Like a lot of fall marathon runners I do most of my training during the summer months. Running in the heat and humidity can have an affect on hydration levels as well as your pace. It is important to understand that it is okay to run slower in the heat.
In my perfect running world I would be able to run at a comfortable 50-55 degrees, with low humidity, and partly cloud skies.
Here are a few reasons why our pace slows down during the heat:
1. Changes in blood flow and sweating.
Our body has the ability to cool itself in hot conditions by sweating. Technically, it is the evaporation of sweat that causes us to cool, not the sweating itself.
In order for sweating and cooling to occur, the body must increase blood flow to the surface of the skin. Blood is the major carrier of heat, and sweat is composed of plasma, which comes from blood. Therefore when we sweat at high rates, we are actually losing blood volume,(as well as electrolytes). This is why proper hydration is crucial.
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. Higher demand on the heart.
During exercise we want our heart to pump as much blood as possible in each beat so our heart rate does not skyrocket to sustain a given workload. However, what happens in the heat is not ideal for our hearts.
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, our 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 mode kicks in.
Muscles engaged in activity suffer because they are not getting as much oxygen from our blood. For endurance athletes (marathon runners), we depend highly on oxygen in our blood because it allows us to be able to run for longer amounts of time. However, with less oxygen in our blood due to the heat we must rely on our anaerobic modes of producing energy. Anaerobic simply means without oxygen. Anaerobic exercise is common in power lifters and sprints (quick, fast movements).
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.
Our heart is working hard during running in the heat, but it is pumping less blood than normal. Therefore our muscles are not getting as much oxygen. These responses play a role in decreasing our VO2 Max (the maximum amount of oxygen that our body can take in and use as fuel for our working muscles).
Because our VO2 Max decreases our bodies are working at a higher percent of VO2 Max at any intensity. In more simple terms, relative stress increases at a given workload because our bodies are less efficient in the heat.
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 our bodies cool off through the evaporation of sweat. The higher the humidity the more saturated the air is with water, making it more difficult to allow our sweat to evaporate.
Here is a helpful chart from Runner Academy. It shoes how pace slows down as the heat increases. The chart is depicting someone who runs an 8-minute mile in idea conditions (50 degrees).
For more information check out this ARTICLE from Runner Academy.