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Digest #4599
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Tue Jul 24, 2012 12:15 pm (PDT) . Posted by:
"CoachJ1@aol.com" coachj12002
>I'm not sure who is reporting what in this debate, but if the statement
attributed to John Buckley is correct, it seems that he doesn't understand
how storage of elastic energy works:
John Buckley is a clinical biomechanist at the University of Bradford
Division of Medical Bradford.
Buckley wrote the following as part of his comment on the JAP
Point/Counterpoint relative to the Pistorius research:
"A runner and prosthesis comprise a mass-spring system with nearly constant
natural frequency. If the prosthesis has high stiffness, the system has a
high frequency and a short period. If it has low stiffness, the system has
a low frequency and a long period. In the first quarter period, kinetic
energy is stored as
elastic energy in the carbon fiber keel. In the second quarter period,
this elastic energy is returned as kinetic energy. Optimal contact time is
therefore one-half the natural period of the system. Ground contact time is
determined by a runner's speed and leg compliance, with the actual contact
time matching the optimal time at only one speed."
Cavagna also wrote a response, which supported the Weyand/Bundle position:
"At high running speeds, a large fraction of the power developed each step
during the push appears to be
sustained by elastic energy stored within muscle-tendon units during the
brake . Elastic storage and recovery is improved at high speeds by
privileging the role of tendon relative to muscle at the expense of a high muscle
activation Replacing muscle-tendon units with a passive, inexpensive,
elastic structure may result in more efficient elastic rebound by increasing the
power developed at low cost during the push.
At low running speeds, the step frequency f is advantageously tuned to the
resonant frequency of the bouncing system fs With increasing running
speed, f increases less than fs to contain the power spent to reset the limbs at
each step If the half period of the bouncing system is measured in Fig. 1
of
Weyand and Bundle , as the time where the vertical force exceeds body
weight, the resonant frequency fs of the bouncing system results 60% greater
than the step frequency f in the intact-limb subject and 30% greater in the
amputee.If this is confirmed by measuring f and fs at different running
speeds, the advantage of a reduced mass of the lower limb may be considered.
These two observations favor the hypothesis that artificial limbs may make
artificially fast running speeds possible, even if, as stated by Kram et
al, this hypothesis cannot be statistically proven.
Ken Jakalski
Lisle HS
Lisle, Illinois USA
[Non-text portions of this message have been removed]
attributed to John Buckley is correct, it seems that he doesn't understand
how storage of elastic energy works:
John Buckley is a clinical biomechanist at the University of Bradford
Division of Medical Bradford.
Buckley wrote the following as part of his comment on the JAP
Point/Counterpoint relative to the Pistorius research:
"A runner and prosthesis comprise a mass-spring system with nearly constant
natural frequency. If the prosthesis has high stiffness, the system has a
high frequency and a short period. If it has low stiffness, the system has
a low frequency and a long period. In the first quarter period, kinetic
energy is stored as
elastic energy in the carbon fiber keel. In the second quarter period,
this elastic energy is returned as kinetic energy. Optimal contact time is
therefore one-half the natural period of the system. Ground contact time is
determined by a runner's speed and leg compliance, with the actual contact
time matching the optimal time at only one speed."
Cavagna also wrote a response, which supported the Weyand/Bundle position:
"At high running speeds, a large fraction of the power developed each step
during the push appears to be
sustained by elastic energy stored within muscle-tendon units during the
brake . Elastic storage and recovery is improved at high speeds by
privileging the role of tendon relative to muscle at the expense of a high muscle
activation Replacing muscle-tendon units with a passive, inexpensive,
elastic structure may result in more efficient elastic rebound by increasing the
power developed at low cost during the push.
At low running speeds, the step frequency f is advantageously tuned to the
resonant frequency of the bouncing system fs With increasing running
speed, f increases less than fs to contain the power spent to reset the limbs at
each step If the half period of the bouncing system is measured in Fig. 1
of
Weyand and Bundle , as the time where the vertical force exceeds body
weight, the resonant frequency fs of the bouncing system results 60% greater
than the step frequency f in the intact-limb subject and 30% greater in the
amputee.If this is confirmed by measuring f and fs at different running
speeds, the advantage of a reduced mass of the lower limb may be considered.
These two observations favor the hypothesis that artificial limbs may make
artificially fast running speeds possible, even if, as stated by Kram et
al, this hypothesis cannot be statistically proven.
Ken Jakalski
Lisle HS
Lisle, Illinois USA
[Non-text portions of this message have been removed]
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