Thermoregulation and Muscle Performance

Craig: On my 60th birthday, I did a thousand pushups.

Brent: But how does the heat extraction actually occur?

Craig: That was our amazing discovery.

Brent: It feels a little bit like a no cost steroid. I can tell you the outcome is better.

Brent: Welcome to Death Clock. I'm your host, Brent. Friends, today we speak with Doctor Craig Heller about thermoregulation and performance. Doctor Heller is a professor of biology at Stanford, and he has spent his entire career studying thermoregulation, circadian rhythms, and human performance. What thermoregulation means in this context is the cooling or heating of the hands, feet, and face—the hairless surfaces on the human body—and the impact that has on performance.

Brent: It turns out that if you properly cool the hands, you'll be able to lift more weights and run further. Your performance will improve fairly dramatically—much more than if you were taking steroids. While steroids will improve all of those things, they're also really bad for you. The techniques that Doctor Heller describes, however, are not bad for you.

Brent: They don't have the negative side effects of something like taking a steroid. He's a wonderful guest who spent his entire career studying this topic. Personally, I'm going to go out and try some of what he talks about and report back. He's a great guest, and I hope you enjoy.

Brent: Doctor Craig Heller, welcome to the show.

Craig: Thank you. Good to be here.

Brent: Today we're going to be talking about the response of the human body to changes in temperature—thermoregulation, I think, would be the right way to say it. But before we jump into all that, give us a sense of your background.

Craig: My background is mostly in physiology and neurobiology. I work in a couple of different areas. One is the neurobiology of sleep and circadian rhythms, specifically the roles they play in learning and memory, where I focus on Down syndrome. The other half of my life is spent in human thermal physiology, especially as it relates to human performance.

Craig: So we have made some critical discoveries that have dramatic effects on people who are conditioning, training, and competing. Right now, we're continuing to investigate our initial discoveries and ideas to improve the product.

Brent: How far back does this go? It seems like, at least in the longevity community, the popularity of saunas and cold plunges is relatively recent. I don't know if we've been looking at the impact of the body being warmer or cooler on performance for longer, say over the last five years. Where does the study of all this start?

Craig: Well, it's always important to distinguish between skin or peripheral temperature, which is our major source of thermal sensation, and deep core temperature, which has a dramatic effect on the physiology of all our body organs. With respect to deep core temperature, we literally live on the cutting edge of life and death. We regulate our body temperature at 37°C (98.6°F), and going a few degrees above that creates dramatic problems.

Craig: Going too many degrees above results in heat stroke and thermal death. Similarly, if we go a couple degrees below the optimal temperature, we lose the ability to think and function, which can lead to fatal consequences. I like to say that we live on the thermal edge of life and death, so it's important to understand all the factors that contribute to our core body temperature.

Craig: Our skin temperature gives us information about what we're experiencing. But frequently, like in a cold plunge, it's totally separated from the temperature of the body core.

Brent: When you're thinking about temperature as it relates to performance—say for an athlete—how do you view the difference? Is it as simple as not wanting to mess with core temperature and focusing on skin temperature, or do we adjust core temperature slightly within its tight spectrum?

Craig: Well, we alter core temperature to bring it back to normal. The major discovery we made years ago is that muscle fatigue and failure are largely due to a rise in muscle temperature. We have developed ways of extracting that heat generated by intense exercise out of the muscle.

Craig: If we do that, the muscle keeps working. For example, I've had freshman women do over 800 pushups. On my 60th birthday, I did a thousand pushups.

Brent: What are you actually doing to the muscle? The muscle heats up, and that's where fatigue happens. Say I hit 60 pushups and can't do any more. Now you help control the heat of that muscle, allowing me to do 500 or a thousand. What exactly are you doing?

Craig: The muscle has a failsafe mechanism. When a muscle works, it produces an enormous amount of heat, and the only way that heat exits the muscle is through blood flow. During exercise, heat production in the muscle can increase 50- to 60-fold, whereas blood flow cannot match that increase.

