Heat and heat conditioning
|June 28, 2013
|Posted by Melinda under Vet & Sports Medicine
Please someone explain to me why I can write posts for this blog day after day after day for a paltry $5/month (that’s what I make on Google ads), but I can’t even think of a good subject, let along actually START my Ride and Tie Vet Student scholarship worth $1,000.
In the comments yesterday Crysta (author of the blog Go Diego Go) brought up heat conditioning, which was a tangental point to the discussion of multidays versus doing the mileage in one chunk and the effect of hot afternoons versus rest.
As it’s no less than 105 degrees here until we get a break of temps in the 90’s for fourth of July, I think now is an excellent time to talk about heat and heat conditioning.
The heat of the afternoon, even with appropriate heat conditioning, takes the toll on a horse and rider (which is why I think a one day 100 in the summer may be easier physiologically than the same mileage over 36 hours – see previous post on the subject).
The cumulative heat load that builds up over a hot afternoon has to be dissipated whether or not the horse and rider is conditioned for the heat or not. The heat conditioned team can do so “better” and has less of a risk of ending up in distress……but energy and physiological resources still have to be “spent” to keep the body cool.
There is a neutral operating temperature range for all species where no extra energy has to be spent to maintain body temperature (shivering, sweating etc.). The animal’s metabolic rate within this temperature zone is steady and the same as the resting metabolic rate – ie no extra energy expended. Above and below this temperature range the metabolic rate rises as various physiological mechanisms “kick in” to either cool or warm the animal. I think it’s natural to think about the calories or energy expended to keep ourselves warm when we fall below the thermal neutral zone (TNZ), however metabolic rate and energy expended increases when we go above the TNZ too!
***It is important to keep in mind that this “neutral zone” is in a NON-exercising animal.
A naked human TNZ is 28-30 degrees C, while a clothed one 22-25 degrees C. One resource I have states the TNZ for humans as 33-35 degrees C (but whether this with clothing or in the birthday suit I’m not sure?).
For those of you (like me) that are a bit C–>F challenged:
22*C = 71.6*F
25*C = 77*F
28*C = 82.4*F
30*C = 86*F
33*C = 91.4*F
35*C = 95*F
For the sake of this discussion I’m going to assume that we are all wearing clothing and let’s call the human TNZ where we can maintain body temperature without expending energy as 80*F.
The equine TNZ is much lower than humans. Depending on the time of year (thus coat) and body condition, most of my sources peg the equine TNZ as 30-50*F for a horse in temperate climates.
In both the human and the horse the TNZ drops when you add exercising…….The physiologic structures that are propelling the horse down the trail and the rider up and down in the saddle, such as muscles, are generating heat. The environment has to be that much colder to compensate for the increase in body heat, OR some sort of physiologic mechanism needs to kick in to actively disperse that extra heat.
Depending on the coolness of the ride, the rider *might* be in their TNZ (if the ride is 60*F, that might be the right temperature for an exercising endurance rider to stay within the thermal neutral zone).
However, it can safely be assumed that when doing endurance rides, the horse is absolutely expending energy to stay cool and maintain a normal core temperature. If you consider that at least for those of us in California’s central valley, we are regularly doing summer rides where the temps are in the mid 90’s or higher in the afternoon – that is a LOT of energy expended by the horse to cooling mechanisms during the ride.
No wonder I feel beat and the horse looks tired after a hot ride!
Some of the mechanisms a horse employs as they compete in temperatures above their thermal neutral zone are:
-blood vessel dilation
-increased respiration – accounts for about 25% of heat dissipation in the horse
-increased heart rate
-increased blood flow to skin (which transports heat from body core to the skin, which it unloads into the environment – a process greatly accelerated by animals with a sweating mechanism), and sweating (as an interesting side note…….Did you know that camels sweat? And that marsupials and rodents don’t sweat but moisten their bodies by salivating and licking themselves?????????) In a concept that comes up over and over in biology, as the body size of an animal increases, the relative amount of surface area decreases………which makes horses relatively inefficient at dissipating large amounts of heat through the skin….which is also the single most important way for a horse to dissipate heat!!!!! Evaporation through sweating accounts for about 65% of heat dissipation in the horse.
