Dehydration Makes That Effort Feel Harder

And it's not just because of the physiological responses inside your body. Dehydration gets inside your head.

Photo: Hannah Dewitt

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Each month, Dr. Jeffrey Sankoff looks at a recent study or body of research to talk to the researchers, explain the process behind it, and break down the findings.

This month: A meta-analysis looks at studies done on dehydration and perceived effort to answer the questions of why RPE (rate of perceived effort) increases—beyond physiological responses—as dehydration increases during exercise. 

Training and racing for triathlon can often be a challenge. First, there is the physical aspect of getting the body to adapt to the increasing load needed to get faster and go further. Then, there is the mental skills and fortitude needed to push the body through those increasingly difficult workouts. This latter aspect of training is often under appreciated. Many athletes believe that in order to become a better swimmer, biker, or runner they just need to train more—but few consider that in order to be able to train with higher quality, they have to first leverage the ability of their brain to resist the urge to tell their muscles to stop.

It has long been understood that how we perceive effort has a lot to do with how long we will be able to sustain that effort. Before we had reliable metrics like power meters or heart rate monitors, the rate of perceived exertion (RPE) was the most widely used method for athletes to give feedback to coaches on how hard they were training and for coaches to prescribe workout intensities. (And RPE still has immense value in training and racing—even with all the data tools available.)

Developed by Gunnar Borg, RPE has been extensively validated to be universally applicable across sports, age groups, gender, and populations to a reliable and reproducible means for assessing or prescribing exercise intensity. One of the most important things to recognize about RPE though is that because it is entirely subjective, it can mean very different things to different people.

For example, one athlete may report an RPE of 8/10 when cycling at a given percentage of their FTP and a different athlete may report an RPE of 6/10 for exactly the same effort. The difference between those two athletes is in how they perceive their effort—and that perception relates entirely to their brains.

The ability to perceive effort differently is frequently cited as the reason why beginners in endurance sport don’t stay with it. If a new athlete perceives the workouts as being too difficult, they are less likely to keep going. Similarly it is also proposed as a reason for why some experienced athletes have success when others may not. An athlete who can push harder at the end of a race because she perceives the effort as being easier than others will see better results than one who perceives that same effort as much more difficult.

There are a number of physiological and psychological stressors that can impact RPE, so it follows that if those stressors can be kept to a minimum athletes can improve their ability to perform. Along with fatigue, life stress, fueling, and temperature, one of the physiological factors that has been hypothesized to impact RPE is hydration status.

Hydration has been shown to have a role in affecting physical performance by virtue of compromising cardiovascular and thermoregulatory systems. With increasing levels of dehydration, heart rate must increase to maintain oxygen and nutrient delivery, and core temperature rises because of an impaired ability to offload thermal energy via normal processes that require water.

It is also being suggested that independent of these physical reactions, dehydration additionally has an impact on the psychological perception of effort and that this can manifest in decreased performance as well—ie., your dehydration makes the effort feel and seem harder.

A recent study published in the Journal of Exercise Science and Fitness collated the results of 16 studies that have evaluated this question in effort to answer several questions: 1) Does RPE change in response to exercise-induced dehydration? 2) What is the magnitude of the change as dehydration increases? 3) Is the magnitude of the change important? 4) Are there any identifiable factors that may moderate the relationship between dehydration and RPE? and 5) How is cardiovascular strain caused by dehydration related to RPE?

This kind of meta-analysis is always impacted by the size and quality of the studies that it pools together, and in this case the studies were quite small and the quality of the experiments in each was variable. A total of 147 athletes were included, of whom only two were women, and the vast majority of the studies employed cycling as the exercise to be studied. Still, despite these important limitations some broad conclusions were made that I believe are important to consider.

RELATED: How Sweat and Hydration Needs Differ for Men and Women

First and foremost, the researchers showed that RPE did indeed increase for the same level of effort when athletes were dehydrated. However, the reported increase was quite small until dehydration level reached 3%. Only at that point did the difference in RPE become greater than one point (on the scale of 10) for the same level of effort.

The authors argued that this difference is likely important in certain contexts. For example, for recreational athletes who are getting started in endurance sport, an athlete who perceives an effort as even slightly more difficult is less likely to to keep persisting with the activity than if they perceived it to be less hard. Alternatively, for a competitive athlete needing to exert themselves in order to obtain a personal best, that difference in RPE could also be important. They admitted though that this difference is small and no studies have been done to demonstrate that there are any performance differences associated with the observed RPE difference. This remains purely speculative.

Interestingly, when looking at all of the available data, the authors could find no correlation between RPE and exertion specifically related to humidity, exercise duration, exercise intensity, aerobic capacity, or ambient temperature—indicating that, in isolation, none of these explain the difference in RPE seen as dehydration increases. 

There was, however, an association with increased heart rate as both RPE and the level of dehydration increased. This is not surprising as dehydration is known to cause an increase in heart rate. Interestingly though, while heart rate and RPE tracked quite closely, some studies demonstrated that this was not always the case and so heart rate could not be said to be the determining factor for why RPE increased with dehydration, only that it is closely associated.

What then can be taken away from all of this and how can athletes use this information?

As I previously noted, hydration status is very important for maintaining both cardiovascular and thermoregulatory functions. This is particularly true when exercising in warmer environments, when athletes face the double-edged sword of being more prone to dehydration and being more reliant on the proper functioning of both of those systems. Knowing that the perception of effort is also impacted by hydration status, however slightly, only serves to further highlight the importance of prioritizing hydration going into and during an event.

Clearly, on hotter days when you are prone to becoming dehydrated late in the race is also the time when perception of effort is highest as well. By focusing as much as possible on good hydration practices and keeping yourself at or below the 3% dehydration status for as long as possible, you may be able to mitigate this issue and give yourself an advantage over other athletes.

Essentially, by continuously taking in adequate fluids and electrolytes, it may be possible to go harder for longer without perceiving it that way.

RELATED: Are You Doing Thirst Right? The Science Says Probably Not

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