Re: positive and negative work

[Larry Cartwright <exit60@CABLESPEED.COM>]

Larry Smith wrote:
> A student asked to day "If I don't do any work holding a heavy object, why
> do I get tired after a while?"  Is John's answer something I should pass
> on?  What's the chemistry of getting tired?

The short answer I got from a biologist friend is that muscles tire from
effort (i.e. exerting force) regardless of whether there is motion or
not (i.e. whether or not macroscopic work is done).  This is exacerbated
by the tendency for muscles to work in opposing pairs for stability's
sake.

Even when a muscle is macroscopically motionless, there is work being
done on a microscopic scale within the muscles as individual muscle
cells twitch on and off.  This becomes more pronounced and visible as
"tiredness" grows.

The chemistry of "tiredness" is complex.  It can set in as a result of a
local or systemwide shortage of blood sugar, shortage of blood oxygen,
or a shortage or excess of various elements and compounds.  For example,
a shortage of calcium can cause spasming and cramping within a stressed
muscle.  A lot of muscle "tiredness" and soreness is apparently caused
by reaction byproducts such as carbon dioxide and lactates which
accumulate in a muscle faster than the circulation can carry them away.

Best wishes,

Larry

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Larry Cartwright                    Retired Physics Teacher
<exit60@cablespeed.com>              Charlotte MI 48813 USA
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Using Cues & Clues:  Aoccdrnig to a rscheearer at an
Elingsh uinervtisy, the oredr of the ltteers in a wrod
deson't gretaly mttaer as lnog as frist and lsat ltteers
are in the rghit pclae.  As you can see, spimle txet can
sitll be raed eevn wehn the oedrr of ohetr leettrs has been
canghed.  Polpee geranelly raed wrods and phasres as whloes,
not jsut as agerrannemts of lteters.