Obviously, there are lots of different factors that can create
or contribute to anaerobic conditions---weight the horse is carrying,
terrain, fitness, skill of the rider, respiratory or cardiac conditions
(like pneumonia or other diseases), speed, even how well the tack fits
(if a poorly fitting saddle impairs blood flow to a muscle, it may not
receive sufficient oxygen, right?). As you've seen, there's alot of
controversy about the correlation between heart rate and anaerobic
threshold and whether or not it's affected by fitness of the horse (I'm
still trying to dig up the research, guys!)
Why exactly 4 mmol/L, what happens
> there ? What happens with the lactat level i.g. when heart rate rises ?
> Does it goes up slowly or jumping when the "threshold" is reached ?
>
>Equine exercise physiologists sort of arbitrarily call a lactic acid concentration of 4 mmol the "anaerobic threshold". 4 mmol was kinda
just settled on and agreed "this is it" just so everyone was talking
about the same thing. However, 4 mmol of lactic acid in one athlete
may be bettered tolerated than in another athlete, so it should be
looked at as more of a general label, not a hard and fast rule of
exactly where the anaerobic threshold is. This is my own just
speculation why they chose 4 mmol, but I would guess from reading the
research (lots and lots and lots of it) that they chose 4 mmol based on
roughly where some effect of lactates begins to be seen in the muscle
cell and begin to affect its metabolism. It's really important to
understand that lactates are a normal part of glycolytic energy
production that are recycled back into glucose for more energy
production later. However, sometimes in instances of high intensity
exercise when insufficient oxygen is being delivered, more LA is
produced than the muscle cell can get rid of by passive diffusion
(that's fancy-shmancy concept for just something moving from an area of
high concentration (the muscle cell) to an area of low concentration
(the blood passing by in an adjoining capillary), like a tea bag in a
cup of hot water diffuses the tea from the bag to the surrounding
water.)
In that case, LA begins to build up within the muscle cell. Because
lactates are, after all, an acid, the pH within the muscle begins to
drop (becomes more acidic). The muscle cell enzymes and "workings" are
pretty specific about the pH range in which they're willing to work, and
if the pH gets too acid, they just slow down and eventually stop doing
their thing, ie energy production. The burn you feel during exercise is
that LA building up in the muscle. If muscle cell metabolism is totally
disrupted, muscle contraction simply stops because the energy production
to drive it has stopped. Anyway, I suspect "4 mmol" is just the point
at which the muscle cell BEGINS to be affected by LA buildup, though
this is a long ways away from total disruption levels. But, I may be
wrong, so I'd welcome any input from some other exercise physiologists
or biochemists :->.
Lactates accumulate according to a curve, they don't "jump" per se like
an on/off switch. The slope of the curve depends on how hard the horse
is working and how far into anaerobic he is. An endurance horse just
barely over his anaerobic threshold would show a slope that is very long
and gradual. A horse that goes from a standstill to a fast run and
stays there as long as he can (like during a Thoroughbred race) is going
to show a slope that is extremely steep and is going to reach extremely
high levels well into the 20's. Actually, just for trivias sakes, the
highest lactates recorded are in SEA TURTLES, who go for extremely long
periods underwater without breathing and can tolerate lactic acid
concentrations of over 100 mmol. (That's not a typo. 100.) So much for
cocktail party trivia.
>
> Because the rider is typically unable to measure the lactat level during
> the ride, a usefull clue seems to be RESIRATION of the horse during the
> run and after break off the workout, isn't it? When respiration is low,
> and heart rate returns to normal quickly, there could be no oxygen
> deficit, therefore NO anaerobic work (even when the heart rate was 180
> recently), or am I wrong ?
Well, he might have still been somewhat into anaerobic but just paid off
his "oxygen debt" quickly, which isn't unusual in a fit horse.
Remember, using the anaerobic system isn't an on/off switch. If you
have a credit line of $10,000, but you only need $100 to pay your bills,
do you borrow $100 or $10,000? You only borrow what you need, because
it's too expensive to borrow the entire credit line and you don't need
the whole thing. The anaerobic system is like that---you may not need
much to pay the energy bill, but you're still operating above the
anaerobic threshold. If a horse is fit, it's not going to take him long
to pay back his "debt".
