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perry75
05-22-2007, 08:35 AM
I am also in agreement with Ton that it is only a matter of time before
an amputee will break a world record in track and field. This is not to
say that an amputee who masters the use of his/her prosthesis both
physically and cognitively to this level is not a remarkable athlete,
and who is perhaps in even better shape than other athletes in the
field. But the question still remains if the athlete is using the
prosthesis to an advantage. Although both analysis methods suggested by
Ton (measuring joint torques) and Jason (measuring metabolic rates)
would no doubt offer insight to the question, these will not, in my
view, address the important concern: whether or not there is an advantage.

Measuring metabolic and physiological cost to assess biomechanical
efficiency tells us precisely that, how "efficient" their running was
from a biomechanical standpoint. Is it not possible for a person who
runs less efficiently (or more efficiently) to still be faster over a
100m distance? Ton has already pointed out that studies show "higher
oxygen uptake in amputee gait may not be relevant to this issue."

And some thoughts on measuring joint torques: whether or not an amputee
athlete stores more energy from the hip and knee in the flexible keel is
(I believe) less important than whether or not this power can be
released to an advantage for 100 meter race. For example, you can jump
all day and never finish the race. Ignoring differences in sprinting
style, we are concerned with components of power that contribute to
ground reaction forces in the direction of motion, and you would have to
show that a larger net difference between fore-aft power resulted
directly from components of the prosthesis. Aside from the lack of
distal leg mass in amputee sprinters, there is an obvious lack of
damping from biological tissue. A simple experiment, that I'm pretty
sure has already been done for able bodied individuals, would be to
impose a sinusoidal torque of increasing frequency about the
co-contracted ankle joint and compare resulting ankle kinematics between
sprinters having biological and prosthetic components. I suspect the
damping in biological ankles would show a much lower cutoff frequency
than the "cheetah" foot, for example, and would further support the
argument that enough leg strength combined with small distal leg masses
could allow an amputee to make use of energy storage/return in
prostheses at higher loading frequencies (i.e., during faster running).

One other comment that I would like to clarify regarding the phrase
"purity of sport", as I agree this can take on some controversial
interpretations. The use that I intend is not meant as a rigid grasp on
the origins of the sport; rather, I am referring to the preservation of
honor and integrity in the games, maintained in part through making sure
that any allowed use of advanced or other technologies are rightfully
allowed to all same-event same-class participants. I like the
suggestion, that has now been made several times, to allow amputees who
make Olympic qualifying times to compete in a designated class. This is
really the only "fair" allowance, and certainly one of few that there is
time to implement by the 2008 Olympics.

_____________
Joel C. Perry
Research Engineer
Biorobotics Laboratory
University of Washington
brl.ee.washington.edu



van den Bogert, Ton wrote:

>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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