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spurkiss46
05-22-2007, 01:44 AM
The approach suggested by Dr. van den Bogert would help elucidate what is
actually happening during running - important for all those who like to
know how & why things work (most of us in this field i think!).

One aspect i haven't yet seen directly addressed in the discussion is the
idea of "equivalency":
if the protheses impart a disadvantage during acceleration, but an
advantage at the end of the race (when muscles fatigue) - is there a way of
establishing if these two phases are "equivalent" parts of the race?
Obviously the longer the race, the greater the effect of fatigue. But if
there were a distance where the two portions were "equivalent", then the
prostheses would theoretically confer no net advantage. This is assuming
prosthetic characteristics (weight, etc.) were controlled for, and that
there was a reasonably straightforward way of defining "advantage" (perhaps
by comparing average speed during that portion of the race?? I don't know
enough about racing to offer an opinion on this).

I realize that this is a simplistic view but thought it might provide
another avenue of discussion.

Sheila Purkiss
PhD Candidate
University of Toronto




"van den Bogert,
Ton"
To
Sent by: * BIOMCH-L@nic.surfnet.nl
Biomechanics and cc
Movement Science
listserver Subject



05/18/2007 12:30
PM


Please respond to
"van den Bogert,
Ton"







I am surprised by the attempts to argue that this prosthetic technology
provides no advantage. Why not allow the idea that this technology
works better (for specific movement tasks, e.g. 100 m sprint) than the
natural limb? Would that not be much more inspiring to those with
disabilities (and to scientists)? The remarkable performance of Mr.
Pistorius shows that this idea must be seriously considered.

First of all, energy cost is probably not a major consideration for a
100 m sprint. Studies showing higher oxygen uptake in amputee gait may
not be relevant to this issue. In sprinting, the main goal is to
maximize total net power output of all muscles, in order to increase
kinetic energy (in the first phase of the race) and to do work against
air drag (in the last part of the race).

If my scientific intuition is right, it is only a matter of time before
an amputee will break a world record in track and field. At that point,
there will have to be separate competitions and records for able-bodied
athletes, as Andy Ruina suggested. This is not really different from
wheelchair athletes in marathons. They are much faster than the
runners, but are considered to be in a different category. Question is
(for the sports organizations) whether to wait until a gold medal or a
world record forces the issue.

If science is to help determine whether or not a prosthetic foot is
advantageous, here is my theory, a hypothesis and a proposal for how to
test it.

Theory

I have not looked at the biomechanics literature on sprinting, but I
suspect that it is known that ankle plantarflexion has a substantial
contribution to mechanical power output. The prosthetic "ankle" has no
net positive power output, so this would seem to be a great disadvantage
to the amputee athlete. However, we can't look at joints in isolation.
The prosthetic foot may allow the hip and knee extensors to function
differently.

It is theoretically possible that a compliant foot allows hip and knee
extensors to produce more net power over the gait cycle. This is quite
similar to the principle of a muscle acting in series with a compliant
tendon. See, for example, Glen Lichtwark's recent paper in J Biomech
[1]. The basic idea is that muscle fibers can shorten while the tendon
lengthens and stores energy. When considering a limb, we have multiple
joints in series. In the amputee, therefore, knee and hip extension may
start earlier in the stance phase than in the able-bodied athlete,
storing the extra energy in the compliant foot. That energy will be
released at the end of the stance phase.

Hypotheses

(1) Joint power at knee and hip are greater in an amputee sprinter than
in an able-bodied sprinter.
(2) This increase in knee and hip power is greater than the power
produced at the ankle in an able-bodied athlete.

Methods

The analysis can be done with standard inverse dynamics methods. It
will be necessary to get data for the entire 100 m race. It will he
difficult to get force plate data for every stride. However, sprinting
has no double support phase, so this can be done quite well with a whole
body model which does not require force plate data.

It is probably best to compare this highly trained amputee to a group of
able-bodied sprinters who have similar 100 m times. If they run the
same speed, hypothesis (2) should probably be worded as "equal to"
instead of "greater than". This is because the same speed implies that
total mechanical power output is the same.

Why is this important?

If this theory turns out to be correct, we have a scientific basis for
optimizing the performance of these devices and for developing novel
movement strategies to use them. We will make great strides (:-) in
improving locomotor function for all amputees, not just elite athletes.
If we keep insisting that "of course" these devices are not as good as
real feet, we have little incentive for research and we are telling them
that, "of course" they will always be handicapped.

References

[1] Lichtwark GA, Wilson AM (2007) Is Achilles tendon compliance
optimised for maximum muscle efficiency during locomotion? J Biomech
40: 1768-1775.

--

Ton van den Bogert
Department of Biomedical Engineering
Cleveland Clinic Foundation
http://www.lerner.ccf.org/bme/bogert/
(apologies for the advertising below, I have no control over this)

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