What a great post, it was nice of you to take so much time to write such
a good article on P&R's, heart rates, etc:->.
I just wanted to add in my .02 that while you're exercising and watching
heart rates, anything below about 158 bpm is considered aerobic---
that is, the body is able to supply sufficient oxygen to maintain
that level of work more or less indefinitely, (until some other form
of fatigue sets in, like you've run out of glycogen). The 158 threshold
is consistent for all horses regardless of their level of fitness, breed,
age or whatever. What will change according to fitness is the SPEED the horse
can travel at 158 bpm---some fit horses can gallop on the flat and never come close
to 158, for some unfit pasture potatoes, 158 bpms may be a slow trot!
Above 158, you're going into the anaerobic levels of exercise, where
the body cannot supply enough oxygen to fuel the manufacture of
energy for the level of work you're doing. The body then supplements
energy production with another system (glycolysis) which is not as efficient,
but does not require oxygen to supply the required energy. However, this
anaerobic system is also where lactic acid accumulates as a by-product.
Lactic acid, of course, contributes greatly to fatigue as the accumulation
disrupts the workings of the muscle cell and produces pain (the "burn").
Therefore, at work levels above 158 bpm, lactic acid will begin to
accumulate, although at a fairly slow rate. As the heart rate gets higher
and higher, the rate of lactic acid accumulation increases until at
a heart rate of around 200, lactic acid is increasing at a very high rate.
So, bottom line, is while you're conditioning or racing, you should try to
keep the heart rate below 158 as much as possible. You can spend some
time in the anaerobic level to either condition the horse to it prior
to a ride, or when climbing over rocks or racing into the finish during
a ride), just remember that you have a very limited amount of time you can
"put the pedal to the metal" before you run out of gas!
Happy trails and good luck. :->
Susan Evans, also Cato, Katy, Lady and Cato
Equine Research Center
California State Polytechnic University, Pomona