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unknown user
10-21-1991, 03:00 AM
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
U-Mass/Lowell