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Re: POSITIVE work in cyclic motion

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  • Re: POSITIVE work in cyclic motion

    Hi netters,

    Much has been said about the work done by a swimmer tethered at the
    waist swimming _stationary_ in a pool. In this case I would like
    to echo Paolo de Leva's comments regarding the principle of work.

    > ... the concepts of work ... in ... application to the study of
    > human motion is so complex that deserves either:
    > a) a lot of study and a humble, doubtful attitude.
    > or
    > b) a lot of study, tons of effective thinking, and a good mind.

    For practical reasons, I would suggest that it is safer to stick to
    (a) :-)

    As for the _study_ here are some useful references.

    1. Knuttgen HG. Force, work, power and exercise. Medicine and
    Science in Sports (and Exercise), 10(3):227-228, 1978.

    Here the basic distinction is drawn that work has a particular
    definition which is often confused with exercise or "effort".

    Work = force x displacement.

    Displacement is a vector quantity therefore in cyclic movements the
    work should be zero. (Note the _should be_ :-)

    The mechanical definition of the term work differs from the common
    usage of the term work; for example to "work-out" in the gym.
    If the weight-lifter returns the weights to the rack where he found
    them, he (should have done) done zero work. He may have performed
    many hours of exercise, but has (probably) done zero work. It is
    perhaps unwise to insist too loudly/repeatedly that this is only a
    "small semantic technicality", particularly within earshot of a
    steriod-filled weight-lifter. (This is where the humble attitude
    comes into it :-).

    > Peter Davidson wrote:
    > > ...Regarding your question on the amount of work done
    > > during tethered swimming, I believe the answer is zero.

    > Paulo de Leva wrote:
    > I totally disgaree. It is true that the work done BY THE SWIMMER
    > ON THE WEIGHT is zero, because the weight does not move. However,
    > there's a lot of positive work done BY THE SWIMMER ON THE WATER.

    > Peter wrote:
    > > But the swimmer moves the water, so is not the swimmer doing work
    > > against the water? To determine this work, you would need to know
    > > the flow rate (volume (mass) and velocity)

    This brings in the second point of confusion about work -- it is time
    independant. Power is work x time. Again from the reference
    material, it is better to refer to power when time is involved. By
    introducing time (flowrate, velocity, instantaneous dislacements...)
    we have digressed, and are no longer discussing work, but power.

    The distinction needs to be borne in mind. Taken to its logical
    conclusion imagine the swimmer swimming so slowly that the water is
    hardly disturbed. The water that is pushed aside returns to fill
    the gap. The _water_ has not displaced but individual molecules
    have. You therefore you have to consider whether the swimmer is doing
    work on the homogeneous _water_ or on the water molecules! The work
    done on the _water_ is zero because it has not displaced (except for
    that splashed out of the pool, of course ;-). However, this IS
    measureable as a displacement of the weight.

    The ultimate end point of any work is motion between perfectly
    frictionless movements between molecules, which has another
    name - - HEAT. Heat seems to come into it quite a bit (resulting in
    heated debates, hot and sweaty exercises, people getting hot under
    the collar, etc etc..) therefore let's look at the thermodynaic
    definition of work as contained in the article:-

    2. Webb P, Saris WHM, Schoffelen PFM, Van Ingen Schenua GJ,
    Ten Hoor F. The mechanical work of walking: a calorimetric
    study. Medicine and Science in Sports and Exercise, 20(4):331-
    337, 1988.

    Work = the energy transferred from a system NOT in the form of heat.

    Now things get complicated because we cannot easily determine what
    portion of the energy content of any given system ultimately
    downgrades to heat (the term ultimately is safely used here because
    we do know that work in independant of time). We also do
    not know how much energy is stored in the system as body
    fat, muscle glucogen etc. For this purpose, a thorough energy
    balance here is essential; if one term is neglected or erroneously
    estimated, then the whole calculation may be invalidated. (This is
    where the tons of thinking and a good mind become necessary!)

    I will pass on that one, but refer to another reference which
    contains as complete an energy balance as I have seen anywhere.

    3. Ward-Smith AJ. A mathematical theory of running, based on the
    first law of thermodynamics, and its application to world-class
    athletes. Journal of Biomechanics, 18(5):337-349, 1985.

    The measurement of these quanties can be estimated or measured in a
    whole-room calorimeter over a period of days (which allows the
    energies to reach equilbrium).

    The interesting conclusion of study [2] is that work IS done
    during walking, but it is not done during cycling (please excuse any
    misinterpretations here, as I am working (sic) from memory).
    These results imply that not all the energy released from the human
    body reappears as heat while walking; but all the energy reappears as
    heat during cycling. Work can be seen as the mechanical transfer
    of work across a boundary; it probably will dissipate as heat
    within the receiving body, but may not :-).

    At this stage, based on all this evidence -- and a great deal more
    that I have not discussed -- I would hazard a guess that the work
    done during swimming is also zero.

    I base this assessment (in very, very, very, very broad terms on
    the fact that cycling consists of cyclic motions as does swimming,
    whereas walking is an oscillating movement :-). This conclusion is
    based entirely on speculation -- and the wise reader should treat it
    with the utmost sceptism... however I believe it to be correct.

    If you want to test this assumption on a swimmer, you would have to
    dress them in a thermal wetsuit and do the whole body calorimeter
    experiment. The emersion of the body in water is so well suited
    (sic) to a calorimetry experiment, that it would not surprise me if
    it has not been done already.

    Craig Nevin
    Biomedical Engineer
    Department of Physiology/Sports Science
    University of Cape Town, South Africa