> 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!)
As fitness improves, the anaerobic threshold, in relation to heartrate,
rises.
>> 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 ?
At a certain point, lactate increases with heartrate increases go from a
linear relationship to a parabolic relationship--that is a little more
heartrate begins to produce a lot more lactate. This linear/curvilinear
relationship crosses over at approximately 4 mmol/liter. That's why it's
called the anaerobic threshold.
>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.
Actually, 4 mmol is nothing. Our racing Thoroughbreds, glycogen loaded, will
demonstrate lactates in excess of 35 mmol. All racehorses deliver at least 17
mmol in a short race. Typical lactates post 1-mile race hover around 22 mmol.
Fatigue in these horses is due more to local substrate depletion than high
lactate numbers. In fact, those horses displaying the highest post-race
lactates are invariably the winners.
>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 :->.
Another way to determine the lactic acid threshold is that heartrate
deflection point. That is, as speed increases, heartrate increases in a
linear fashion. However, at the LA threshold, heartrate loses its linear
relationship with speed--because anaerobic metabolism takes on the principal
energy production, oxygen delivery becomes less important, and heartrate no
longer reflects the speed challenge.
>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.
If the slope is linear, the horse has not crossed the LA threshold.
> > 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 ?
Respiration rate is more a reflection of body temp than lactate level. In hot
temperatures, heartrate will recover while respiration rate stays
high--leading to alkalosis, not acidosis.
> > 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.
There is a plateau that the recovering heartrate will hit at about 60-90
seconds after the work has ceased. It is this "recovery" heartrate that can
give you a reasonable estimate of the level of anaerobic work performed by
the horse. Other factors come into play when simply sitting waiting for
heartrate to drop below 100--body heat, for example, and
agitation/discomfort.
>
> 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
i>nstances 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.
Right!
>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.
I would wager that NO endurace horse ever came into a vet check in a
condition of acidosis. You have to be very careful about this, because if you
treat a horse for acidosis and he's actually alkalotic, you'll kill him.
I see a problem here. You guys are worried about something you need not worry
about. Your fatigue, whenever you encounter it, is not coming from lactic
acidosis. It's coming from fuel depletion--and, in some cases, alkalosis.
> 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).
More recent work, the German AESM proceedings, indicate that recovery
heartrate is a very good indicator of lactic acid production during anaerobic
work.
>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.
Again, anaerobic threshold is not a consideration in an event like this--and
in fact, crossing that threshold with bursts of near-maximal effort may be
beneficial. The problems are going to be muscle substrate depletion and body
heat. Eventually, alkalosis (too much oxygen due to rapid breathing).
ti