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View Full Version : Re: Oscar Pistorius - Science and Engineering vs Training



Van Den Bogert, Ton
08-01-2007, 07:47 AM
This is a belated response to Jamie Carruthers' posting of July 13. It
is useful to read Jamie's full article on
http://scienceofsport.blogspot.com.

Jamie's analysis makes sense to me and I agree that this technology
should be seen as potentially performance enhancing, until we know for
sure that it is or is not.

The often quoted argument that natural limbs have an energy return of
240% is quite misleading. Only a small part of that amount is actually
a return of stored energy. Most of the positive work must be generated
at considerable metabolic cost.

For sprinting, average external power output is what matters, not
efficiency. The loss of net power output from ankle muscles therefore
seems to be a disadvantage for an amputee. However, I think that such
loss can easily be compensated for by a better utilization of the
remaining muscles. This argument can be supported by a simple
calculation. In able bodied sprint running, net power output is about
350 W in the second half of the race (e.g. Ward-Smith, J Sport Sci
1999). The same two legs can produce roughly 1000 W external power in a
10-second bicycle ergometer test. So obviously there is extra power
capacity in the muscles which is not used in running. This is because
of suboptimal mechanical boundary conditions, requiring muscles to work
at high shortening speeds. It is conceivable that a well designed
prosthetic foot can make some of that extra capacity available by
allowing knee and hip extensor muscles to start shortening earlier in
the stance phase, which would lower their peak shortening speed. With
650 W unused capacity to play with, the loss of power from the ankle
muscles can probably be overcome. This is just a hypothesis, but it
should be possible to quantify these effects through a comprehensive
biomechanical analysis. Such an analysis would consider joint power
patterns at hip and knee, but also muscle shortening velocities from
kinematic analysis, and perhaps even individual muscle forces and
powers.

Some of the discussion has focused on potential advantages of less mass
and less air drag. Even if these are eliminated by making these equal
to able-bodied limbs, the potential advantage of energy storage remains
and should not be overlooked. If this is indeed an advantage, it can
only be eliminated by requiring a minimum stiffness in the prosthesis.
This would also reduce the problems with stability. Oscar Pistorius was
disqualified in a recent 400 m race for going out of his lane.

I agree with Jamie that the IAAF must be careful that they do not create
a precedent that allows footwear technology to create similar advantages
for able-bodied athletes. It should be possible to prevent this by not
allowing this technology in able-bodied athletes. Which still leaves
amputees with a possibly unfair advantage. Jamie suggests that an
athlete like this is necessarily a very rare occurance, that someone is
a double amputee before learning to walk, and has the talent and
resources to pursue an athletic career. If Jamie's estimate is
correct, we may be able to accept the fact that occasionally someone is
born who has that advantage. Sports performance is already influenced
by genetic factors anyway. The fear is, of course, that there will be a
substantial group of individuals who can do this. Then what do you do?
Make the same equipment legal for able-bodied athletes, for fairness, or
have separate categories in the Olympics for amputee (or double amputee)
runners?

--

Ton van den Bogert

--

Department of Biomedical Engineering
Cleveland Clinic Foundation
http://www.lerner.ccf.org/bme/bogert/

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