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.