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Hein Daanen
10-15-1991, 01:26 AM
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

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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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