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Up and Down Hills

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  • Up and Down Hills

    I would like to make one comment and raise one question with regard tothe
    biomechanical paradox fostered by Ian Stokes, further discussed by
    Hein Daanen and George Havenith, with a poorly thoughtout response by
    myself in between.

    The comment addresses the variability of metabolic rates, both in
    terms of the data presented and in terms of the energy expended by a
    single person, especially in descent. I used Astrand and Rodahl
    (Textbook of Work Physiology, McGraw-Hill, 1977) for comparison data.
    I found the following for an average (70-75 kg) person:

    grade velocity metabolic rate
    level 0% 1.39 m/s 349 W
    level 0 1.94 559
    level 0 2.5 768
    ascent 5 1.25 436
    ascent 15 1.25 698

    Which compare to the data given in the two other postings:

    grade velocity metabolic rate
    ascent 10% 1.4 m/s 1165 W
    ascent 25 0.67 636
    descent 10 1.4 235
    descent 25 1.33 352

    For the ascent, the variability does not seem to be simply related to
    grade and velocity. We would need to look at all of the data in the
    two sets previously described in order to adequately compare them.

    The major point I wish to make here is about descent. Energy
    expenditure is decreased by as much as 25% when going downhill
    compared to level walking, except on steep grades at low speeds where
    energy consumption may be higher than on level surfaces (Astrand and
    Rodahl). The energy expended by the body in descent is greater for
    slower velocities because of the energy needed to eccentrically
    contract the muscles in order to slow the body down and counteract
    gravity. Faster speeds are harder on the joints but require less
    energy. The comment by Hein Daanen and George Havenith that the
    results are very dependent on the example is thus very important.

    The question I have in simple terms is this: If a rock is dropped from
    the edge of a cliff, where does the potential energy it contains
    initially, end up after a relatively inelastic collison with the
    ground? Is part of this energy transferred to the ground (other than
    frictional losses due to shear) or is it all dissipated as heat
    through the rock? If it is, in part, transferred to the ground, then
    it is also true for the hiker who uses less eccentric muscle activity
    in his descent and part of his initial potential energy is transferred
    to the ground with each of his many collisions with the ground.

    Bryan Buchholz