Category Archives: Heat

Age and Exercise-induced Muscle Damage can increase risk of heat stress.

Muscle damage and heat and age.

Many people dislike working out in the heat.  There are also serious safety concerns to consider.  Heat stress, or course, can kill . . . it can also make people sick.  Those who recover from heat stroke, the more severe form of heat illness, can suffer life-long difficulties with regulation of body temperature.  Older people, and presumably older athletes, are more vulnerable to heat stress.  There may be many reasons for this.  Older people may sweat less and they may sweat less efficiently (Inoie 1996).  Vasodilation, an important cooling mechanism that shunts blood flow to the surface where it can cool, is less efficient in older adults as well (Smith et al. 2013). Thirdly, older athletes may be slower to recover from exercise-induced muscle damage.  Exercise-induced muscle damage can increase production of pyrogens, specifically interleukin-6, tumor necrosis actor and interleukin-1-beta (Fortes et al. 2013). This research is important (and new at least as of 2013). The next section of this article addresses risk factors for heat illness.  Please skip ahead if you know all this stuff already.

Risk Factors for Heat IIlness other than Muscle Damage

There are many known risk factors for heat illnesses.  Among them are:

  • Being in poor physical condition
  • Being overweight
  • Various medical conditions
  • Already being under stress from lack of sleep
  • Being over-dressed
  • Working out in hot, humid conditions
  • Not being heat acclimated

Some people will still get heat stroke or heat illness even though they have not been suffering from any of the above conditions.  The research discussed here is an attempt to determine if exercise-induced muscle damage increases risk of heat injuries.  The hypothesis is that muscle damage increases inflammation, which upregulates productions of pyrogens, which adds additional heat stress.   Additional heat stress, induced by the pyrogens, may be enough to tip an individual into heat illness or heat stroke.  Pyrogens, for those who don’t know, are chemical agents that trigger fevers.

There is good evidence that exercise can cause fever.  And that fevers can be blocked by anti-bodies that oppose inflammatory agents.  A previous study collected plasma following an exercise protocol and injected it into rats.  The rats then developed fevers. A second group of rats were injected with human plasma collected from donors prior to exercise, and no rat fevers developed.

Protocol for heat and muscle damage study:

Subjects for the muscle damage study were 13 young men (not heat acclimated).  The muscle damage was induced by having subjects run downhill (at a -10% gradient) for 60 minutes (For those who commented that a -10% grade run for 60 minutes couldn’t possibly cause muscle damage . . . it will if you are not adapted to running downhill.)   The second protocol required that subjects run at a +1% gradient, that was not muscle damaging.  Subjects performed the tests twice, 14 days apart, in a counterbalanced manner.  Researchers assessed evidence of heat strain 30 minutes after the protocol and again 24 hours after completion of the protocol.  The time point of 24 hours post-protocol was chosen to coincide with peak (or close to peak) inflammatory response.  Please see the figure for clarification.   For full details please read the original paper (reference below).

Heat Stress Experimental Protocol
Experimental protocol



Rectal temperatures were higher 30 minutes after the downhill runs, even though both protocols were of the same exercise intensity.  In addition, Interleukin-6 was elevated following the downhill run and subjects reported feeling “hotter”.  Rectal temperatures had decreased 24 hrs later, but remained slightly higher than normal (0.17 degrees C).  That 0.17 degrees is probably not physiologically significant, but interesting that it was still elevated an entire day after the workout.  Researchers also acknowledged that eccentric exercise impairs glucose synthesis, therefore it remains possible that diminished glucose may have contributed to the results found in the study.

What does this mean for us?

