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Steve Piazza
09-05-2008, 01:15 AM
At NACOB last month, we presented data that supports Ton's expectation that
sprinters would benefit from having smaller moment arms. We measured tendon
excursion with ultrasound to estimate the plantarflexion moment arms of the
Achilles' tendon in collegiate sprinters and height-matched non-sprinters
and found the sprinters' moment arms to be 25% smaller.

Why smaller moment arms might confer an advantage to sprinters, and when
that advantage occurs, is an interesting question. We made a simple (but
anthropomorphic) 2-segment, 1-muscle model of a sprint pushoff and found
that smaller moment arms conferred an advantage even at slower speeds
consistent with the start of the race. The simulated "plantarflexion"
moment is indeed lower when smaller moment arms are imposed, but the
force-velocity-related benefits of having reduced shortening velocity result
in contact being maintained with the ground for longer, thus enabling
greater forward impulse generation.

Steve Piazza
Sabrina Lee
Penn State University


One of the commentators on NBC suggested that he may have larger muscle
>>
> moment arms, and this was also mentioned by Bengt Saltin to a New York
> Times reporter:
> http://www.nytimes.com/2008/08/20/sports/olympics/20bodies.html. There
> was no mention of any MRI data to support this idea, though.
>
> Theoretically, I would expect that a smaller (not larger) muscle moment
> arm would be advantageous (except at the start) because it allows
> muscles to operate at lower speed during the sprint. In the later parts
> of the race, where Bolt does especially well, high joint angular
> velocity is more important than high joint moment. With long legs and
> small muscle moment arm, it is like having a higher gear on the bicycle.
>
> Ton van den Bogert
>
> --
>
> A.J. (Ton) van den Bogert, PhD
> Department of Biomedical Engineering
> Cleveland Clinic Foundation
> http://www.lerner.ccf.org/bme/bogert/
>
>