An article was published in the March edition of the Journal of Strength and Conditioning Research entitled, Heat Exposure and Hypohydration Exacerbate Physiological Strain During Load Carrying by Adams, et al. It was the first study performed to specifically assess the impact of load-carrying (in the form of a 45-pound ruck) on how the body responds to conditions of heat and dehydration (aka, hypohydration, or not replacing lost fluids). This post summarizes the research.
It's well-known that either high temperatures or dehydration cause a strain on the body during exercise and it's obvious to most of us that combining the two would make matters even worse. The research performed by Adams, et al. sought to evaluate the individual impacts of high temperatures and dehydration as well as the combined effects of the two when a load (45-pound ruck) is added during exercise vs. no load. (Non-loaded data is from prior experiments). Although we can all reasonably assume that the effects of heat and dehydration would be more profound when a ruck is added, this study provides insight on just how much those factors change with a loaded ruck.
12 moderately-trained people participated in this study and were asked to ruck with a 45-pound load on a treadmill for 90 minutes. The conditions tested were:
Hydrated subjects were allowed to drink to thirst the day before and day of the test and were asked to drink an extra 500mL of water on top of that the night before and morning of the test. During the test, these subjects were given fluids every 15 minutes.
Dehydrated subjects were fluid-restricted for 20-22 hours before the test and were required to perform 60 minutes of exercise the day before the test to ensure that they had lost 1-2% of their body mass due to dehydration prior to testing. During the test these subjects were not given fluids.
Exercise intensity was held constant across each test. All subjects participated in all 4 arms of the test.
Test subjects were evaluated for core temperature, heart rate, mood state and visual vigilance (awareness).
To summarize, dehydration and heat have a greater effect on you when you are rucking than if you are exercising at the same pace without a load. Heat, dehydration and load together create an added negative effect on core body temperature and heart rate. This puts you at greater risk for heat illness, decreased inability to control your core temperature once it starts to elevate and can induce a greater level of suck in terms of perceived exertion and fatigue. This study reenforces the importance of managing hydration before and during your ruck and taking opportunities to cool your body during rucking events.
Heat Exposure and Hypohydration Exacerbate Physiological Strain During Load Carrying
Adams, Elizabeth L.; Casa, Douglas J.; Huggins, Robert A.; DeMartini-Nolan, Julie K.; Stearns, Rebecca L.; Kennedy, Rachel M.; Bosworth, Megan M.; DiStefano, Lindsay J.; Armstrong, Lawrence E.; Maresh, Carl M. LessThe Journal of Strength & Conditioning Research. 33(3):727-735, March 2019.
About the Author
I have completed 38 GORUCK events including 3-GORUCK HCL's, and have done a variety of other endurance events including Ironman Maryland 140.6 two times. I am a Certified Strength and Conditioning Specialist and health coach, scientist, minimalist, nature lover and single mom but on a more basic level, I'm just a woman in her 40's that likes to push her limits and write stuff sometimes. This blog is a way of sharing some of my experiences and insights.