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Having lifted weight for 15 years, I have a problem with the freefall model.
It does not fit the conditions of the problem given and in practice will
destroy joints due to the jolt caused by stopping the falling weight. Moving
the weights (or whole body) at constant speed on the both positive (up) and
negative (down) at constant speed is considered to be good form.
Why does it feel different? My opinion is that when I am trying to maintain
this ideal of constant speed the positive stroke requires that I transfer my
chemical potential energy into gravitational potential energy (simple view
of the main process). It becomes hard when I deplete the stores of CPE
energy. Failure occurs when little is available. On the neg. stroke (many
consider this to be the most important in muscle gain since the speed of
descent can be lengthened considerably) I must maintain a constant speed,
but I must absorb GPE energy from the weights. The difference is that the
weights will always provide energy on the way down, not so on the way up.
This allows me to complete the negative stroke no matter what - easier
mentally. Some electronic weight machines (the brand name escapes me - Life
something) actually are programmed to take advantage of this and provide
about a 10% increase in load during the negative stroke.
Also-
A pullup is a complex motion from a weightlifters view point. No one set of
muscles is isolated. Different muscles come into play at different points in
the motion. Completing (point just before the max. height) a pullup requires
considerable strain on the lats (muscles under each arm) since this strain
occurs just at the top of the stroke and lats are typically less developed
than biceps and delts, the downstroke will feel much better as the load is
relieved from those quick to fail muscles back to other stronger muscles.
This can be demo-ed for any student who cannot actually do a pullup by using
a lat pull down machine. This machine will also show how changing the
spacing between the hands gripping the bar will dramatically change effort
required to pull down the bar.
HTH
Scott