Selected Topics in Athletic Training

Accuracy and Reliability of Core Temperature Measurement Tools
 in Identifying Heat-Related Illnesses in Athletes

Lesley Willis
California University of Pennsylvania


The recognition of injuries and illnesses is a responsibility of the Certified Athletic Trainer. Heat-related illnesses (HRIs) can be more effectively prevented when they are recognized at an early stage. Core temperature is one indicator of HRI, but the accuracy of measurement tools is questionable. Certified Athletic Trainers can recognize these conditions more efficiently in athletes by analyzing the accuracy and reliability of current and developing core temperature measurement tools in conjunction with other indicators of HRI.

Continuum of Heat-Related Illnesses

Heat-Related Illnesses (HRIs) are plotted along a continuum that leads from mild to severe conditions.1 With early recognition by a Certified Athletic Trainer, a mild or moderate HRI can be prevented from progressing to a severe and possibly life threatening condition. Signs and symptoms along with core temperature should be assessed to determine the presence of HRIs. Core temperature is the only objective measure that a Certified Athletic Trainer can use to detect a HRI. Mild HRIs are not related to an increased core temperature and should be diagnosed symptomatically, as recommended by the National Athletic Trainers� Association (NATA) position statement on exertional heat illnesses.1 The mild HRIs are heat edema and heat rash; although they are uncomfortable, they are not life-threatening. The HRIs are considered to be moderate are heat syncope and heat cramps. Heat syncope does not result from an increased core temperature, but heat cramps are the body�s earliest indicator of such an elevation.1, 2 Early recognition at this phase is a key factor in effective prevention of progression along the continuum to a severe HRI. Severe HRIs are heat exhaustion and heat stroke. These life-threatening conditions can be recognized by an increased core temperature. The core temperature during heat exhaustion is between 37�C and 40�C (98.6�F-104�F).2 Heat stroke is recognized by an increased core temperature over 40�C (104�F) and can be categorized based on the circumstances causing the illness.1, 2 Classic heat stroke is based on environmental factors alone that cause an increased core temperature; exertional heat stroke is caused by exercise which causes the body�s core temperature to increase in any environmental condition.2 Tachycardia, altered mental status, emesis, hyperventilation, and seizure are some of the most severe and apparent signs and symptoms of heat stroke.1 If the athlete continues without treatment, heat stroke can ultimately lead to coma and death.1 Any core temperature over 40�C (104�F) is considered a medical emergency and should be treated by cooling the athlete immediately followed by hospitalization with constant monitoring of vitals.1,2 When Certified Athletic Trainers are comfortable monitoring core temperature with accurate and reliable devices, earlier recognition and prevention can occur more often, possibly resulting in fewer athletes suffering severe HRIs.

The Gold Standard and the New Generation

The gold standard of core temperature measurement is currently rectal thermometry, which measures the temperature 8-10 cm inside the rectum. Rectal thermometry is recommended by both the NATA and the American College of Sports Medicine (ACSM) for recognition of HRIs.1, 3 The rectal thermometer has been shown in studies to be the most accurate and reliable thermometer during exercise and is used in most studies as the control variable to test other types of thermometers.4-6 Rectal temperatures can be measured by using a flexible thermometer that is inserted 8-10 cm into the rectum and secured by applying medical tape to the wire lead. The measurement site is considered to be accurate and easily accessible before, during, and after exercise. The reading device is easy to carry or attach to clothing during exercise; it can be left in place during exercise since the thermometer itself is inserted into the rectum, to allow immediate core temperature measurement at any time during exercise.

One other type of thermometer has been shown to be accurate and reliable during exercise in current research is the ingestible telemetric pill, measures intestinal temperature.4, 7-10 Telemetry is the automated recording of specified measurements that are transmitted electronically. The ingestible telemetric pill measures core temperature by recording temperature measurements and then transmits that reading to a receiver via radio, infrared, or ultrasonic signal.11 In studies by Casa (2007) and Ganio (2009), ingestible telemetry was tested against rectal thermometry and other common thermometers. These studies reported significant positive correlations between ingestible telemetry and rectal thermometry and proclaimed the pill to be both accurate and reliable.4, 7 The theory behind ingestible telemetry and intestinal temperature is that measurements closer to the core will be more accurate. Although this type of thermometer has been examined in many studies, more research is necessary before it should be included in recommendations by the NATA and ACSM.

