I am going to make a bold attempt to bring together two
very interesting discussion lines: Marker placement and the
Oscar Pistorius debate.

Regarding running with a prosthesis I think there is one
element that has not been mentioned and is probably one of
the most critical to the amputee’s ability to function with
a prosthesis. The suspension of a prosthesis is imperfect
relative to the skeletal anatomy. The best prosthetic
suspension available (neglecting osteo-integration) will
minimize movement of the socket relative to the skin.
There is essentially a 6 degree of freedom joint between
the prosthetic socket and the residual skeleton.
Displacement of the socket relative to the skeleton
depends on many factors: amount of surrounding soft tissue,
anatomy of that soft tissue (stiffness and voluntary
control), surface geometry of the socket, stiffness of the
socket, type of prosthetic suspension, weight of the
prosthesis, and inertial forces acting to pull the
prosthesis distally i.e. high lower limb swing phase

Of course, one of the principle jobs of the prosthetist is
to manage and minimize movement at this joint in a way that
is comfortable to the amputee. Athletes will also use
secondary and even tertiary suspension methods to further
minimize movement, but the joint is still there.
Obviously, the less movement at this ‘joint’ the easier it
is to neglect the joint when doing kinetic and kinematic
calculations for the prosthetic side of the body.

So the questions are:
1. How significant is this joint and what are the dangers
of neglecting it in terms of gait analysis and inverse
dynamics calculations?
2. Is there a way to account for this joint during gait
studies of amputees, especially amputees with fleshy limbs
where larger movements between the skeleton and socket may
3. Is there a way to determine the cost to the amputee
caused by movement at this joint or voluntary control of
4. Are we safe in the commonly held assumption that the
existence of this joint is inherently disadvantageous to
the amputee?

I have my suspicions, but suspect there are people much
better equipped than I to attempt to answer these

Thank you,


Jason Wening, MS, CP
Clinical Research Director
Certified Prosthetist
Scheck and Siress Prosthetics, Orthotics, & Pedorthics
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