Re: Heart Rate

[Kim <mathew@redhorsetech.com>]

Bob Morris wrote:

> >Truman: To some extent you are correct; <<<One final point at a canter the
> >horse may actually take in more oxygen
> > in its lungs than when it trots.  So it order to supply the oxygen to
> > the muscles at the trot it has to raise it heart rate.>>>
> 
> This intake of oxygen is a function of the stride also. The heart and lungs
> tend to work in rhythm with the stride. (pace is a word with so many
> different meanings is why I did not use it) Trot is a faster stride i.e.
> more movements of a leg per minute and the heart and lungs use that rhythm
> to assist in functioning. Thus a faster heart beat. Canter with fewer
> strides (but longer) elicits a slower heart beat. Remember also, it is not
> just the number of beats that supplies the oxygen to the body but the
> volume of the beat (stroke). \

This is not a *direct* answer to any of these posts, but just adds a few
more ideas to the discussion.  References provided at the end of the
post.

The inhale/exhale cycles of the canter and gallop are determined by
locomotor/respiratory coupling (LRC) relationship (1, 2, 3), unless you
are in a situation where the horse needs to pant for cooling purposes or
is starting from a standstill to a gallop (for a few strides, anyway). 
There are a couple of explanations for this LRC relationship, but in
general, the point given is that the respiratory cycle is coupled with
the stride.  

Since the forelegs of the horse are not attached to the clavicle,
extrinsic muscles attach the forelegs to the rib cage and trunk of the
horse.  So, while the cantering horse completes a stride, the thoratic
cage's shape gets altered in the process. Other ideas also suggest that
the thrust of the horse, compression of the muscles of the trunk, etc.
add to this LRC relationship.  No matter the cause, if you ride your
horse at a canter, notice that the breathing is in sync with the stride
frequency (if the horse is not panting, sprinting from a standstill
(horse may hold his breath), etc.).  The faster the stride cycle is
completed, the faster the rate of breathing.  Fatigued horses may also
use their neck and head as a "lever" of sorts to increase the volume of
air taken in per stride.  Watch a tired horse as he canters --- his head
will drop lower and raise higher during the stride, which according to
Howell (1944) occurs from the horse using brachiocephalus muscles to
help move the offside foreleg.  Combining these muscles with the trunk
muscles already in use, the horse *in theory* should be able to draw in
a greater volume of oxygen during the suspension phase of this gait when
he needs it the most.

At the trot, this LRC relationship does not usually exist since the
diagonals left and right serratus ventralis muscles are contracting
alternately. No one has really found a relationship between respiration
cycling and the trot that I know of.

The amount of oxygen taken in by the horse depends on both tidal volume
and respiratory rate.  The tidal volume seems to have a linear
relationship with with stride length, and the maximal flow rate
increases with the horse's speed (4).  

-----------

On Thu, 24 Sep 1998 16:58:23 -0600 Jerald Thiessen <thiesj@tdbank.ca>
wrote:
> 
> So next question then is if the heart rate is based on the stride and not on the the
> oxygen requirement, then would the horse possibly not be using all the oxygen and
> there for going at  an aerobic burn at the trot ,while at the same speed at the
> canter the horse may be anaerobic and there for explaining the higher heart rate at
> the trot while maintaining  the ability to pulse down quickly or quicker than if he
> was cantered???
> 

Forgive me if this gets answered differently than what you hoped (I
think the language interpretation function of my weary brain has finally
shut down (uhh, what's English???) ;-)), but as supported by the info.
above, the heart rate is not directly related to the horse's stride, but
instead is related to the ventilation demands of the body which *then*
is fufilled by the mechanical actions of the horse.  The horse needs "X"
amount of oxgen for a particular gait at a particular speed, and his
stride length, air intake volume, and respiration frequency (as well as
other factors) will fill his needs, otherwise, he will work
"anerobically" --- which, BTW, is a debated term to use in itself ;-)

If the horse can meet or exceed his oxygen requirements, he is working
"aerobically".  This can occur at the trot or canter --- it just depends
on whether you can keep him within that optimal range of speed for a
particular gait (regarding oxygen consumption).  In other words, you can
also "go anerobic" at the trot --- just push him at a fast working trot
where the lungs cannot take in enough oxygen to meet the demands of the
slow twitch and conditioned fast twitch, high oxidative muscle fibers of
the horse.  He will have to use more of the fast twitch muscle fibers
that are best suited for anerobic situations --- but now I see that I am
going off in a tangent!  Sorry 'bout that!

In short, you can "go aerobic" or "go anerobic" in either the trot OR
the canter; just depends on the speed the horse works at and the
conditioning level of the involved muscles and circulatory system.

Another cup of coffee and back to work ;-)

Kim (and Lee)

Red Horse Technologies


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REFERENCES:

1.  Attenburrow, DP: Respiratory sounds recorded by radio-stethoscope
from normal horses at exercise.  Equine Vet. J. 10:176-179, 1978.

2.  Bramble, DM and DR Carrier: Running and breathing in mammels.
Science 219:251-256, 1983.

3.  Hornicke, H and R Meixner: Depth and frequency of breathing in
exercising horses.  Proc. Int. Union Physiol. Sci. 13:332, 1977.

4.  Hornicke, H: Respiratory air flow in the horse and its relation to
locomotion.  In Proceedings of the Fourth Annual Meeting of the
Association for Equine Sports Medicine, 1984, pp. 56-57.

5.  Howell, AB: Speed in Animals.  Chigago, University of Chicago Press,
1944.