Re: Muscle work

["Paul O. Johnson" <pojhome@SWBELL.NET>]

Your muscle force is directed upward during both halves of the cycle. But
your displacement is up during the first half and down during the second
half. Therefore, the work performed by your muscles (force x displacement)
is positive during the first half and negative during the second half. So
the total work done BY your muscles is zero.

Paul O. Johnson

----- Original Message -----
From: "Tucker Hiatt" <hiattu00@USFCA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Saturday, July 21, 2001 1:05 PM
Subject: Muscle work


> When I (barely) do a pull-up, my arm and shoulder muscles do plenty
> of work.  I'd like to understand why the constant-speed "upstroke"
> seems harder to perform than the constant-speed "downstroke".
>
> The only forces at work on my body are gravity and a complex,
> collective force that I hope I can characterize as "muscle force".
> The TOTAL work done by BOTH forces acting on my body during one
> complete cycle must be zero since my change in KE is zero.  Since
> gravity is a conservative force, the total back-and-forth work that
> it does must be zero.  That means that my muscles must, collectively,
> also do work that totals to zero.
>
> 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?
>
> When I try to pay close attention to the agony level during both
> motions, I am surprised how similar they are when I truly manage to
> descend at constant speed.  But still there is a difference.  Why?
>
> I appreciate that we may need to think about how individual muscle
> fibers do work.  But, even then, it seems that we have a minor
> mystery, here.
>
> ... Any help?
>
> Thanks, in advance.
>
> - Tucker
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