OCR presents unique challenges – that is part of why we choose it. In its extremes, thermoregulation in adverse environments ranks right up there with “mud” and “grip strength” as key challenges that a course can provide. OCR is the one place where we are asked to stay cool enough to run in a wetsuit yet warm enough to continually climb in and out of cold water, sometimes through the night. Addressed here are the definitions relevant to hypothermia in OCR, prevention and treatment.

In conducting research I realized that my sources, such as Wilderness Medical Society Expert Panel on Accidental Hypothermia (Zafren et al. 2014), journals such as Aviation, Space and Environmental Medicine or High Altitude Medicine and Biology had one thing in common; their patients were not deliberately climbing in and out of cold water for over 24 hours. Their patients were entirely accidental. Assessing and treating a patient that has been submerged in frigid, fresh water because they fell through the ice is inherently different than the conscious OCR athlete who is more interested in miles completed than core body temperature. However, I believe the two overlap completely and the more proactively you manage your thermoregulation the better your performance.

The human body has the amazing ability to maintain core temperature within its comfortable range of 37 degrees Celcius +/- 0.5 degrees or 98.6 +/-0.9 degrees Fahrenheit. When it’s hot, we sweat and use the cooling effect of evaporation and when it is cold, we shiver. The purpose of shivering is to generate body heat through activation of the muscles. Short term shivering as a response to rapid cooling of the skin (think ice water in July) is not hypothermia, instead it is “cold stress”. In cold stress there is no change in core body temperature. Even shivering people may not have experienced an actual change in core body temperature. “Many alert, shivering patients who are well nourished and not exhausted are not hypothermic” (Zafren et al. 2014). Since the techniques to measure core body temperature are quite invasive, expensive and unrealistic in the field your medical team is going to rely on their clinical judgement to assess your condition. Over a series of events you may notice a pattern of key medical staff or possibly the reverse is true, where a young EMT is looking at your shivering self as their first dose of our kind of crazy.

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In contrast to cold stress, the term hypothermia means an actual, measurable decrease in core body temperature and it is defined as mild, moderate, or severe. “As your brain cools down your personality and communication can quickly change. You may be irritable, confused, apathetic, lethargic, have poor decision making and appear obtunded” (Zafren et al. 2014). While the traits of irritability and fatigue are pretty normal in prolonged activity, it’s the confusion you show and the potentially lousy decisions you may make that actually scare your medical team.

Hypothermia classifications: 

Mild/ HT I—clear consciousness with shivering: 35°C to 32°C or 95 – 89.6 F

Moderate/ HT II—impaired consciousness without shivering: 32°C to 28°C or 89.6 – 82.4 F

Severe/ HT III—unconscious: 28°C to 24°C or 82.4 F (Zafren et al, 2014).

A key difference between HT I and HT II is the end of shivering. The most likely reason an OCR athlete would stop shivering is an improvement in environmental stressors and an increase in core body temperature. However, there is the possibility that the reason they have stopped shivering is that they are actually worse. Shivering is generating heat through the movement of the muscles. If the body has run out of fuel to the extent that it no longer has the energy to sustain that effort you will stop shivering and that is one indicator that you have transitioned into a much more serious hypothermia.

It is our collective insistence to climb in and out of cold water that puts us at the biggest risk for hypothermia because cold water cools the body up to 25 times faster than the same air temperature. In cold water immersion your body heat is essentially “stolen” by the cold water around you through a complicated physics equation that I won’t attempt to tackle. Just know, if you are going to hit the wall in OCR it is most likely going to be in or immediately after a water obstacle.

“Hitting the wall” (also called “bonking”) means you have overspent your energy bank account and have maxed out your energy stores. You have done this by over exercising and under consumption of nutrition. Glycogen is stored in small amounts in the muscles but mostly in the liver. First your body uses the energy stored in muscles then moves on to the liver. When blood sugar levels indicate the need, the pancreas messages the liver to unload its glycogen stores as blood sugar. This is great, and it is usually substantial to carry us through. However, in prolonged OCR, the balance of energy expenditure and energy intake can be enough to level you. When you hit the wall (and I have, it’s awful) the sensation is that your legs are attached but you can’t make them move. You could have all the drive of ten horses and you cannot will the body to move forward. Your body is smart, and no way it is going to allow you to compromise the energy to continue life’s basic functions so in a jam, your body will knock you down. Flat. Until you have taken in adequate nutrition and earned your body’s approval to engage in activity, flat is where you will remain. I have seen expert endurance athletes disqualify themselves when this energy balance is wrecked.