Craig: As a result, heavy anaerobic workouts can cause very high muscle temperatures, leading to muscle fiber death. This condition is called rhabdomyolysis, which is very serious for many athletes. We aren't always aware of this danger because we have a tremendous capacity to push through and tell ourselves to work just a little harder.

Craig: Pushing through can actually be the exact wrong thing to do. This failsafe mechanism shuts the muscle off when it reaches dangerous temperatures. That switch essentially halts the production of ATP—adenosine triphosphate—which is the muscle's source of energy to contract. Without ATP, there is no energy.

Craig: You can't do one more rep or pull-up. If you extract that heat, the enzymes inactivated by temperature reactivate, feeding fuel back into the muscle. Doing this repeatedly over time increases your total work volume.

Craig: Increasing work volume gives you a conditioning effect. We've seen people—including professional athletes—dramatically increase their capacity for a particular activity through conditioning with heat extraction.

Brent: How does the heat extraction actually occur? While doing a thousand pushups, are you wearing something or have you ingested something?

Craig: That was our amazing discovery, and it came quite by accident. An anesthesiologist friend of mine who knew I worked on animal thermoregulation, hibernation, and bears said, "You think you know so much about temperature regulation?"

Craig: "I bet you couldn't fix a problem we have in the recovery room: patients come out of surgery hypothermic, and it takes nurses hours to get them to stop shivering." This is a very difficult problem because skin is a very good insulator. If blood stops flowing to the skin, you might warm the skin surface, but that heat doesn't reach the body core.

Craig: So we had a crazy idea: place an arm in a negative pressure, partial-vacuum environment to pull blood into it. We could then heat that arm, and the warm blood would return to heat the body core. It worked beautifully.

Craig: Unbelievably, the first patient returned to normal temperature in under ten minutes with no shivering at all. We couldn't understand how we transferred so much heat through one arm. It turned out to be even more interesting: it had nothing to do with the arm itself, only the hand. We then dug into old literature on the circulatory system of limbs, hands, and arms.

Craig: We found that non-hairy skin—the palms of the hands, soles of the feet, and upper face—contains special blood vessels that shunt blood directly from arteries to veins, bypassing capillaries. When these shunts are open, large volumes of blood flow from the arteries into extensive venous networks in the skin.

Craig: That's where we lose heat. You can tell someone's thermal status simply by shaking their hand.

Brent: So in the case of pushups, you are cooling the hand?

Craig: Yes. In anesthesia recovery, we were applying heat. Once we discovered these blood vessels, we realized we were leveraging a natural mammalian adaptation for heat loss in reverse. Why are these surfaces specialized for heat loss? Mammals have fur, and while humans don't, we inherited the same vascular system.

Craig: The non-hairy skin areas are major sites of heat loss. Placing the hand under slight negative pressure dilates those blood vessels further, allowing us to extract or insert significant heat. Much of our research is available on our website, CoolMitt.com, if you'd like to dive deeper.

Craig: When extracting heat for athletes today, we no longer use negative pressure or vacuum systems. People exercising hard are already vasodilated, so vacuum adds little benefit. Removing that feature allows us to cool both hands simultaneously instead of just one.

Craig: You can place both hands on a heat sink without needing a free hand to operate a vacuum. It's a straightforward approach, though certain elements are critical. For instance, you must use a circulating heat exchanger to avoid boundary layers and maintain the precise temperature required.

Craig: You shouldn't use a bucket of ice water, as extreme cold causes vasoconstriction. In an overheated person, that actually causes core temperature to continue rising because the heat loss pathway shuts down.

Brent: Is that why taking my hands out of a hot tub allows me to stay in longer? I used to tell people to try that, but I never knew if it was just an old wives' tale—even though it seemed to work.

