This two-part series looks at why heat and humidity affect running performance and how to acclimatize to hot conditions.
Written by: Tawnee Prazak
It’s no secret that exercising in the heat is downright hard. Throw in humidity and it gets even worse. But why? Below are five physiological reasons the body suffers when temperatures are soaring.
1) Changes in blood flow and sweating
The human body has the ability to cool itself in hot conditions by sweating. Actually, it’s the evaporation of sweat that causes cooling, not sweating itself. (This is important with regard to humidity.)
For sweating and cooling to occur, the body must increase blood flow to the surface of the skin. Why blood? Blood is the major carrier of heat, and sweat is composed of plasma, which comes from blood. So when we sweat at high rates, we’re actually losing blood volume, not to mention electrolytes. This is why proper hydration is vital.
Increased skin blood flow is problematic for an endurance athlete because less blood is available to working muscles and vital organs such as the heart, making exercise more difficult.
2) More demand on the heart
Ideally, during exercise we want the heart to pump out as much blood as possible in each beat so that the heart rate doesn’t skyrocket to sustain a given workload. What happens in the heat, however, is less than ideal.
With more blood at the periphery, there is less blood flowing to the heart. This decreases cardiac filling and stroke volume—the amount of blood that is pumped from the heart. To compensate, heart rate increases to sustain the workload. As a result, the relative intensity of exercise increases, more stress is placed on the heart and we max out sooner. In other words, an 8-minute pace may feel like a 6-minute pace because the heart is working that much harder.
Also, if blood volume decreases from high sweat rates (a loss of plasma) there is an increase in blood viscosity—a higher concentration of red blood cells—which puts more stress on the heart and vessels.
3) Working muscles suffer and anaerobic modes kick in
When blood volume is split amongst competing interests during exercise in heat, the next victim is active muscle.
Muscles engaged in activity suffer because they aren’t getting as much oxygen from the blood. For endurance athletes oxygen is gold; it’s the fuel that allows us to sustain exercise for longer durations, and without it we’re forced to rely more on pain-inducing anaerobic (without oxygen) modes of producing energy.
Increased anaerobic energy production affects exercise at all intensities and causes a slew of issues including higher total energy expenditure and blood lactate accumulation. Also, carbohydrates are used for energy more than lipids (fat), and since carbohydrate fuel stores are extremely limited in the body, exhaustion is reached much sooner.
In the end, this shift from aerobic to anaerobic modes will generally result in a faster onset of muscular fatigue.
4) VO2 max decreases
The heart is hard at work but pumping out less blood than usual; the muscles aren’t getting as much oxygen. These responses play a role in decreasing the all-important VO2 max, which is the maximum amount of oxygen that the body can take in and use to fuel working muscles. Again, for endurance athletes, the higher VO2 max, the better; it’s a marker of exercise efficiency.
Because VO2 max decreases, at any exercise intensity you’ll be working at a higher percent of VO2 max. In other words, relative stress increases at a given workload because you’re less efficient.
5) Humidity hinders cooling
Relative humidity is the amount of water in the air compared to the theoretical maximum amount of water in the air and it directly influences sweating and cooling.
Remember that the body cools itself with the evaporation of sweat—not the sweating itself. The more humid it is, the more saturated the air becomes with water, and the harder it becomes to evaporate sweat. With less evaporation of sweat, we don’t cool as well. Plus, that sweat remains on the skin, making it seem like you’re sweating more, but you’re not—that’s the lack of evaporation.
The bottom line? As temperature increases exercise costs more energy and you’ll poop out sooner.
So those are the body’s acute responses to exercise in the heat (the problem). Stay tuned for Part 2 on chronic heat exposure and acclimatization (the solution) in the next edition of Running Hot.
Tawnee Prazak is a certified triathlon coach, exercise science grad student and triathlete. Find out more at www.tawneeprazak.com.