One important finding was that pyrogens (interleukin-6) were highest 30 minutes after the protocol.  They remained elevated above normal 24 hours later, but had declined considerably.  Given this information, one can imagine that individuals completing multiple workouts in quick sucession (as happens during crossfit competitions) may experience staggered accumulations of pyrogens.  This would leave athletes at greatest risk of heat illness or heat stroke at the end of the workout cycle (or sequence of WODs).  If you are a Crossfit trainer, competitor or interested spectator you may wish to take any potential increased vulnerability to heat stroke or heat illness into consideration. People at risk of rhabdomyolysis are likely at increased risk of heat illness as well.  Fever, after all, is one of several signs of rhabdo. The question may be raised (and its an interesting one): Will taking anti-inflammatories protect someone from heat stroke or heat illness? I would need to read more on that, but off the top, possibly not.  Ibuprofen, for example, increases production of Interleukin-6, which would make the situation worse.  Aspirin?  Maybe, but it may end up causing other problems.

Keeping cool.

A few people have asked for more information on how to keep cool.  Most of these are pretty well-known:

  • Avoid dark colors if you are working out outdoors
  • Drink plenty of water
  • Keep air circulating if you are indoors or use air-conditioning
  • Dress in thin, light, minimal clothing (while avoiding sunburns).

There has been fairly recent research on the effects of hand cooling on internal temperature.  You can read about it here.  I’ve tried running in the heat while holding frozen water bottles.  I think it works.  Lastly, while we all “know” about how to keep cool, a lot of people skip steps.

For those wanting a simpler synopsis of the article and a little more information on heat stroke and heat illness .  .  .


Exercise induced Muscle Damage, Heat and Rhabdomyolosis

CrossFit Workout and Heat Stress
CrossFit Workout At CrossFit Seven in Fort Worth, TX


Exercise-induced Muscle Damage, Rhabdo and Heat

Most people in CrossFit are probably well aware of risks of rhabdomyolosis.  Rhabdomyolosis occurs when muscles are damaged severely.  Broken down proteins enter the blood stream and can clog up the kidneys.  You can get muscle damage without rhabdomyolosis.  It happens all the time.  Most of the time it is minor and part of training.  However, small amounts of muscle damage may increase a person’s risk of heat illness.

Heat Illnesses include:

  • Heatstroke – is life-threatening.   Temperature can shoot up to the point of brain damage and death.  A person with heat stroke may have dry skin, strong pulse and feel dizzy.
  • Heat exhaustion – not as bad as heat stroke, but can come before heatstroke.  People with heat exhaustion may sweat heavily, have rapid breathing, and a fast pulse.
  • Heat cramps
  • Heat rash

New research shows “exercise-induced muscle damage” increases risk of heat illness.   This is different from exercise-induced heat illness, which may not involve any muscle damage.  The study looked at runners exercising under hot, humid conditions.  Thirty minutes of exercise in hot, humid conditions increased levels of pyrogens in blood over controls.  Pyrogens are substances that cause fever.  Working out in the heat with a fever seems like a particularly bad idea.  The pyrogens subjects’ blood included interleukin 6 which is associated with inflammation. Pyrogens remained higher 24 hours later. This might mean that the risk of heat stress may build a little more every day.  If you do a crossfit workout, or train at anything every day your risk of heat illness may increase a little more every day. People adapt to exercise and heat exposure.  We become better at handling heat over time. The study was done with athletes who were not heat acclimated.  Still, there is reason to be careful.  Subjects exercising in the heat also experienced more muscle soreness the next day.

