Wow! All this talking is sure gettin' you guys hot and bothered! :)
From: "malmo" <[EMAIL PROTECTED]> Reply-To: "malmo" <[EMAIL PROTECTED]> To: "'Arthur Snoke'" <[EMAIL PROTECTED]>, <[EMAIL PROTECTED]> Subject: RE: t-and-f:the bigger they are, the harder they cool Date: Thu, 26 Aug 2004 14:37:36 -0400
Bingo!
You're balancing heat production, which is proportional to body mass,
against heat dissipation, which is proportional to surface area. Since mass
(volume) increases with the cube of the radius, and surface area only
increases with the square of the radius, it should be clear that weight (and
heat production) will increase faster than the heat dissipation properties
afforded by surface area.
malmo
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_ui ds=11211124&dopt=Abstract
Advantages of smaller body mass during distance running in warm, humid environments.
Marino FE, Mbambo Z, Kortekaas E, Wilson G, Lambert MI, Noakes TD, Dennis SC.
Human Movement Studies Unit, Charles Sturt University, Bathurst, NSW, Australia. [EMAIL PROTECTED]
The purpose of this study was to examine the extent to which lighter runners
might be more advantaged than larger, heavier runners during prolonged
running in warm humid conditions. Sixteen highly trained runners with a
range of body masses (55-90 kg) ran on a motorised treadmill on three
separate occasions at 15, 25 or 35 degrees C, 60% relative humidity and 15
km x h(-1) wind speed. The protocol consisted of a 30-min run at 70% peak
treadmill running speed (sub-max) followed by a self-paced 8-km performance
run. At the end of the submax and 8-km run, rectal temperature was higher at
35 degrees C (39.5+/-0.4 degrees C, P<0.05) compared with 15 degrees C
(38.6+/-0.4 degrees C) and 25 degrees C (39.1+/-0.4 degrees C) conditions.
Time to complete the 8-km run at 35 degrees C was 30.4+/-2.9 min (P<0.05)
compared with 27.0+/-1.5 min at 15 degrees C and 27.4+/-1.5 min at 25
degrees C. Heat storage determined from rectal and mean skin temperatures
was positively correlated with body mass (r=0.74, P<0.0008) at 35 degrees C
but only moderately correlated at 25 degrees C (r=0.50, P<0.04), whereas no
correlation was evident at 15 degrees C. Potential evaporation estimated
from sweat rates was positively associated with body mass (r=0.71, P<0.002)
at 35 degrees C. In addition, the decreased rate of heat production and mean
running speed during the 8-km performance run were significantly correlated
with body mass (r=-0.61, P<0.02 and r=-0.77, P<0.0004, respectively). It is
concluded that, compared to heavier runners, those with a lower body mass
have a distinct thermal advantage when running in conditions in which
heat-dissipation mechanisms are at their limit. Lighter runners produce and
store less heat at the same running speed; hence they can run faster or
further before reaching a limiting rectal temperature.
PMID: 11211124 [PubMed - indexed for MEDLINE]
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Arthur Snoke Sent: Thursday, August 26, 2004 12:19 PM To: [EMAIL PROTECTED] Subject: RE: t-and-f:the bigger they are, the harder they cool
Yes, the mechanical work expended is proportional to the weight, so a heavier runner expends more energy to run up a hill than does a lighter runner. But that does not explain why the same runner, running at the same pace on the same course (and with the same air resiatance and friction) will suffer more if he/she is heavier when the temperature is 90F compared to 40F.
One reason is that weight is proportional to volume while heat loss is more
related to the surface area. Although body shape does not exactly scale
with weight, it is probably reasonable to say that in general heavier people
have relatively less surface area than lighter people so they cannot
dissipate heat as well. As the temperature increases, neat loss by both
conduction and by radiation is less efficient, then, for heavier people so
they are more likely to overheat.
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