All performances are influenced by air temperature and humidity. When at rest, air temperature impacts resting HR. Resting HR will be lowest between 60-70 degrees F (16-21 C). Colder than 60 degrees F (16 C) and Resting HR will steadily rise by as much as 50% at 44 degrees F (7 C). This is how the body keeps blood pumping and keeps the core warmer. On the flip side, above 70 degrees F (21 C) Resting HR continues to escalate to nearly 300% higher by the time the air temperature reaches 120 degrees F (49 C). So, with this as the starting point, let’s dive in.
Every physical performance depends on a series of chemical reactions in muscls and nerves. These reactions are very sensitive to fluctuations in temperature. Any change in internal body temperature will brign with it consequences. Muscular activity as well as high or low external temperatures will change the body’s internal temperature. In simple terms, when the body’s internal temperature is higher, the processes which occur within the body will be faster; when it is lower, these processes will be slower. During exercise, heat production in the body rises; the harder we are working the more it rises. Heat regulation is therefore taxed and is imperative to keep in control. In one study, subjects sat motionless in a sauna for 15 minutes. The temperature in the sauna was set at 176 degrees F (80 C). During those 15 minutes, the subjects average Resting HR rose from 50 beats per minute to 90.
In high external temperatures and humidiy, the body is more heavily tasked than with high temperatures alone. This is because when there is more moisture in the air, our sweat does not evaporate nearly as quickly or as well, which negatively impacts our body’s ability to cool itself down. The large amount of heat emitted by muscular activity leads to higher internal body temperature which, even to a minimal degree will become a performance-limiting factor. An increase in body temperature from 98.6 degrees F (37 C) to 100.4 degrees F (38 C) will increase HR 10-15 beats.
Perspiration is an important mechanism for releasing heat. As we sweat, fluid loss occurs. During endurance workouts, body temperature may rise to 104 degrees F (40 C) or higher. As we sweat, we lose body weight. Even a 1% decrease in body weight via fluid loss will start to negatively impact performance. Think about this: If you don’t drink during a workout, or if you don’t drink enough, you could lose up to 3% of your body weight during a 1-2hr workout, depending on temperature and exertion. Another study with runners demonstrates this. The group of athletes was split into 2 groups and exercised at 70% of their VO2max. Think of this as toward the high end of your aerobic zone but definitely comfortable. One group didn’t drink at all; the other group consumed 5-8 ounces (150-250ml) of liquid every 15 minutes throughout their exercise. The goal was to exercise at 70% of VO2max until exhaustion.
In the first 45-50 minutes, both groups experienced an equal rise in HR to an average of 145 beats per minute. It was at around 50 minutes that the impact of dehydration started kicking in. In the group that did not drink at all, HR continued to rise steadily to an average of 170 and exhaustion was reached on average at 2 hours. In the group that regularly hydrated, HR plateaued between 145-150 and exhaustion was reached beyond 3 hours.
In another study, test subjects rode a cycling ergometer two times, four days apart, each time for 60 minutes. Temperature and humidity remained consistent one test to the next. Workload was also consistent. In the first test, the athletes did not drink nor did they have any means for cooling themselves off. The HR curve of this group showed a steady increase with an average Max HR of 167 at the end of the 60 minutes. In the second test, the athletes did not drink, either. However, they had a fan blowing on them and cool, wet sponges were administered regularly. The cooling effect showed a steady increase in HR for the first 15 minutes to an average of 140, followed by a plateau for the remaining 45 minutes. In this study, a difference of 27 beats per minute occurred just from keeping the body cooled off. Imaging coupling proper hydration with remaining cooler!
So, there’s hydrating and there is regulating body temperature. The last piece of this puzzle is fueling. The body has enough glycogen stored in the working muscles to last anywhere from 75 to 180 minutes. Typically, count on 90 to 120 minutes before your muscles are tapped out. And, once they are tapped out, you’re done. You will bonk and continuing to push will become first uncomfortable and then excruciating. In yet another study, subjects rode a cycling ergometer at 70% of VO2max (again, high-end aerobic effort) for 2 hours on 2 separate occasions. During that time, they either consumed no calories or a total of 800 calories (200g) as 400 (100g) per hour. This was a carbohydrate solution. During the test when they consumed carbohydrates, they experienced a 7% improvement in performance. Had the test been longer, say 3-4 hours, the improvement would have been even greater.
The punchline? It is imperative to both drink and fuel enough during workouts. Both will allow your body to better regulate its furnace so HR will remain lower and output will remain higher, resulting in markedly better performances. On top of this, if you can find ways to cool yourself off by dousing yourself with cool water, you will further reduce your core temperature, lower your HR and allow for greater performances. So, how much to drink and fuel? The rule of thumb I provide my athletes is 16-24 ounces (0.5 to 0.7 liters) of liquid and 250-350 calories (60 to 90g) per hour. You can stretch it to about 28 ounces (0.8 liters), but not beyond this. You will never be able to fully replace all your sweat or all the calories you burn. However, these ranges allow for sufficient replenishment so that you can keep your body firing on all cylinders. What you want to avoid is two scenarios: (1) being in a position where you have to ration your liquid; and (2) find yourself under-fueled because you are forgetting to regularly consume.
So, there you have it. Hopefully, you now not only better understand hydration and fueling, but also comprehend precisely why it is critical to stay on top of both during your training and racing. If you have found your performances lacking to-date, becoming stricter about your replenishment habits could yield great improvements. One thing I used to do is set my watch to beep every 10 minutes as a reminder to drink and eat. I could hydrate and fuel more often if I needed it, but every 10 minutes was my maximal gap. If you need a reminder before this becomes a habit, give it a try.