Conclusion for CrossFit Trainers and CrossFit Athletes

Muscle damage may increase risk of heat stress.  Masters athletes may be at greater risk of heat illness.  If you are a Masters Athlete and notice that you are having a harder time coping with the heat, it is not “all in your head.”  Heat adaptation also happens.  It may take longer than it did when you were in your 20s.  Be patient.  Understanding physiology may help.  It helps for me. Notes:  The featured photo is from CrossFit Heath’s recent fund-raising Masters Crossfit Competition organized by World War Fit.   Bradford CD, Cotter JD, Thorburn MS, Walker RJ, & Gerrard DF (2007). Exercise can be pyrogenic in humans. American journal of physiology. Regulatory, integrative and comparative physiology, 292 (1) PMID: 17197641 Fortes MB, Di Felice U, Dolci A, Junglee NA, Crockford MJ, West L, Hillier-Smith R, Macdonald JH, & Walsh NP (2013). Muscle Damaging Exercise Increases Heat Strain during Subsequent Exercise Heat Stress. Medicine and science in sports and exercise PMID: 23559121 Inoue Y (1996). Longitudinal effects of age on heat-activated sweat gland density and output in healthy active older men. European journal of applied physiology and occupational physiology, 74 (1-2), 72-7 PMID: 8891503 Smith CJ, Alexander LM, & Kenney WL (2013). Nonuniform, age-related decrements in regional sweating and skin blood flow. American journal of physiology. Regulatory, integrative and comparative physiology, 305 (8) PMID: 23926135

Physiology of Fatigue: What are we fighting when we try to push through a challenging workout?

Why are workouts so hard?

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We are fighting a lot when we push ourselves through workouts that are challenging. There are times we may be fighting a bad attitude, discouragement, lack of confidence, drive or our own personalities, but we are, at times, also fighting a very complex regulatory system designed to protect us from severe self-induced damage.

Fatigue and Temperature

Fatigue can be defined as reaching a point where the body seeks to slow down or stop. Exhaustion is that point where a person (or animal) is unable to continue. The most important factor driving suppression of motor activity is believed to be brain temperature. In an untrained person, exhaustion may occur when core body temperature reaches 100 to 102F(~38 to 39C) while a highly trained person may not reach exhaustion until body temperature has reached 104F (~40C).

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Interestingly, it’s not only humans who are stopped at these temperatures. Internal temperatures of ~104 oF will stop other animals whether sprinters (Cheetahs) or the generally more placid and possibly endurance-oriented (Goats) (Taylor and Rowntree 1973). And yes, I’m sure you’re wondering: temperatures were measured rectally, and the animals ran on a treadmill while wearing masks so oxygen and carbon dioxide levels could be assessed. The research team also cranked the heat up. Cheetahs ran for shorter periods when the room was hot. The authors of this paper concluded that the duration of a Cheetah’s sprint is limited by core temperature, which is influenced by air temperature. Keep this in mind when you are working out in the summer with no air-conditioning. There are other factors that are also thought to play roles in regulation of intense physical output. Working muscles send feedback to the brain, and in most of us, they are not yelling “Go! Go! Go!” At first they are saying things like “we need more oxygen over here” and “pump the heart faster.” Unfortunately you maximum output can only go on for as long as you have the necessary materials to keep the system running. Your maximum obtainable heart rate will matter. That may be one you cannot make “just do it.” although you can improve your ability to pump blood with training.

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Blood concentrations of important factors or metabolites, and depletion of working materials, are also monitored by the brain. Changes in concentrations and availability of neurotransmitters, endorphins, cytokines, along with a build-up of ammonia in the brain, occur during continued intense exercise. Cerebral energy use increases requiring more oxygen, while blood flow will decrease by about 20% due to constriction of brain arterioles. Low oxygen, loss of neurotransmitters, and accumulation of waste products can cause a problem that is truly “all in your head” but a real problem none the less. An increased need for oxygen and fuel in the brain may be part of what causes someone to want to slow down or stop.

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Another strong woman shirt for strong women. Be fit and wear an awesome shirt. For strong women who love art, irony and kettlebells

Practice improves physiology and performance.

Increasing oxygen intake may improve performance not necessarily by providing muscles with additional oxygen, but in providing the brain with what it needs to keep the system running. Depletion of brain glycogen and excessive use of lactate as an alternative brain fuel may also signal fatigue. This may happen faster in untrained athletes. Physical training is, after all, about much more than simple strength and endurance. It includes getting all systems, including subtler aspects of physiology like the ability to dissipate heat, produce lactate, carry oxygen and oxygenate the brain, to work as efficiently as possible. We can reach our limits, but our brains rarely stupid enough to allow us to go beyond them and recklessly run our bodies off the edge of a cliff. The brain also likes to know what’s going on and practice (going through the motions) and rehearsal are important to performance. Rehearsing movements before a WOD may be as important as traditional warming up. It preps your system for what it is about to do and lets it know what is coming. Even imagining movements may help improve strength output and performance (Jeukendrup et al. 1996).

