Athletic development specialists dedicated to the art and science of excellence in movement

Concepts for Hot Weather Training


As we turn the calendar to June, hot weather is now upon us in Arizona and is on the horizon in many other places.  Most training accommodations for hot weather are fairly obvious: drink enough, replenish electrolytes, don’t do stupid workouts, and avoid the heat when possible.  In terms of fueling, this post is not the place for recommendations since everyone’s sweat rate and content of sweat is different.   The basic mechanisms behind heat and its effects on the body are also well established  For instance:

“Sweat evaporation limits the rise in core temperature, but dehydration will impair cardiovascular function, leading to a fall in blood flow to muscle, skin and other tissues.” (Maughan 2010)

In general, common sense would suggest avoiding hot races if finish time is important, but certain races cannot be avoided (qualifiers, Trials, national championships).  Professionals can’t always skip races if they need the money to pay the bills and national championships are….championships!  Otherwise, racing and intense training during heat (unless you’re preparing for a specific championship race with hot weather) is just poor planning and reflects a lack of discipline to wait for better conditions. 

However, a few factors regarding heat performance are less obvious than hydration and nutrition.  We’ll cover those factors in this post; diurnal variation, psychology, and fitness levels.

Diurnal variation….The body’s inherent diurnal temperature variation is often overlooked, in part because the afternoon is usually hotter than the morning.  With two heating mechanisms occurring simultaneously, we’re more likely to note the more apparent one (air temperature).  As for diurnal variation, in most people body temperature naturally rises throughout the day.  Although the natural rise in body temperature can aid afternoon performance and even allow for a truncated warmup in afternoon/evening workouts or competitions, it can have a negative effect when paired with high ambient air temperatures.     

“[H]igher afternoon body temperatures can reduce the heat storage capacity and result in a reduction in exercise capacity in hot environments. In addition, in parallel to the circadian variations in muscle contractility and central temperature, exercise capacity in hot environment may also be affected by the diurnal variations in melatonin concentration and in the onset of peripheral vasodilatation and sweating.” (Racinas 2010)

Key implication is how hard the body is working to achieve certain result.  Yes, we can monitor physiological output via performance markers but the actual effect on the athlete may change based on how hard the body is working to sustain itself.  These mechanisms are not always reflected in short term performance markers.  As such, the coach and athlete must keep other factors in mind when interpreting data.

One relatively new frontier in this area is the nervous system.  “[E]volutionary perspective suggests that psychological safeguards should also protect individuals prior to catastrophic hyperthermia, and exposure to hot environments or elevated body temperature may directly attenuate central drive for exercise even well before the attainment of a critical limiting central temperature. Voluntary exercise tolerance or pacing may be influenced by a complex integration of peripheral and central thermal afferents, with regional differences in thermosensitivity across the skin surface and individual variability due to age and fitness.” (Cheung 2007)  In other words, the body is programmed to avoid overheating.  Just as our cars come equipped with a heat gauge, so does our brain. 

Furthermore, performance impairments in both cold and hot environment are related to a modification in neural drive due to protective adaptations, central and peripheral failures. (Racinas 2010) Tucker (2004) studied male cyclists who performed two self-paced time trials: one in cool conditions, the other in hot conditions.  Authors noted that subjects’ core body temperatures were similar before starting each time trial. 

Key finding: during the hot time trial, power output and iEMG (muscle activity) measurements dropped BEFORE any abnormal increase in rectal temperature, heart rate, or perception of effort.  By the end of the time trial, rectal temperature had increased significantly.  However, these finding reveal the cyclists shut down effort even before their body was measurably affected by the heat!!   This adjustment was thought to “form part of an anticipatory response which adjusts muscle recruitment and power output to reduce heat production, thereby ensuring that thermal homeostasis is maintained during exercise in the heat.” (Tucker)

Fitness levels also affect heat response, though the data is mixed.  Chueng (2000) noted that “long term improvements in physical fitness appear to provide some degree of protection. Individuals with higher proportions of body fat have a lower heat tolerance because of a reduced capacity to store heat.” 

(Side note…Cheung also observed that “Women not using oral contraceptives are at a thermoregulatory disadvantage during the luteal phase of the menstrual cycle. The use of oral contraceptives eliminates any differences in heat tolerance throughout the menstrual cycle but tolerance is reduced during the quasi-follicular phase compared with non-users.”….OK, not related to fitness levels, but since someone determined it was important enough to study and report, their observations are worth mentioning here!)

