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  • Going Up and Down

    Dear Biomch-l

    The biomechanical paradox issued by Ian Stokes appears to us as a
    thermophysiological one.
    In the quantitative example the speed of descent was twice the climbing
    speed. In this case the heat production of the descent was indeed higher
    than the ascent (610 versus 509 Watts for a 80 kg person). However, as
    noted by Ian Stokes, the convection term was higher in the descent. This
    term is calculated to cause a difference of 83 W for a nude individual and
    only 13 W for a dressed person (ambient temperature: 18 degrees C, skin
    temperature: 33 degrees C, clothing insulation: .75 clo = .116 meter square
    degrees C/W). This is not enough to explain the entire difference in the
    amout of liberated heat, but may be of importance for subjective temperatu-
    re sensation due to local skin cooling. If we also take the difference in
    frictional losses into account, we can conclude for this example that the
    net heat production rate of the ascent about equals the descent with a
    speed twice as high.

    Note: the ascent time is double the descent time, thus the total liberated
    energy during the ascent will be about twice that of the descent.

    As soon as the person stops at the end of the descent the kinetic energy
    gain will be zero. Thus, this factor does not play a role.

    The results above are very dependent on the chosen example. Another
    example, in which the ascent and descent speed are equal, shows the
    essential differences.
    Speed: 5 km/hr = 1.4 m/s. Grade: 10 percent.
    Person: 80 kg.
    Metabolic rate ascent: 1165 W
    Metabolic rate descent: 235 W
    (source: Ergonomics - Determination of metabolic heat production. ISO
    standard 8996)
    Potential energy gain or loss: 194 W
    Heat production ascent: 1165 - 194 = 971 W
    Heat production descent: 235 + 194 = 429 W.
    Which shows differences as expected.


    Hein Daanen and George Havenith

    ================================================== ======================
    Hein A.M. Daanen
    George Havenith Thermophysiology Group
    TNO Institute for Perception Phone: +31 3463 56334
    P.O. Box 23 Fax: +31 3463 54635
    3769 ZG Soesterberg E-mail: hein@izf.tno.nl
    The Netherlands or: uunet!hp4nl.nluug.nl!tnosoes!hein
    ================================================== ======================
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