CrossFit training, rational mental toughness.

Womens crossfit shirt by WODMASTERS with Mona Lisa and her Kettlebells
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We can improve performance intelligently rather than fight what we imagine to be a lack of mental toughness, or allow ourselves be discouraged. We can keep cool and well-hydrated. We can be patient enough to recognize that our physiological and biochemical systems are becoming more efficient as we train, even if our speed or strength has plateaued, and not give up on long-term goals. Finally, encouragement and cheers can help people achieve their maximal level of oxygen consumption (Nybo & Secher 2004) and maximum performance. This may be especially true if they are new to Crossfit and have type A personalities. New Crossfitters may be putting superhuman efforts into their workouts and should be congratulated and admired for these as much as our seasoned champions.

Taylor CR, & Rowntree VJ (1973). Temperature regulation and heat balance in running cheetahs: a strategy for sprinters? The American journal of physiology, 224 (4), 848-51 PMID: 4698801

JEUKENDRUP, A., SARIS, W., BROUNS, F., & KESTER, A. (1996). A new validated endurance performance test Medicine & Science in Sports & Exercise, 28 (2), 266-270 DOI: 10.1097/00005768-199602000-00017

Nybo, L., & Secher, N. (2004). Cerebral perturbations provoked by prolonged exercise Progress in Neurobiology, 72 (4), 223-261 DOI: 10.1016/j.pneurobio.2004.03.005

Taylor CR, & Rowntree VJ (1973). Temperature regulation and heat balance in running cheetahs: a strategy for sprinters? The American journal of physiology, 224 (4), 848-51 PMID: 4698801 Nybo, L., & Secher, N. (2004). Cerebral perturbations provoked by prolonged exercise Progress in Neurobiology, 72 (4), 223-261 DOI: 10.1016/

CrossFit training: Heat dissipation is key to athletic performance

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Work on the effects of heat on athletic performance continues (see earlier post “What are we fighting when we try to push through a challenging workout” for in depth discussion). This is an important area of research for most of us because heat may be the limiting factor in performance. You body will try its best to make you stop exercising when your brain temperature reaches a certain level. Muscle cells will also start to function poorly when they are heat stressed. This is due, in part of in whole, to the sensitivity of enzymes which tend to require a pretty narrow temperature range in order to work. Exercise can raise muscle temperature above this range causing enzyme function to drop and muscle cell resources to plummet. Cooling to normal temperature will allow enzymes to return to optimal function and will allow you to resume activity faster.

Cooling speeds recovery “better than steroids.”  At CrossFit Seven in Fort Worth, TX.

Many animals, including humans, have heat transfer mechanisms. Excess body heat can be directed outwards by directing blood flow to the surface of highly perfused regions. Dog tongues are examples here, as are the ears of jack rabbits. In people facial tissue and tissue in the palms of the hands and soles of the feet serve as heat dissipators. This is probably why some people get so red in the face during runs and workouts. It may also be why some people swear by barefoot running. They may enjoy the feel of it, but they may just plain be able to go longer, harder and faster because their feet are uninsulated by thick layers of synthetic cushioning.

Researchers at Stanford University are working on a glove (not a pair of gloves; apparently one works just fine) that is effective at rapid cooling. It involves circulating cool water around the hand while applying gentle suction. It looks a lot like having a hand encased in an ironing board: not quite ready for in-play use. Let us know you devise something lower tech and tell us how it worked for you.

Grahn DA, Cao VH, & Heller HC (2005). Heat extraction through the palm of one hand improves aerobic exercise endurance in a hot environment. Journal of applied physiology (Bethesda, Md. : 1985), 99 (3), 972-8 PMID: 15879169