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  • Re: Oscar Pistorius - Science and Engineering vs Training

    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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