Welcome to 8th grade physics! Take a seat and we’ll talk about heat.
In this earlier post, we talked about the problem of hot and bothered athletes. All other forces being equal, nothing slows an athlete down more than the gradual buildup of heat in the body. To avoid overheating (and death!), the body has a variety of ways to dissipate heat during exercise.
And here they are.
Conduction: This is the direct transfer of heat energy between two solid objects. It’s pretty intuitive that putting a cube of ice against your skin makes you feel cold, but for our purposes it’s important to understand why. The ice cube is cold because it has a low thermal energy state. When the energy is removed from those water molecules, they stop moving as quickly and assume the solid state of ice. Transferring thermal energy into it will increase the motion of the molecules and turn it back into water. Putting the ice against your leg creates an imbalance of heat energy between the two objects in contact (you and the ice). The laws of thermodynamics dictate that heat energy will naturally move from a high-energy area to a lower one. Your leg starts to feel cold because all the heat energy at your skin surface suddenly begins rushing into the cube. Once the heat energy of the entire leg-cube system is balanced, your skin will stop feeling cold and you’ll have a lukewarm puddle of water on your leg. This is important to understand because it means conduction is only effective as a thermoregulatory method so long as whatever object you place against your body is substantially cool enough to cause heat to leave your body faster than normal. We’ll discuss what constitutes a substantial rate later.
Convection: This is very similar to conduction, except the heat is transferred to a fluid (in the triathlete’s case, water or air) instead of a solid. Why the distinction? Because a fluid field like the atmosphere or your local pool is so large compared to your body that you can’t change its energy state the way you can with something small like an ice cube. The air and water around you is moving so much that you are also essentially “refreshing” its cooling properties on a constant basis. These are the two major aspects that make convection so effective. You have some conduction by transferring tiny amounts of heat to those air molecules directly in contact with your skin, but more important is the advection component, which is the flow of air sweeping heat off your skin surface like a hurricane blowing debris around.
The key point for athletes is that the motion of the air matters more than its temperature. For those who are curious and do a little digging, you may find sources that seem to contradict this. Here’s why. If you simply look up “convective heat transfer,” you’ll find equations and coefficients that deal with standard conditions, such as walking or running. But athletes on bikes are not working under standard conditions. The airflow around you on a bike is so great that it qualifies as “forced convection,” which means you are literally creating air conditioning for yourself. While the equations work the same, the coefficient of heat transfer is much larger, so you are working with completely different numbers. To see what this means in practical terms, try turning the A/C down in your house someday and riding your trainer at a race pace without a fan on you. You’ll quickly learn the power of cold wind vs. static cold air!
Radiation: Ladies, gentleman—you are positively radiant! We all are, in fact. Heat doesn’t always need to transfer from your body into the molecules of the air or some object in contact with you. Sometimes, it just beams into space all by itself, just like light, which is also a form of energy. The difference is that heat energy beams at a wavelength that isn’t visible to the human eye, known as infrared. You may have heard of infrared (often called “IR”) devices used for night vision by the military. These sensors are uniquely adapted to detecting energy in this wavelength. The amount of heat you radiate depends primarily on the difference between your skin surface temperature and the outside air. All told, the average person emits about 100 watts of heat energy while standing still in room temperature conditions. By comparison, the average athlete generates between 900-1300 watts during exercise.
Transpiration (Evaporation): In other words, sweat. You might think that sweat needs no introduction to you as an athlete. You get hot, you sweat, it evaporates and you cool off. True enough, but there are a couple of subtleties in the process that make all the difference to you as an athlete. First of all, you don’t sweat because your skin gets hot. The physiological response that turns on the waterworks is actually a response to your core temperature. That means your body is building up heat all the way through. This has tremendous implications when you consider just how quickly you can start sweating in some situations like waiting in line for an amusement park ride or walking across a parking lot on an extremely hot day. That’s how quickly you start to cook inside!
The speed with which this happens once again relies heavily on air temperature. If the air around you is warmer than your skin, you no longer get rid of heat energy by way of radiation or convection. Instead, you take on heat from your surroundings! When the other three forms of heat transfer fail, evaporation is all you have left. Sweat beads up on your skin, absorbing heat from your body through conduction. Eventually, the little water droplets take on so much heat that they begin to turn into vapor. When those molecules float off of your skin, they take the heat energy with them. It’s literally our body’s last line of defense against dying in the heat, but anyone who’s ever crossed a finish line drenched in sweat understands how limited this mechanism is. Sweat only helps you when it’s evaporating. Any sweat still on your body isn’t moving heat energy off your body. Once again, the process of evaporation is dictated by the environmental conditions, namely the heat and a new enemy—humidity.
The laws of thermodynamics are harsh, indeed. That’s why the hot races are always the most punishing. Given that the proposition of heat dissipation relies so heavily on mother nature, is there really anything that clothing technology can do about it? Now that we have an understanding of the fundamentals, we can begin to answer that question.
If you’re interested in getting faster, you’ll be fascinated by FASTER: Demystifying the Science of Triathlon Speed. In Faster, astronautical engineer and triathlon journalist Jim Gourley explores the science of triathlon to see what truly makes you faster—and busts the myths and doublespeak that waste your money and slow down your racing. With this knowledge on your side, you can make simple changes that add up to free speed and faster racing.