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Energy cost in any mammal is divided up into the energy to sustain life,
energy associated w/digestion,  and the cost of doing work.   The majority
of the energy cost on level ground is to propel the horse.  Lacking any
vertical component even highly trained humans are relatively unaffected byu
an increase in weight.  South African Tim Noakes demonstrated that during
level running humans had no great HR response when they carried an addition
5-10 pounds.  Keep in mind that a human carrying 5-10 pounds is
proportionally similar to a horse carryiing 1-200 #.  However, once you go
up a hill this is not true at all.  Mr. Newton changes things quite a bit

At 5 miles/hour on a 5% grade the work rate between a 100# rider & 1000#
horse is ~3300 kg-m/min and nearly 3700 kg-m/min for a 200# rider.  The
cost per unit total mass will be identical but the total energy cost is
greater for carrying the heavier rider.  10% more work must be done
requiring 10% more energy and giving off 10% more heat.   You can not
escape this.  Manage it yes, escape it no.

No one is saying a horse can't do it.  No one is saying a HW can't win, but
what I am saying after over 20 years in the field is to explain why it is
more difficult physiologically.  These animals are incredible & do
incredible things, no doubt.  They amaze me.  I thrive at reading about all
of you who are my heros riding things like an XP, multiday rides, etc.  We
are still on 25 milers, working slowly, learning to manage my mass.

Try this:

Take 2 people riding: one who weighs on the lighter end of the spectrum one
on the heavier.  Measure your horses heart rate on level ground at a walk
and trot/for a mile each.  Switch horses & do the same.  Do this several
times in random order so that you have collected sufficient data to
estimate HR response to 2 gaits by 2 differnt riders on 2 differnt horses,
but on the same day so that ewnvironment does not cloud your data.  A good
measure would be each horse/rider combo 3 times.  Next repeat this going up
a consistent level grade several times each, each of 2 gaits.  On level
ground the heart rate reponse of the horse will be increased in response to
increase speed (walk to trot), but largely unaffected by the difference in
the mass of the rider.  However, on the hill the heart rate response to
weight will be significantly higher for the heavier person.  It is not the
comparison of one horse against another it is the comparison of the mass of
the rider on any horse.  Why is heart rate higher.  Heat.  The heart is the
water pump that circulates heat to the skin for cooling.

Notice I did not say this bad, that this will cause failure, etc, no value
judgement being made, I am simply stating the biological fact that weight
is a penalty in sustained vertical movement.  Sure this can be managed.  In
fact Lari Shea points out that when she hops off a horse to walk a hill her
horse gets much less of a break than when a HW hops off and walks.  Savvy
people know how to manage the penalty of weight, but they can't escape it.
At least not as long as we have gravity.  Now on the moon.........well
there is another story.

Jon K. Linderman, Ph.D., FACSM
Assistant Professor of Health and Sport Science
University of Dayton
300 College Park
Dayton, OH 45469-1210
Voice:(937) 229-4207
FAX: (937) 229-4244

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