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Re: Muscle work



I have some further thoughts and observations regarding the muscle
work and power of typical exercises. But first, let me reiterate the
origin of my concern ...

I do a classroom lab that's designed to give students a decent
intuitive understanding of work and power. In the course of
performing various calisthenics/exercises, students measure muscle
force, displacement, and time, and then they roughly calculate the
muscle work done and the power output.

Often, the muscle motion under examination is repetitive. Push-ups
and pull-ups are particular favorites. I instruct the students to
complete each half-cycle of their exercise at constant speed, if
possible. (And I acknowledge that we are ignoring the work done
during turn-around times.) This means that a student does work
during the downstroke as well as during the upstroke. A key question
is: How much? DOES A STUDENT DO AS MUCH WORK ON THE WAY DOWN AS ON
THE WAY UP?

I have thought about the insights offered on Phys-L, and I have tried
to observe my own "agony level" carefully during a pull-up. Here are
my tentative conclusions:

1) The subjectively measured, constant-speed muscle force that I
apply is the same halfway up and halfway down. And these, in turn,
are the same as the muscle force applied when I'm stationary at the
halfway point.

2) Ascending and descending at the same constant speed is not a
natural exercise motion. The natural tendency is to go slightly
faster on the upstroke! (Of course, if one "free falls" during the
downstroke, all bets are off. But this is not a natural motion,
either, unless one is near exhaustion.)

3) Long before exhaustion, power is probably the key
agony-determining variable; the faster work is done, the more it
"hurts". ("Agony" and "hurt" are not the right words, here; I'm
trying to be cute. But I don't quite know what words do describe the
subjective experience of doing a pull-up.)

4) When the upstroke and downstroke are completed at the same
constant speed, it's remarkable how similar the agony levels are.

5) The turn-around motions at top and bottom are neither
energetically negligible nor equivalent. At the top of a pull-up,
the muscles involved have the least leverage, so the top turn-around
motion requires more muscle force and work than the turn-around at
the bottom. More important, the bottom turn-around involves passing
through the "dead hang" state which requires -- however briefly -- no
muscle force. The top turn-around has no such relaxing moment.

6) Psychologically, the top turn-around is something to be dreaded
much more than the bottom turn-around (for the physical reasons cited
in #5). So the upstroke is subjectively experienced as "bad but
getting worse," while the downstroke is "bad but getting better."

Strangely, I think assertion #6 may offer the best explanation for
why the upstroke is "harder" than the downstroke in constant-speed
cyclic exercises like pull-ups.

- Tucker
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Tucker Hiatt, Director
Wonderfest
P.O. Box 887
(39 Fernhill Avenue)
Ross, CA 94957
hiattu00@usfca.edu
415-577-1126 (voice)
415-454-2535 (fax)
http://www.wonderfest.org

Truth is a great flirt. - Franz Liszt
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