When core and skin temperatures are elevated, the body must work hard to cool down, resulting in performance loss. The increased blood flow to the skin for cooling comes at the expense of blood and oxygen transport to the muscles. As a result, we observe a higher heart rate at a given workload, and the maximum heart rate is reached earlier.
Additionally, heat-related performance losses can be associated with changes at the muscle and brain levels. For example, the perception of heat (i.e. how hot and how uncomfortable one feels) plays a significant role in performance decline. This is why psychological adaptations to the heat may be as important as physiological adaptations.
So, multiple factors contribute to heat-related performance loss. In this article, we will focus on one of the main causes: cardiovascular strain. This is the strain that is put on the heart and blood vessels.
Cardiovascular Strain
Higher Heart Rate
At low to moderate exercise intensities in cool conditions, many athletes can predict their heart rate at a given pace/power. And as their pace/power increases or decreases, their heart rate rises or drops in a predictable way.
However, when heat strain increases during exercise, the heart must beat faster to facilitate blood flow to both the muscles for exercise and the skin for cooling. So, for a given pace/power, heart rate is substantially higher than it is in cool conditions.
Cardiac Drift
In addition, during exercise in the heat, there will be increased cardiac drift. The term “cardiac drift” (or decoupling) refers to the upward drift in heart rate during exercise at a constant power/pace. While cardiac drift can also occur in cool conditions, heat exacerbates that drift.
The main causes of cardiac drift are overheating and dehydration. The increase in heat strain elevates heart rate to facilitate blood flow to the skin for cooling. Dehydration reduces the blood volume, even further increasing heart rate, and thus cardiac drift.
Fatigue also plays a substantial role. When main muscle groups get fatigued, smaller muscle groups are recruited to maintain pace/power – this inefficiency raises heart rate. A standard fitness benchmark is being able to train at a steady moderate intensity in cool conditions with no more than a 5% raise in heart rate from start to finish – any more drift than that is considered a sign of poor endurance.
Performance Loss Due to Cardiovascular Strain
The increased heart rate during exercise in the heat is considered performance loss. At moderate endurance efforts, the body can withstand the increased heart rate for lengthy durations, although perceived effort will likely increase. And at higher intensities where heart rate is already considerably elevated, the body may not be able to sustain a heart rate that is increased even further. As a result, pace/power must decrease.
Individuality
Heat-induced performance loss is highly individual. Some will experience great performance losses during exercise in the heat, while others will be able to keep performance loss minimal.
During steady-state moderate efforts in hot conditions, athletes can learn to predict cardiovascular strain by monitoring Heat Strain Index. For example, an athlete may learn that a Heat Strain Index of 0.5 causes no increase in heart rate, a sustained 4.5 leads to a 10% increase in heart rate, and a sustained value of 6.5 results in a 20% increase, relative to a Heat Strain Index of 0.
In other words, Heat Strain Index enables athletes to predict heart rate based on pace/power during exercise in the heat. This is useful information both for heat training and for racing and will be discussed in greater detail in later articles. See Individuality of Heat Zones.
References
Flouris AD, Schlader ZJ. Human behavioral thermoregulation during exercise in the heat. Scand J Med Sci Sports. 2015;25:52–64. https://doi.org/10.1111/sms.12349.
de Korte, JQ, Bongers CCWG, Hopman MTE, Eijsvogels TMH. Exercise performance and thermoregulatory responses of elite athletes exercising in the heat: Outcomes of the ThermoTokyo study. Sports Med. 2021;51(11):2423–36. https://doi.org/10.1007/s40279-021-01530-w.
Nybo L, Rasmussen P, Sawka MN. Performance in the heat - physiological factors of importance for hyperthermia-induced fatigue. Compr Physiol. 2014;4(2):657–89. https://doi.org/10.1002/cphy.c130012.
Périard JD, Eijsvogels TMH, Daanen HAM. Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiol Rev. 2021;101(4):1873–979. https://doi.org/10.1152/physrev.00038.2020.
Wingo JE, Ganio MS, Cureton KJ. Cardiovascular drift during heat stress: implications for exercise prescription. Exerc Sport Sci Rev. 40: 88–94, 2012. https://doi.org/0.1097/JES.0b013e31824c43af.