Craig: Yes, that's true. I recently returned from a meeting in Europe during very hot weather. It still amazes me that hotel rooms keep thick comforters even in the summer. To stay cool, you simply stick your hands and feet out.

Brent: Right, we've all figured that out through trial and error. Is there any known benefit to exercising in the heat?

Craig: Exercise increases circulation and overall activity across many body systems, which is beneficial. As for whether doing so in a hot environment is better than in a neutral or cold environment, I would favor heavier exercise in cooler conditions as more beneficial, though I don't have direct data for that.

Craig: I have no, no proof of that.

Brent: All these performance-related optimizations tied to body or skin temperature rely on cooling hairless surfaces. By cooling those areas, you prevent the muscle from overheating as quickly, allowing you to sustain exertion longer.

Craig: The critical factor is pulling heat out of the body core. Cooling the overall skin surface extracts some heat, but skin is an effective insulator. Targeting non-hairy surfaces allows you to extract a considerable amount of heat to offset what's generated during exercise.

Craig: You also prevent steep temperature spikes in muscles and joints that cause delayed onset muscle soreness (DOMS). Extracting heat during exercise leaves people with no muscle soreness. This differs from taking a cold plunge after heavy exercise, where you've already experienced hyperthermia and are merely accelerating recovery. Preventing the hyperthermic spike altogether makes DOMS much less likely.

Craig: You're just recovering faster. But if you prevent that hypothermic, spike during the exercise, you're less likely to have delayed onset muscle soreness.

Brent: How is this practically applied? I went for a trail run this morning, and tomorrow I'm doing strength training at 24 Hour Fitness—I think tomorrow is leg day. If I wake up and do leg day as a control, what does that look like compared to using Doctor Heller's protocol?

Brent: And then if I'm experimenting with the doctor helper protocol, how does my how does tomorrow morning look different on leg day.

Craig: Our CoolMitt equipment is currently used episodically because we don't have a commercial wearable version yet, though we have built wearable prototypes. For example, we prototyped systems simulating conditions for healthcare workers treating Ebola in Sierra Leone. Operating in extreme heat while fully encapsulated in protective suits, workers overheat very quickly.

Craig: We developed a wearable prototype to fit inside the suit, allowing them to work safely for hours in heat instead of just 20 or 30 minutes. Right now, our equipment is used episodically—such as between sets in the gym, or during breaks in practice and competition when athletes come off the field.

Craig: Our football team uses it during turnovers when players come off the field. Our women's soccer team used it during a semifinal match in hot weather in Arizona last spring. People expected Stanford to struggle without heat acclimation, but they handled the conditions extremely well.

Craig: Many Arizona players went down due to heat illness, but none of the Stanford players did. Regulations mandated water breaks every 15 minutes due to the high ambient temperature, so our athletes used the cooling system during those breaks and performed perfectly.

Brent: So it's a device with a cooling unit, tubing, and mitts for the hands. For tomorrow's leg day, I put the mitts on my hands—regardless of whether I'm training legs or arms.

Brent: I might use the CoolMitt before starting my workout and then again between sets. This reduces the heat generated by my muscles, allowing them to sustain exertion longer.

Brent: Is there any downside? One might assume muscles heat up for a protective reason. By artificially cooling them, am I increasing the risk of a muscle tear or interfering with a natural physiological response?

Craig: The rise in temperature is simply a byproduct of activity. Take a long-distance race where you warm up beforehand: the term "warm-up" is misleading, as its main purpose is to increase flexibility.

Craig: Energy metabolism benefits when muscle reaches an optimal temperature, which a warm-up can briefly achieve. Once you continue producing heat, muscle capacity drops rapidly on the downside of that thermal curve, reducing your ability to contract muscles and perform work.

Craig: Temperature rise isn't designed to protect your body; it's a byproduct of exercise. That's why we have cooling mechanisms like sweating to protect us from hyperthermia.

Brent: So performance increases without a physiological cost, aside from buying the CoolMitt device itself?

