Re: Heart Rate

[Kim <mathew@redhorsetech.com>]

On Wed, 23 Sep 1998 12:04:02 -0600, Jerald Thiessen <thiesj@tdbank.ca>
wrote:

> I was having a discussion the other day on a training ride with a friend
> and we started talking about stress on a horses legs and cardiovascular
> systems. I  believe that trotting puts more stress on the legs but was
> easier on the cardiovascular system.My friend disagreed with me and
> raised a very good point. If trotting is easier on the cardiovascular
> system then why  does the heart rate go up when I pull the horse from a
> canter(120bpm) to  a trot (140bpm) and vice a versus when trotting if
> we go to a canter the heart rate drops.

-=-=-=-=-=-=-=-=-=-=-=-=-=-=

Every gait has a point, or speed, which delivers the most work for the
least amount of oxygen (which would determine cardiac output).  Each
gait has a certain speed which uses the least amount of oxygen compared
to other speeds at that -same- gait.  You could push the horse to a
faster speed at that gait, but he would require more oxygen to maintain
that speed.  And, on the other end of the spectrum, you could slow a
horse down at the same gait, and he would require more oxygen because he
is not working efficiently --- he would require more oxygen to move 1
meter down the trail.

So, each gait has it's own "optimal" speed, which requires the least
amount of oxygen for the horse.  Endurance riders will find this speed
after time --- this is the pace your horse tends to "settle" into during
a consistant-speed conditioning ride.  

Now, if you compare the oxygen requirements at a trot to a canter AT THE
SAME SPEED, one of these gaits is going to be very inefficent in oxygen
use.  This depends on the particular horse and the speed which you are
comparing.  Seems like in Jerald's example, the horse was moving faster
than the "optimal" point for his trot, which increased his heart rate
compared to the next step up, which was the canter.  Since the canter
had a lower heart rate at the same speed, this speed was better suited
for this gait (in terms of oxygen needs).  Horses, when left to their
own devices, will automatically select the optimal gait for a particular
speed --- it's just more comfortable for them.  But, humans ask them to
collect a canter to a very slow speed, or preform an extended trot while
showing, etc. where both are inefficient uses of oxygen for the horse. 
But, you usually won't find this occurring often with endurance horses
for a long period of time ;-)

Regarding forces applied by both gaits:

The trot and the canter do not have a *huge* difference in the overall
amount of force, but what is interesting is how individual legs receive
this force.  Since the trot is *just about* symmetrical (two legs
contact the ground at once, with just a split second difference), the
horse does not experience a huge force "spike" when the initial hoof
hits the ground. The second hoof takes up a good portion of that force
as soon as it hits as well.    

The canter, on the other hand, does have an inital "spike" when the
horse moves from suspension (in the air) to when the hind hoof hits the
ground.  The timing between the first and second hoof contacting the
ground is greater.  Also, measurements have shown that while the
non-lead foreleg of the cantering horse recieves more force compared to
the trot, the lead foreleg actually sees a reduction of about 20%.

The gallop is a whole different story, and can generate a whole lot of
force --- I'll stay out of that one since it usually is not the choice
of gait for endurance riders :)

If anyone needs the research references, just let me know.  Hope it
helps!

Kim (and the ground-poundin' Lee)