Dear Steve,

You wrote:

.."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."

This is a surprising result, and I cannot guess the mechanism through which
a shorter moment arm (with consequent reduced shortening velocity and higher
force) produces a longer contact time, although I have read your abstract

It is not as simple as it seems. With a fixed displacement (range of motion)
in a given direction (e.g. "forward"), if you push harder in that direction
throughout the pushoff, your time of force application (contact time)
becomes smaller, and vice versa.

Thus, one way to maintain contact "with the ground for longer" would be to
push less hard throughout the pushoff (as if your leg were a spring with
lower stiffness). But if you did so, even though the contact time would
become longer, your impulse would become smaller (the increase in time of
force application would not suffice to compensate for the decrease in

Another way would be to change the range of motion: you would obtain a
longer range of motion in the forward direction if your leg were more tilted
forward at the end of the extension, or more flexed at the beginning of the
extension (hip and knee flexion is limited, however, by "short term
fatigue", otherwise everybody would jump from full squat...).

A third way would be to push harder in the first part of the pushoff, and
less hard in the second part. This may give a larger final velocity and a
larger impulse, even with a fixed range of motion...

Your abstract does not explain this mechanism. Do you have an explanation?
Also, what are the simulation constraints? Was the orientation of the ground
reaction force (GRF) fixed? And what about the moment arm of the GRF
relative to the ankle rotation axis? Subjects with a longer arm for the
muscle force and longer muscle fascicles may also have a longer arm for the
GRF (i.e a longer foot). A longer foot would increase range of motion...

Thank you,

Paolo de Leva
University of Rome - Foro Italico.

-----Messaggio originale-----
Da: * Biomechanics and Movement Science listserver
[mailto:BIOMCH-L@NIC.SURFNET.NL] Per conto di Steve Piazza
Inviato: Friday, September 05, 2008 5:15 PM
Oggetto: Re: Bolt

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:
> 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
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