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).
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.
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
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