
acting in a direction from the innominate bone to the femur of the
stance side, resulting in a moment opposite to the gravitional moment.
Some people call this process HAT adduction because the hip joint and
HAT are being aligned by the hip abductors, but I find this term
somewhat confusing so I don't use it. Arthrokinematically, it is very
similar to hip abduction except the acetabulum glides superiorly on
the femoral head.

Well, I hope this helps out. Good luck!


+--------------------------------------------------------+
Arnel Aguinaldo, BSBE
Biomechanical Engineer








The abductors apply a force which
acts along an obliuque line that joins the gretaer trochanter (and its
neighbourhoods, depending on the mucles you take into account) to the
pelvis. Therefore, its force can be split itno an horizontal and a
vertical component. Now, the muscle applies to its two extremities a
force that is equal in direction and magnitude, but aligned in two
opposite ways. The abductors muscles apply:
- a downwards oriented vertical component and a medial-to-lateral
horizontal
component to the hip bones
- an upwards oriented vertical component and a lateral-to-medial
horizontal
component to the femur
thus pulling the head inside the socket. I don't have the paper of
Bombelli here right now, but I guess this is what he wrote. Of course
there is also an antero-posterior component. If you are interested in
motre papers on this subject, you should check the works of Patriarco,
Crowninshiled, Bergmann, J. O'Connor and myself.


Luca Cristofolini

I am not familiar with the references you provided. I think, however,
the apparent contradiction may not really be a contradiction at all.
The issue here may be a matter of frame of reference. The hip
abductors have attachments proximally on the ilium and distally on the
greater trochanter. The forces imposed on the ilium act to pull the
ilium inferiorly and laterally. These same forces, therefore, pull
the acetabulum inferiorly and laterally into the femoral head. If the
frame of reference, however, is the effect of the hip abductors on the
distal femoral attachment, then the action of these forces is to pull
the greater trochanter superiorly and medially. These same forces,
therefore, pull the femoral head superiorly and medially into the
acetabulum.


Michael T. Gross, PT, PhD

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