There are three ways to prevent this calamity; layering, movement and nutrition. Every stitch on your body should be made up of a synthetic or natural wool fiber. As a whitewater guide we referred to cotton as “the death fabric” because of the way it will pull your body heat, seemingly, to dry itself. Start with polypropylene bra, underwear and socks. Follow with any synthetic compression gear. There are varying thicknesses, own a variety. Follow with wetsuit (or drysuit!) layer and finally a wind barrier. Not trying to sound like a mom (although I am a mom) but your head, feet and hands will appreciate their own synthetic layer. Probably my favorite piece of gear I own is the 1mm dive hood that I can pull on before entering the water and slide off where it stays around my neck when I am back on land and warmer. Too often I see the velcro swim caps laying unloved along the course route because removing them just creates the opportunity to lose them.

Using movement to prevent hypothermia is a little tricky because not moving would slow the rate of energy loss. However, as any veteran of a 24 hour OCR endurance event can tell you, the pit is both your best asset and your riskiest move. When you have spent the last two miles freezing and stumbling towards the Reese’s peanut butter cups that await you it is hard to walk away when you are finally reunited. But my question is, how would your performance be affected if you had the candy *with you* two miles ago?

In regards to nutrition the best advice is probably going to be gained 1) from your own experience and what you can tolerate at that moment and 2) from experts, including registered dieticians. Bring a lot of variety and give yourself options in the pit: salty, sweat, chewy, crunchy, liquid. If you don’t like it, don’t bring it. Dr. Charles Hill is the medical director for the Leadville 100 mile race in Colorado and on YouTube you can find his hilarious twelve minute mandatory safety briefing the night before the event. To paraphrase he says “I don’t care what kind of paleo, low carb, tree hugging shenanigans got you here, from this point forward you need to eat mostly sugar. Simple sugars that are easily converted to energy for your muscles and brain. You can forget everything else”.

People’s aversions to carrying food on course run the gamut from weight, destruction of the item, concerns about trying to eat it with dirty hands, lack of garbage cans and the fear of a bowel movement. However, in the wilderness, eating food ~ specifically foods with a significant percentage of their calories from fat ~ is like putting on a layer of clothing. If you have ever been cold weather or snow camping you know that the power move for a good night’s sleep is two tablespoons of peanut butter at bedtime. While that is not possible on course just know that the calorie content in a waffle -type snack is upwards of 50% from fat calories and packs perfectly against your pecs inside your wetsuit. The gels and chews are a quick 20-30 mg of easily processed sugars and fit nicely in the wrist of a wetsuit. Ladies, a sleeve of blocks fits nicely between the girls, just don’t try and put it back half eaten and expect good results. Best to eat a few blocks yourself and give a few away.

If your projected time back to your bucket of goodies is X consider multiplying that times 2-3 to account for a rolled ankle, a hurt friend, or the unexpected. If you don’t eat what you brought out on course it is pretty much guaranteed that you will find an athlete that would happily accept the treat. The most important thing you can do for yourself in cold weather OCR is expect the unexpected. Have the layers and the calories on hand to adjust to your changing environment. What you think your body will do, and what it will actually do on race day may be two very different things.

Field treatment of hypothermia:

1) Protect from further cooling: In the pit throw a blanket or swim jacket over your shoulders. If it’s raining, spend your pit time under a pop up. It may seem small, but added up little interventions make a big difference. Also, protecting from further cooling means all sides, don’t even think about sitting on the ground without a barrier between you and the earth. A thin camping mat or foam mat work well and are light in your luggage.

2) Remove wet clothing: Not relevant. Better to add a layer than take anything off. If you are in a bad way and your event accommodates it you can remove your wet clothes and recharge as needed. Instead of feeling bad about it, know that act of smart resting may be what allows you to finish the event, even if it means donning a freezing wetsuit at sunrise.

3) Passive rewarming: Calorie replacement and protect from heat loss. Eat something! Anything you think you can handle. Seriously.

4) Active re-warming: An athlete that requires active rewarming has transitioned into moderate hypothermia, has a core body temperature of sub 90 degrees and their care will take place in the med tent or the nearest hospital. They are utterly and completely debilitated. They need warm IV fluids, heat packs and warm air convection. (Zafren et al. 2014).


Zafren, K., Giesbrecht, G., Danzi, D., Brugger, H., Sagalyn, E., Weiss, E…Grisomm, C. (2014). Wilderness Medical Society Practice Guidelines for the Out-of-Hospital Evaluation and Treatment of Accidental Hypothermia. Wilderness and Environmental Medicine, 25 (4), 425-445. https://doi.org/10.1016/j.wem.2014.09.002

This article is provided as information only. It does not replace the counsel and advice of your physician. The OCR Report takes no responsibility for your personal safety.

While you are out on the course, DO. NOT. GET. HURT.


    • Totally happy to do it! I learn with everything I write. Sorry I did not see your comment until just now. Hope your WTM and any other cold adventures have been great.


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