Re: Muscle work

[William Beaty <billb@ESKIMO.COM>]

On Sat, 21 Jul 2001, Tucker Hiatt wrote:

> Let's suppose that I am able to raise and lower myself at constant
> speed.  This requires that that initial and final moments of pull-up
> acceleration be negligibly small -- a reasonable assumption, I hope.
> Then, don't my muscles do precisely the same (magnitude of) work
> during the upstroke as during the downstroke?

If your muscles do positive work on the first half of a pull-up, and also
perform POSITIVE work on the downstroke, then you'd expect it would feel
the same in both directions, and also expect it to give 2x net work, not
zero net work.

On the other hand, if you store potential energy when lifting yourself,
and then your muscles recover it all while descending, then your muscles
should feel EQUALLY GOOD on the way back down, equal to the badness they
felt on the way up, and with good + bad summing to zero.

But in reality, muscles don't recover much energy when doing negative
work. Otherwise you'd be able to wind up a human being as if it was a
spring-wound toy, like the way sunlight can "wind up" a plant, un-burning
the water and CO2 to make carbohydrates.  (Hey, a weird science fiction
idea:  mechanically driven chemical reactions and plants that can have
evolved to feed themselves with little windmills.)

Now hold a large book at arm's length without moving it.  You're doing
zero work, so why is it painful, and why do your muscles tire?




I've always imagined muscles to be like a grinder wheel which is held
against a long vertical rod.  To keep the rod lifted to a certain
position, the grinder wheel produces huge amounts of frictional heating
which have little do to with the work done on the rod.  This "muscle"
mostly performs work upon itself, not upon the outside world, mostly
creating a temperature rise.  Ease up on the grinder wheel to let the rod
fall again, and the recovered energy is insignificant when compared to the
heating produced as the grinder wheel applies a force to the rod.

I wonder, do muscles recover ANY energy when doing negative work?  If your
muscles could act like "programmable rubber bands", then whenever you
tightened them, the spring-like character could store some energy if
muscles were later stretched by an external force, and then they would
release that energy when the external force was lessened.  Ideally you
could inject a bunch of work to perform your first pull-up violently to
fling yourself up to the desired height.  Afterwards, your tightened
muscles would act like springs, allowing you to effortlessly perform many
pull-ups as your body oscillated up and down like a spring/mass system.
Maybe it's easier to do pull-ups if you tighten your muscles and let
yourself "bounce" at the lower excursion, which would convert your
downwards velocity into upwards velocity without your muscles needing to
do as much work upon your body.


((((((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))))
William J. Beaty                                  SCIENCE HOBBYIST website
billb@eskimo.com                                  http://www.amasci.com
EE/programmer/sci-exhibits          science projects, tesla, weird science
Seattle, WA   206-789-0775          freenrg-L taoshum-L vortex-L webhead-L