Craig: You work alongside your thermoregulatory system. You cannot overcool or make yourself hypothermic because lowering local temperature below the set point causes the thermostat to shut off vascular blood flow.

Brent: There's a physiological governor that shuts the door if hands get too cold to protect core temperature. What do critics say about this? Some professional athletes use CoolMitt, but why isn't adoption at 100%?

Brent: What are the what are the critics saying?

Craig: There aren't real scientific critics, though some flawed experiments claim it doesn't work. One notable paper from an Air Force lab claimed cooling yielded no benefit while studying sprinters.

Craig: The issue was that sprint protocols didn't raise temperature enough to cause heat impairment. Cooling can only address heat-induced impairment; if performance isn't thermal-limited—say, if you can't do two pull-ups—cooling won't help.

Craig: You must reach a point of thermal impairment for cooling to work. Broad adoption takes time because introducing new technology faces natural skepticism. People say they might try it later.

Craig: My favorite response to skeptics is: try it on yourself and see whether it works. For instance, we had an incredible track coach here who was a three-time Olympian thrower for Great Britain.

Craig: One of his student-athletes worked in my lab and wanted to test the protocol on the team. I told him to consult his coach first. The next day, he said the coach wanted to meet me. I went to talk to this imposing man.

Craig: He listened to our explanation and wanted to try it himself during his 7:00 PM gym session. He was doing bench presses or squats with an immense amount of weight.

Craig: He knew his limits from a lifetime of lifting. He completed his first set of six reps, used the cooling device, and completed a second set of six. Surprised, he cooled again and hit six more reps, stating that at age 42, that matched his personal best.

Craig: He then completed a fourth set of six reps. He continued using palm cooling throughout that winter, competed at age 43 in the Commonwealth Games, and won bronze in the discus.

Brent: Experienced athletes know their exact baseline performance, making them ideal subjects. Exceeding those limits leaves no ambiguity. It feels like a steroid with no physiological downside.

Brent: Steroids yield similar performance gains by delaying muscle fatigue, but they carry severe negative health effects and are illegal. Heat extraction offers a safe way to do more reps and gain strength faster.

Craig: The outcome is actually better. Reviewing studies on bench press conditioning with anabolic steroids shows roughly 1% strength improvement per week. With palm cooling, we routinely see higher weekly percentage improvements.

Brent: How should this fit into a routine? Should you cool before, during, or after exercise?

Craig: You recover.

Brent: So doing it before, during, and after aids recovery?

Craig: It depends on the activity. During gym workouts, you can cool between sets during standard rest intervals by placing your hands in the device for a couple of minutes before the next set.

Craig: This leads to a dramatic increase in work volume. To test optimal temperature and vacuum parameters early on, we ran trials with my research assistant, a dedicated athlete who enjoyed pull-ups.

Craig: We tested him doing ten sets of pull-ups to failure with three-minute rest intervals to raise his core temperature and see how quickly we could extract heat. Over six weeks of cooling only at the end of workouts, his total pull-ups across ten sets increased from 100 to 180.

Craig: During the next six weeks, we cooled him after every other set, and his volume increased from 180 to 618 pull-ups.

Brent: Cooling between sets yields greater benefits than cooling only at the end. Can ultramarathoners use this during aid station stops to run faster and sustain pace longer?

Brent: Is this a like hey when you hit the aid station, it's cool for a few minutes, you're already stopping for a few minutes and you're going to you're going to be able to run faster, last longer.

Craig: Yeah. Absolutely.

Brent: So the benefits apply equally to strength and endurance sports?

Craig: One year we set up at the midpoint of the Western States Ultramarathon to treat runners, though it turned out to be the coldest day on record for the event. While we couldn't gather standard cooling data, we recorded thermal video of all runners passing Michigan Bluff.

Craig: Comparing thermal images of the top three runners at midpoint, the lead runner registered the coolest body temperature, the second was warmer, and the third was the warmest.

