Compared to jogging in high 60s/low 70s (F) with a slight breeze and partial clouds, jogging in mid-80s (F) no breeze full sun means a worse pace and also much harder struggle to get that worse pace.
When I google online, I get kinda very generic answers: “your body has to work harder to cool” “sweat is less effective” “extra stress on body”, etc but I don’t understand what the actual biomechanics are that make me less capable when I’m working out. Like, OK, if my body has to sweat more, how does that make my muscles less capable of pushing me through a 3-miler? What does the extra stress actually mean? What is actually getting stressed that prevents me from hitting my normal pace?
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edit: first time posting here and this sub does not disappoint, thanks for such quick responses!
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When you use your muscles, your body uses glucose and oxygen to create adenosine triphosphate (ATP), which essentially then breaks down into adenosine, phosphate, and the energy your cells use to create movement. This release of energy is done in two main parts: Kinetic energy, which is motion energy that actually makes you move, and thermal energy, which is heat.
Your body needs to release this thermal energy, or it would otherwise overheat, which causes the breakdown of essential proteins in your brain, heart, and other places. The human body’s main mechanism to get rid of excess heat is sweat. Sweat is mostly water (around 98%), and the rest is sodium, which comes from salt, and broken down fat. This sweat is secreted from glands in your skin, which then evaporates. Simply speaking, when sweat evaporates, it takes heat with it.
In a sedentary state, your body spends about half of your daily caloric intake on regulating its temperature. You spend more energy every day keeping your body in that 36-38 degree Celsius (97-99 degree Fahrenheit) range than any other singular bodily function, including digestion. Sweating and the release of bodily heat, which is part of this so-called thermal regulation, uses a very large amount of energy.
This being said, when it’s cool outside, your body heats up slower during exercise than it does if it’s hot. Therefore, you have to spend less energy sweating, which gives you more energy to activate your muscles and propel yourself forward. Wind accelerates the cooling effect that sweating brings by more rapidly evaporating the sweat off your skin. This is the reason why, for instance, when you’re outside on a hot day and a breeze blows past you, you feel an instant wave of relief from the heat. The wind rapidly evaporated the sweat off you, causing all that heat to get taken away.
Unless you are extremely conditioned, meaning you have trained your body to act and function in certain manners, your body can’t just use unlimited energy to simultaneously cool yourself down and commit to high energy physical activity in high heat. In higher heat, this means that your body redirects energy from your muscles to your autonomic nervous system, which is responsible for making you sweat. This redirection of energy is what makes you feel exhausted more quickly when running in higher temperature — you physically cannot surpass your body’s automatic response to the stress placed upon it by physical exertion.
Elite runners, such as marathon runners or those who run in hot climates, have dedicated significant time and energy towards training their bodies to withstand more extreme environments. This training is the reason why they can run for extended periods, at higher speeds, in higher temperatures. Of course, genetics plays a role, but that’s a discussion for a different time.
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