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View Full Version : Re: BIOMCH-L Digest - 30 Jan 1999 to 31 Jan 1999 (#1999-28)



Thomas A. Stoffregen
01-31-1999, 11:54 PM
> 1. Biomechanics in Space
>
>Date: Sat, 30 Jan 1999 05:37:20 -0800
>From: Jack Sujovolsky
>Subject: Biomechanics in Space
>
>Hello Group...Is anyone aware if any gait studies, including "GRF"
>were conducted in space? I am curious as to the effect of an abscence
>of gravity towards treadmill running...It is one of the exercises
>prescribed in space. I was specifically thinking about the maintenance
>of bone mass, which is triggered by the pounding our body takes...
>
>Jack Sujovolsky, MS

Astronauts have run on treadmills while in orbit (beginning on Skylab, I
believe), and the space agencies surely have reams of data on this.
However, such running does not take place in the "absence of gravity".

A little physics. In orbital flight there is gravity. The force of
gravity is about 98% as strong for objects in low earth orbit as it is for
objects on the surface. It is gravity that sustains orbital flight; if
there were no gravity the spacecraft and its contents would move only in a
straight line; i.e., away from earth into deep space. People and other
objects are weightless in orbit not because of any absence of gravity, but
because there is no resistance to the extant gravitational force. The same
is true in the terrestrial environment; if you take away the resistive
force (usually provided by the ground), then the object becomes weightless.
Consider jumping. Stand and jump straight up. During the period that you
are airborne you are, literally, weightless. The only difference between
this weightless and that found in orbit is that the latter lasts longer.
Note that in a plane or other aircraft the force of gravity is resisted by
aerodynamic lift created by the wings; people in aircraft are not
weightless (except during the upper peak of parabolic flight, which NASA
uses to "simulate" weightlessness).

You cannot run on a treadmill when you are weightless; pressing against the
treadmill with your feet will cause your body to float away. Treadmill
running is possible on orbit only if the body is restrained in such a way
that the treadmill can resist the forces that you apply to it. This is done
with bungee cords, typically. It is important to note that with this
manipulation the runner is no longer weightless; they have weight
proportional to the force applied by the bungees. It seems unlikely that
the bungees apply a force equal to 1 g, and so on-orbit running probably is
characterized by the body weighing less than its terrestrial weight. This
might make for some interesting differences in running. But it is not
correct, physically, mechanically, bio-mechanically, or otherwise, to talk
about "running in weightlessness".

Tom Stoffregen

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