– As FYI points since these aren’t strictly a physiologic changes, but they are all ways to dissipate heat….wind is your friend 🙂 Have a hot horse that you have sponged and scooped? But the air is just sitting there and not helping you out and now you have a rather wet, coolish horse but want to get cooler? Generate some wind! And put your horse in the shade. And let him drink water. And periodic short walks to help muscles pull heat out of deeper tissues.
All of these mechanisms “cost” something in physiological currency. You don’t get anything free in this world……….In summary: If you are at an endurance ride, your horse is spending energy to keep cool. The hotter the ride or the time of day, the bigger that energy expenditure is. This is a biological fact that has nothing to do with how much “heat conditioning” you have. This is the cost of staying alive and moving forward in the heat.
So, if hot weather is going to cost my horse energy no matter what, what IS the point of heat conditioning?
Heat conditioning allows you (and your horse) to actually function and perform in hot conditions instead of dying. In some cases heat conditioning will reduce the physiologic “cost” of the cooling mechanisms, and in other cases it will just make them more available/effective/active.
How can you heat condition or function in the heat better?
General conditioning – just having muscles that are more fit will impact “heat conditioning”. Fit muscles generate less heat to achieve the perform the same level of “work”. Less heat generated is less heat that needs to be dissipated. Conditioning also expands capillary beds which improves the flow of blood to the skin and muscles which will make the horse more efficient at dumping the heat outside the body to the environment.
Live somewhere hot. This sounds contrite, but it’s true. I’ve grown up my entire life in an area where triple digits for weeks on end is not abnormal. I’ve never had air conditioning, worked outside, and don’t notice hot temps unless it’s a really high humidity (50+) or it’s over 115*F or so.
Wear long sleeves and sweat shirts all the time. Especially if you are working inside with an airconditioner.
Drive with the windows up and no air conditioning. (If Tess is with me I have to make an exception – in that case I’m in a heavy jacket while I drive).
Don’t get sunburnt – I find that how much heat I can handle for how long takes a dramatic downward turn if I allow myself to get sunburnt.
Walk in the shade, run in the sun. This actually works. Try it!
Exercise in the middle of the day. I do all my runs and rides at noon until the weather starts to hit triple digits. Since I’ve started doing this, I haven’t had any problems with the heat.
My horse wears a fly blanket. I don’t think that it makes her any hotter than without it (she’s not any more sweaty under it). But if it does? *shrug* I consider that a fringe benefit.
Exercise in clothes that make you sweat – live in the bay area and running in the afternoon only gives you a high of 75*F? Where a sweat shirt and pants.
Run high intensity intervals- this is how even in cooler weather I raise my core temp and practice living with sweat dripping down my armpits and having my respiration up really high.
Don’t clip for training…..and then +/- clip for competition if necessary
Get younger: From a article in the Equine Veterinary Journal Supplement: “Ageing compromises the ability to handle the combined demand of exercise and thermoregulation in part due to decreased absolute pre-exercise PV.” Or in plain english, as you age your plasma volume (PV) decreases and the decrease in PV is to blame for why old horses reached a core temperature of 40*C faster during exercise and had a greater HR when they reached this temperature.
How long will it take to “heat condition”?
TheHorse.com says that most horses will need at least 3 weeks in a warmer climate to allow their bodies to adapt.
As this post is taking WAY more time than it should (HOURS!!!!) I’m not going to try and find any more sources on this. Three weeks feels about right. Two-three weeks is about how long it takes me acclimate to really hot weather and be active in it. 2-3 weeks is about how long it takes for a physiologic system to do some major shifting in metabolic pathways, gene upregulation etc. 2-3 weeks is how long it takes for vegetables not to taste bitter to me after I cut out all refined sugar in my diet. 2-3 weeks seems to be the magic number for a body to adapt, so that sounds entirely reasonable to me………
Although, after reviewing this post, maybe this wasn’t a complete waste of my time? Maybe I could rework this subject as my Ride n Tie Scholarship submission?
References: here is a partial list of my references – I originally had them within the post, but it started to get a bit messy.