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

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  • Re: BIOMCH-L Digest - 30 Jan 1999 to 31 Jan 1999 (#1999-28)

    > 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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