Craig: The coolest runner won the race in record time, the slightly warmer runner finished three hours back, and the warmest runner dropped out. Body temperature directly impacts performance.

Brent: Is cooling hands chosen primary for convenience, or do feet and face work equally well?

Craig: We've tested all three, and the cooling effect is additive.

Brent: Why can't someone just bring ice packs to the gym instead of using specialized equipment?

Craig: Standard ice packs are too cold. Traditional first aid places ice packs on armpits, groin, and neck. Applying those same ice packs to palms, soles, and face doubles the rate of cooling.

Craig: Traditional placement targets major blood vessels near the surface at the groin and armpits. However, if your car engine overheats, you spray the radiator rather than the hoses. Hands, feet, and face act as the body's radiators.

Craig: These are the radiators. These are these are the tubes.

Brent: Does this apply to treating hypothermia? Would applying heat to hands, feet, and face help rewarm someone with dangerously low core temperature?

Craig: That was our original application for post-surgical anesthesia recovery. Emergency technicians testing our prototypes treated ice fishermen rescued from frozen water who were clinically hypothermic.

Craig: They successfully rewarmed and resuscitated one patient using the device. While anecdotal, it demonstrates that palm heating treats hypothermia effectively, though our current focus remains on performance cooling.

Brent: Placing ice packs on the neck or armpits is outdated practice, as science shows non-hairy skin surfaces are far more effective.

Brent: It's just a question. Do we stop doing it now or in ten years or 20 years? But the science here is clear it's face, hands and feet.

Craig: Cold-water immersion remains the clinical standard for severe hyperthermia because submerging the entire body drops temperature rapidly when an ice bath is available.

Craig: However, a portable palm cooler can be applied immediately upon first contact, often recovering the patient before an ambulance arrives.

Brent: How does this connect to your work on circadian rhythms, sleep, memory, and Down syndrome?

Brent: You got curious or is there some connection between the two?

Craig: My research naturally evolved across related fields. I started in mammalian thermoregulation, which led to hibernation studies, then sleep, circadian rhythms, and ultimately learning and memory.

Craig: One discovery led to the next, and the main challenge is balancing multiple active research threads.

Brent: As a CEO, I relate to exploring diverse domains. What connects sleep and circadian rhythms to Down syndrome?

Brent: And I'm kind of the jack of all trades, master of none. And that that just suits me. I buy, my attention span just requires some variety. But what is the connection of circadian rhythm? Sleep and learning to down syndrome?

Craig: Sleep consolidates short-term memory into long-term memory, and disrupted sleep impairs cognitive performance. We investigated whether learning deficits in Down syndrome relate to circadian or sleep abnormalities using genetic mouse models.

Craig: While circadian rhythms were largely normal, manipulating neurochemicals involved in sleep significantly improved cognitive performance in our models.

Craig: A two-week daily drug treatment restored normal learning and memory in mouse models for months.

Brent: So animals also have down syndrome or some version of it.

Craig: Down syndrome is a triplication of chromosome 21. Genetically engineered mouse models replicate this chromosomal duplication.

Brent: So you've completed early animal trials measuring memory and learning, but clinical human trials haven't started yet?

Craig: Preclinical work in animal models is complete, and we are studying underlying brain mechanisms. A small pilot trial was conducted in Australia, though larger clinical trials are needed.

Craig: It was done in Australia. So, I would love to find a group that wanted to sponsor a clinical trial. That would be, I think, a really worthwhile thing to do.

Brent: Where can listeners learn more? CoolMitt.com has me convinced, and I look forward to testing it out during my own training.

Brent: Where else can people connect with you?

Craig: You can reach me by email at hcheller@stanford.edu.

Brent: Thank you for your time and insights, Doctor Heller.

Craig: I really enjoyed talking to you.

Brent: Death Clock is recorded in Boulder, Colorado; produced by Patrick Gudino; music by Patrick Lee; and hosted by Brent Franson.