The dog days of summer, as they say, are upon us. As an endurance athlete, you should have a basic understanding of how your body responds to the stress of heat, what you can do to better acclimatize to it, and stay safe while doing it.
How Heat Affects Your Body and Performance
Heat places a tremendous load on your cardiovascular system as you try and keep your body temperature in a safe zone. There are two key things to remember, of which you likely already know:
1. Oxygen and nutrients must be delivered in adequate amounts to sustain exercise levels.
2. Your body must maintain a stable temperature and sweating is your body's way of keeping cool.
Keeping these two things in mind, anything that disrupts this delicate balance will affect your performance.
When you exercise, chemicals (hormones and neurotransmitters) are released restricting blood flow to inactive tissues in the body (digestion, reproduction, etc) while increasing blood flow to active muscles. This is a good thing (see # 1 above). When your core or skin temperature increases enough to require cooling, some blood is directed to the skin to promote sweating. This is also good (see #2 above).
Under cooler temperatures, less blood flows to the skin since there is less of a need to sweat allowing more blood to flow freely to the active muscles. During hotter conditions, the skin and active muscle must compete for blood in an effort to maintain the exercise level and stabilize body temperature. As a result, the cardiovascular system must compensate by beating harder and faster to circulate enough blood to sustain your activity. Until adaptation occurs, this negatively impacts your performance. The moral of the story: get adapted to the heat or you'll be in for a long day of racing/training.
Controlling the Heat: Where does it come from?
Understanding where the heat originates will help you in your attempts to manage your body temperature. There are two primary sources for endurance athletes.
1. Metabolism-A good estimate is that your body will convert 25% of metabolic reactions to usable energy. The remaining amount is given off as heat which raises your core temperature.
2. Radiation-Gaining heat through electromagnetic waves is the next method in which you gain heat. Human skin absorbs about 97% of radiant energy which obviously raises your skin temperature.
If both the skin and core temperature are elevated, from the sun and metabolism for example, performance significantly declines.
When you consider the two primary forms of heat gain (and where they occur), you should be asking how you can minimize this effect? A few suggestions:
Adaptations to the Heat
The main way to acclimate to the heat is through exposure. As a result of exposure, your body will create more plasma to blunt the competing effects of skin and muscle blood flow. You will also begin to sweat more, sweat sooner, and more evenly over the entire body. This helps keep temperature low early in the exercise session, a proactive approach instead of a reactive approach to keeping cool. Of course, the major take away from this adaptation is that you need to drink more fluids than you normally would since you are now loosing more than in the unadapted state.
To achieve this adaptation, exposure to mid-day like heat can be incorporated once a week. The heat and humidity levels for morning and evening workouts are usually enough to drive adaptation.
Staying on Top of Your Fluid Intake
Hydration is key to adaptation and staying safe. The best way to monitor your fluid intake is to weigh yourself before and after your workouts. You simply need to replace the amount of fluid you lost during your exercise session. While this process often is followed with calculating a sweat rate, the accuracy of sweat rate really depends on several factors such as the intensity, humidity, type of workout, etc. A better way is to make sure you drink what you've lost.
Scott Flynn, owner, coach and triathlete of 10+ years with Threshold Multisport Coaching, holds a MS in Exercise Science and multiple nationally recognized fitness certifications (CES, CSCS). For more about Threshold coaching packages click here.