HEAT, EXERCISE AND CARDIOVASCULAR RESPONSES
This article is ESPECIALLY for all of you hard core, tough and resilient cyclists who proudly proclaim “THE HEAT DOESN’T BOTHER ME.” We just want to make the case that it’s a different statement than “THE HEAT DOESN’T EFFECT ME.”
Interesting – fascinating FACT. Did you know that depending on your size you have between 60,000-80,000 MILES of blood vessels in your body? And your body has an incredible ability to open and close all those vessels depending on a number of “things” it senses locally and systemically.
The term CARDIOVASCULAR refers generally to the heart as the pump and this astounding number of large and microscopic system of blood vessels that delivers everything every cell needs to survive.
For the sake of brevity we’ll consider some basic aspects of CARDIOVASCULAR changes that occur and place significant demands on the heart as well as other systems when cycling in the heat.
At rest and in a cool comfortable temperature your body circulates about 250-500 milliliters of blood to the peripheral circulation and only 20-25 % of your capillary blood vessels are “open”. What happens when you’re pushing the pedals hard in hot weather is astounding. First of all nearly 100% of those 80,000 miles of blood vessels are wide open and the heart is pumping 15-20 times as much blood per minute as it was at rest.
You don’t have to be exercising at all to experience a change in cardiac output in the heat. Even at rest your heart rate goes up a few to several beats a minute to accommodate stable blood pressure as more capillaries dilate in order to shed heat and keep you cooler.
The problem your heart has while riding in the heat is directing all this additional blood flow to two very important parts of your body – your working muscles and to the peripheral circulation so core temp doesn’t rise too fast. Not enough blood to your skin and you’ll over heat very quickly. Not enough to your working muscles and fatigue, cramping, lactate build up and bonking happens pretty fast. In some cases of extreme exercise in the heat 60 % of the cardiac output is directed to the peripheral circulation to facilitate evaporative cooling. That is a huge loss of blood flow to the demands of your working muscles.
The importance of skin temperature cannot be overemphasized. While it’s true that CORE temperature sets a ceiling on the duration of exercise [most mammals simply stop working when core temps hit 104 F.] pacing, endurance, power output and even neural-muscular impulses are driven more by skin temperature. The reason begins first because our skin is our “first responder” in regards to “sensing” the environmental conditions we will be exercising in. Ever walk out from a cool room into the heat to ride and immediately start “recalculating” the distance and pace you had in mind when it was cool inside? That’s because as soon as your skin feels the heat it’s sending messages to the brain – “NOT SO FAST and NOT SO FAR” today. I’d like to live to see tomorrow.
One of the most significant benefits of heat acclimation is a great capacity to sweat and to begin sweating earlier when exercising in high temperatures. The key advantage is the ability to maintain a lower skin temperature sooner thus lowering the amount of blood flow to the skin. Net benefit more blood to the working muscles and other organ systems which we’ll discuss in a later post.
Remembering the foundational principle of thermoregulation from a previous post, “The harder your body works to cool itself, the faster you fatigue” it becomes clearer why acclimation helps your body stay cool more efficiently.
It’s exactly this principle behind Spruzza as “A Cooler Way to Ride.” Not only does the application of 3-5 mL of water facilitate skin cooling but it’s using an external source of water [not your own sweat] to accomplish this. This is a key reason why in our field study we documented a lower level of dehydration when using Spruzza to stay cooler.
Another key observation and effect of spraying 3-5 mL of water over your face is that normally your sweat rate is only sufficient enough to drop your skin temp by about 5-8 degrees below core temperature but adding additional, external water increases the evaporative cooling capacity such that your skin temp will drop 10-20 degrees below core temperature. The result in not only amazingly refreshing it increases the rate of heat exchange by a factor of two to four fold.
The important conclusion in all this is that even if “the heat doesn’t bother you” it does have a physiological effect on performance, endurance, dehydration and recovery. Several major organ systems are effected as we will look into in later posts.
So no matter how tough you are, think about riding SMART, lowering the work your body does to cool itself. You’ll feel better, ride stronger and recovery faster. Who doesn’t want that?