The physiology of exercise and warm-up in horses.

Here are my notes – with my own comments peppered within – from the enjoyable panel at the PNER Conference 2015 put on by Dr. Erica McKenzie BSc, BVMS, PhD., DACVIM, DACVSMR. Yes, that’s a lot of degrees.

This is the kind of stuff I nerd out on regularly anyway, building on my equine science background. I highly recommend my fellow nerds and wannabee nerds dip into the nitty-gritty of horse physiology and nutrition. It prepares our equestrian athletes for whatever we ask them to do, and makes us better horse people.

The long and short of it: horses are better athletes than humans. They have massive lung fuel, which increases the maximum rate of their oxygen use. They have a large heart comparable to body size and have a higher amount of blood hemoglobin (for carrying oxygen to tissues). They have something called a “contractual spleen”, which essentially allows them to squeeze blood out of their spleens and increase their blood volume during exercise. Wild, right? Horses are also efficient at heat loss through sweating. They sweat a lot, and the ability to shed heat can be a limiting factor for some horses.

A recently sweaty Deli, still in her winter woolies.

A horse’s physiology, including their “fragile” stick-legs among other things (their shoulder contraption is a beautiful work of evolutionary engineering), give them a very energy efficient gait. (Especially when compared to me, because my conformation isn’t great.)

However, when it comes to exercise physiology the source of fuel is central to everything else!

Horses have slow digestion compared to other animals. Endurance racing requires a ton of fuel. Dr. McKenzie enlightened us as to one point: a horse can’t have enough fuel for an endurance race just based on recent food intake. They will need to run on stored fuel to be successful.

Muscle, which can be 55% of an athletic horse’s body weight, is the primary storage of glycogen. When glycogen is burned for fuel, waste products like lactate build up and cause muscle fatigue.  Glycogen is primarily used in the absence of oxygen in the muscles – anaerobic situations. Endurance is largely an aerobic sport, where the horse is (ideally) able to intake enough oxygen to not swap to burning primarily glycogen. Instead, these horses will burn fat to fuel their activity (and horses are built for this, so it’s groovy). This is the reason endurance horses are usually fed high-fat low-sugar diets!

One point I found particularly notable in this panel is that to use fat as fuel, a little glycogen still needs to be burned to keep the fat burning process working! So yes, sugars have their uses for an endurance athlete even if fat should be their primary fuel source (to prevent muscle fatigue). It takes horses a long time to replace lost glycogen in the muscles. If you want to read further on this subject, this article on muscle-glycogen replacement in horses was fascinating (and references the more detailed scientific article).

I’m sure you can see how respiration relates directly to where the horse is getting their fuel and a horse’s ability to take in air can be a limiting factor. However, lung volume (unlike heart rate) is non-responsive to training – as in, a horse’s lungs have a cap on how much oxygen they can breathe in. It’s still a HUGE volume of air!

An equine athlete’s fitness level is also is key in how a they burn what fuel they have. The more fit our partners are, the more slowly they go anaerobic and start using up their glycogen stores. Blood volume also increases when you get fit so that you can afford water loss from sweat. And a decrease in the resting heart rate in a fit equine indicates a larger heart that more efficiently pumps blood. Interestingly, max heart rate is not variable and actually decreases as an equine ages.

So what are the practical applications for this information?

First, horses have higher thermal demands than humans – they lose a lot more electrolytes in sweat and their thirst response is not good. Dr. McKenzie recommends you offer water to your partner right when they stop because there may be a small window within which they feel thirsty. This is also why supplementing electrolytes is so common in endurance races.

Second, training and conditioning plays a huge part in improvement. When exercise starts they are always anaerobic to get the whole process started – which is where an easy warmup comes into play. The more fit a horse is the more slowly they use up muscle glycogen stores.

Third, to avoid bonk/fatigue, avoid using up the fuel stored in your partner’s muscle! It’s also a good diet to give them some time to replenish their fuel stores before asking them for another “race” effort.

Last time: what do you pack in your vet bag?
Next time:  Deli’s big rehabilitation update – and saddle fit issues.