|Energy costs in lifting
Thanks to Ron Daniel for sharing his thoughts on this topic. The following
come to mind at once:
1. The ankle and knee are not pure hinge joints and analysis of the motion
of the lower leg and foot must take into account rotational forces as well
as the linear forces relating to forward drive. This is further compounded
by hip motion.
2. The foot position at toe off is not indicative of drive angle in some
walkers. This is especially true amongst elite walkers who maintain a
relatively passive ground contact for the last few thousandths of a second
prior to toe off. In simplified form; after the drive work is completed, the
foot motion is continued to keep a toe tip in contact for legality.
4. Ron observed that:
" .....a walker in a 20,000 meter race who has a stride length of
one meter is taking 20,000 strides. Let's say that he is able to float
1/10th of a meter. He has now saved himself 10% of the needed strides, or
2000 strides, which is a saving of 10% of the push-off energy and 10% of the
I disagree. The body mass and acceleration due to gravity don't change. The
number of strides decrease and therefore the acceleration required at each
stride must increase to maintain the same velocity. Since F=M.A it follows
that the force must increase and the energy cost will therefore go up in
each stride but we would need to solve the resulting energy curve equation
for each athlete for each velocity in order to predict their break even
energy cost point. This presumes the use of otherwise legal walking form on
a flat surface of constant restitution.
There would also be an increase in energy cost due to braking forces which
are directly proportional to the maximum inter contact height of the whole
body centre of gravity. This may be moderated by alterations in lead leg to
ground contact angle in the lifting versus the no lifting condition. I am
working from the premise that this angle would remain unchanged but the
human nervous sytem has shown incredible degrees of plasticity - we know
that the spring constants for the leg muscles can alter in a single stride
when a different hardness of surface is reached in running - that I doubt it
is valid. It wouldn't be a surprise if we found subliminal adaptations in
muscle tension or limb angle occuring with a transition to lifting.
All off ground motion is parabolic but the individual body segment motions
will have a mirror effect down the biomechanical chain. We can calculate
these using Newton's third law of motion but would need a large array of 3D
sensors and some googlisciously large number crunching machines.
5. There is no point number five.
6. Your feet should get hotter if you lift. This is an amusing and probably
insignificant effect predicted by the second law of thermodynamics.
7. We can over analyze anything, my post being a prima faciae case for this
statement. The practical test of the energy cost of lifting would be to take
a large group of trained subjects and race them over various distances using
straight legged form but with or without lifting. The proof of the
advantages of lifting would then be clear, I believe. It is rather like the
urban myth that "scientists calculated that honey bees should not be able to
fly." Lifting is an advantage and the fact that we can't theoretically
calculate the exact nature of the advantage doesn't make it go away.
8. We can't ignore physiology and do a biomechanical study alone. There is
an advantage in changing the muscle groups used during long races. A short
form break can aid recovery. Such effects don't appear in a stride to stride
I am intrigued by Ron's theoretical finding that small amounts of lifting
increase energy costs but large amounts decrease costs. If anyone has
several thousand dollars in grant money in a shoe box under their bed,
please send it to me and I'll arrange a full scale test of this hypothesis.
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