> When the horse is fast and heavily breathing and heart rate stays long
> time over 100 after stop workout, this appears like oxygen deficit
> and the horse had worked within the ANAEROBIC zone, even when the
> heartrate was not exceeding 160 while running.
>
> Yes, probably the horse was anaerobic, although there are other reasons
why a heart rate might stay elevated other than repaying an oxygen debt,
like excitement, pain, dehydration, colic, etc. There are also
instances in which a heart rate may drop, respiration remain elevated
and there still be no oxygen debt---ie, when the horse is overheated and
using respiration to remove body heat, or often in panters that just
breathe that way, just to drive their owners and the ride vets nuts.
However, let's assume a normal, non-dehydrated, non-bleeding,
relatively calm horse that spent a fair amount of time working in
anaerobic ranges, wherever that may be for that particular horse. Now
you've pulled into a rest stop and he's blowing like a blue whale.
Without getting into a long and boring dissertation on organic
chemistry, which would qualify as Torture as far as I'm concerned,
horses that have been worked well above their anaerobic threshold are
not just sucking in oxygen, they're blowing off a much larger amount of
carbon dioxide than they do at aerobic levels, which is in part one of
the ways they compensate for the increased acid levels produced by
anaerobic exercise. A horse that blows for a long time after exercise
is doing so to re-pay the oxygen debt he has incurred during exercise,
but ALSO to get rid of the metabolic by-products of that exercise.
Blowing off increased amounts of carbon dioxide affects the acid-base
balance within the blood and compensates for the lactates accumulated
during exercise. Ever breathed (while at rest) real hard and fast for a
few seconds until you've hyperventilated and made yourself giddy? The
dizziness is because you've shifted the acid-base balance within your
blood. If your system was already a bit (or even alot) acid because
you'd exercised, breathing hard doesn't make you dizzy, because the
acidity of exercise and the basicity of breathing fast is balancing each
other out.
The HR is also going to continue to stay elevated after anaerobic
exercise to circulate blood to continue to deliver oxygen and help clear
away by-products, although it's important to remember there is a very
poor correlation between HR and lactic acid levels (this is from Rose's
1988 work on max oxygen uptake, O2 debt and deficit and muscle
metabolites in Thoroughbred horses, if anyone's interested).
Actually, one way to truly measure the exact anaerobic threshold in an
individual is by collecting and measuring the gases an exercising horse
is inhaling in and exhaling, which can obviously only be done under
laboratory conditions. During aerobic exercise, a horse breathes in
more oxygen than he is breathing out carbon dioxide. If you divide the
volume of carbon dioxide breathed out by the volume of oxygen breathed
in (Volume CO2/Volume O2), the ratio of the two is less than 1, probably
closer to .70 while at rest (that is for every 1 liter of oxygen the
horse breathed in, he breathed out .70 liters of CO2). This is based on
the type of fuels, mostly fatty acids, being burned during aerobic
exercise and at rest. As the anaerobic threshold is approached, more
and more glucose as fuel is being burned to supply the increasing
demand for energy. A glucose molecule has more carbon in it than fatty
acid and as a result, more carbon dioxide is blown off as a by-product.
The ratio of CO2 expired to oxygen inspired (which, BTW, is called the
Respiratory Quotient, or RQ) will increase until it exceeds 1---that is,
for every one liter of oxygen inspired, the horse is blowing off MORE
than one liter of CO2. I know, the chemistry doesn't look like it
would work out, but it does. The point at which the RQ reaches 1 IS
ABSOLUTELY the anaerobic threshold. Unfortunately, it can't be measured
anywhere but in a laboratory on a high speed treadmill with a whole
bunch of gas exchange equipment that breaks down alot depending on how
badly you needed it that day.
Which brings us back to trying to figure out where anaerobic threshold
is if you don't happen to be riding your fifty mile endurance ride on a
treadmill. I'm still looking up the articles, and will post it when I
do, but hoped this helped a bit in the meantime.
BTW, I apologize if some of the explanation sounds like Romper Room
Riders. I don't mean to sound over-simplified like I think you're all a
bunch of idiots (because I don't), it's just that some riders have a
better understanding of all this than others, and it's probably the
newbies that could use the info more, anyway. There's nothing worse
than trying to follow the finer nuances when you don't quite get the
basics yet---I know, because I've been there! (And still am, on a
regular basis, but aren't we all?) :->
Susan Evans
Equine Resrach Center
Cal Poly University