Selkirk (2001) studied trained and untrained individuals in an exercise test in which subjects exercised until exhaustion or until rectal temperature reached 39.5 degrees C or heart rate reached 95% of maximum.  Results showed that trained individuals could exercise longer until any of the conditions was met.  This result may seem obvious (fit people can exercise longer than unfit people…duh!), but authors also found a correlation between body composition and exercise time, indicating that ratio of lean body mass to fat mass may impact heat storage. 

Several studies have looked at marathon performance, but reached differing conclusions.  Note that in marathons, most subjects in the samples can be characterized as “fit,” unlike lab studies in which you may have fit vs. unfit subjects.  Vihma (2010) studied the Stockholm Marathon from 1980-2008 and found a significant correlation between air temperature and specific humidity with the percentage of drop outs.  Seems obvious, but according to the study this was a novel finding.  As for finishers, slower runners were more affected by unfavorable weather.  Previous studies had established this relationship for sub-3 hour racers, but this study extended the correlation to 4.7 hours.   Interestingly, females were less affected by heat than males, which authors attributed to slower running velocity and/or females larger surface area to body mass ratio, which aids cooling.

Ely (2007, 2008) conducted studies in consecutive years and found somewhat conflicting results.  In 2007, while studying several North American marathons, there was a significant relationship of slower runner being more affected by heat than faster runners.  The relationship was reversed in a study of Japanese women marathoners, as that study revealed the faster runners were more affected by temperature increases in the race than the slower runners.  However, because this was a championship race, the ability of the participants was far more compressed, indicating greater homogeneity.  From these result, we many conclude that when comparing runners of similar abilities, faster runners might be more affected, but when comparing elite or near-elite runners to casual or recreational athletes, heat may affect the slower runners more.  More study is needed in this area given the conflicting results.

Take home points

-Heat is affected by endogenous (internal) variables, not just the heat itself: these variables include diurnal variation, individual psychological perception, and fitness, though the evidence is mixed on the latter.

-Body has innate mechanisms to slow itself down to avoid damage.  The correlation of these mechanisms to actual threat may vary by individual, but if the body is saying something, there’s a reason why.  Summertime heat not a time to be a hero. 

-But if you need to be a hero in a race, plan accordingly for recovery.

-Consider the body’s response to the heat, not simply output.  Subjective responses to heat are critical to account for in future planning.


Selkirk GA, McLellan TM.  Influence of aerobic fitness and body fatness on tolerance to uncompensable heat stress.  J Appl Physiol. 2001 Nov;91(5):2055-63.

Cheung SS, McLellan TM, Tenaglia S.  The thermophysiology of uncompensable heat stress. Physiological manipulations and individual characteristics.  Sports Med. 2000 May;29(5):329-59.

Vihma T.  Effects of weather on the performance of marathon runners.  Int J Biometeorol. 2010 May;54(3):297-306. Epub 2009 Nov 25.

Ely MR, Martin DE, Cheuvront SNMontain SJ. Effect of ambient temperature on marathon pacing is dependent on runner ability.  Med Sci Sports Exerc. 2008 Sep;40(9):1675-80.

Ely MR, Cheuvront SN, Roberts WO, Montain SJ.  Impact of weather on marathon-running performance.  Med Sci Sports Exerc. 2007 Mar;39(3):487-93.

Racinais S.  Different effects of heat exposure upon exercise performance in the morning and afternoon.  Scand J Med Sci Sports. 2010 Oct;20 Suppl 3:80-9. doi: 10.1111/j.1600-0838.2010.01212.x.

Maughan RJ.  Distance running in hot environments: a thermal challenge to the elite runner.  Scand J Med Sci Sports. 2010 Oct;20 Suppl 3:95-102. doi: 10.1111/j.1600-0838.2010.01214.x.

Cheung SS.  Neuropsychological determinants of exercise tolerance in the heat.  Prog Brain Res. 2007;162:45-60.

Racinais S, Oksa J.  Temperature and neuromuscular function.  Scand J Med Sci Sports. 2010 Oct;20 Suppl 3:1-18. doi: 10.1111/j.1600-0838.2010.01204.x.

Tucker R, Rauch L, Harley YX, Noakes TD  Impaired exercise performance in the heat is associated with an anticipatory reduction in skeletal muscle recruitment.  Pflugers Arch. 2004 Jul;448(4):422-30. Epub 2004 May 8.


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