Accuracy and Reliability of Current Tools

Core temperature measurement during exercise is an effective way to objectively recognize HRIs but only if the measurement devices are accurate and reliable. They should also be practical and functional in all environments. Rectal thermometers are the gold standard because they are accurate and reliable in all conditions, as illustrated in two classic studies by Casa (2007) and Ganio (2009). These studies had similar protocols and tested the same thermometer types: oral, axillary, aural, gastrointestinal, forehead, temporal, and rectal. The most notable difference between studies was setting, one was performed outdoors and the other indoors. Both studies showed that of all the thermometers tested, only the rectal and ingestible telemetric pill were accurate and reliable.4, 7 These findings indicate that the environment has little effect on these types of thermometers and further demonstrates their reliability and validity.

Other factors affect the accuracy and reliability of these instruments. The location of a rectal thermometer can be uncomfortable 5 and insertion, as it is an invasive measure, can be viewed as inappropriate touching if not handled with professionalism.6 The lag phenomenon is also a problem, but it does not cause concern of enough significance to make the rectal thermometer inaccurate.4, 7 The lag phenomenon is thought to be caused by poor blood supply to the rectum, the presence of feces at the time of measurement, and postural differences during the recovery period.12 There are also speculations about the practical use of ingestible telemetric pills because of their detailed time schedule and possible diet challenges. The pills should be taken 3-4 hours prior to activity and with a prescribed diet,7 which naturally suggests that this method should be used for prevention and not an emergency situation. Not all types of telemetric pills are disposable with retrieval after exercise difficult, opening a doorway for contamination upon the next use. More research is necessary to fully determine future recommendations for the use of ingestible telemetry pills during exercise.

Other Common Measurement Tools

There are several other common types of thermometers that are used for athletics today. Although most of them are considered to be accurate and reliable in the clinical setting, during exercise their use is questionable. These thermometers include esophageal, aural tympanic, oral, and temporal artery thermometers. Current research proclaims that these tools are not recommended for use during exercise by Certified Athletic Trainers.4, 7, 9, 10, 13 Research showed that esophageal thermometers are accurate and reliable during exercise, but the location of thermometer, at heart level inside the esophagus, can be uncomfortable for the athlete and easily influenced by any substance entering the esophagus.4, 7, 9, 10 Aural tympanic thermometers are easy to use and popular for their non-invasive location,6 but they are not accurate or reliable during exercise.4, 7 Neither oral nor temporal artery thermometers are considered accurate or reliable either. These two types of thermometers have been used by sports medicine professionals for years, but current research shows that too many external factors affect these instruments to create an accurate or reliable measurement.4, 7, 13 Each for their separate reasons, these thermometers are not recommended for use by Certified Athletic Trainers on athletes during exercise.


The recommendations by the NATA and ACSM are for the use of rectal thermometry to recognize HRIs in athletes during exercise. This should not be used in isolation of the signs and symptoms, which also lead to early recognition and treatment for prevention. Although mild HRIs are not related to an increased core temperature, moderate and severe HRIs will present with an increased core temperature. Current research shows that intestinal temperatures, as assessed by an ingestible telemetric pill, are as accurate and reliable as rectal temperatures, but there is not enough research on this device to include it in recommendations for Certified Athletic Trainers.


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  2. Howe AS, Boden BP. Heat-related illness in athletes. Am J Sports Med. 2007;35(8):1384-1395.
  3. Armstrong LE, Casa DJ, Millard-Stafford M, Moran DS, Pyne SW, Roberts WO. American College of Sports Medicine position stand: exertional heat illness during training and competition. Med Sci Sports Exerc.2007;39(3):556-572.
  4. Ganio MS, Brown CM, Casa DJ, et al. Validity and reliability of devices that assess body temperature during indoor exercise in the heat. J Athl Train. 2009;44(2):124-135.
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  7. Casa DJ, Becker SM, Ganio MS, et al. Validity of devices that assess body temperature during outdoor exercise in the heat. J Athl Train.2007;42(3):333-342.
  8. Engels H, Yarandi H, Davis J. Utility of an ingestible capsule for core temperature measurements during body warming. J Exerc Phys Online. 2009;12(2):1-9.
  9. Gant N, Atkinson G, Williams C. The validity and reliability of intestinal temperature during intermittent running. Med Sci Sports Exerc. 2006;38(11):1926-1931.
  10. O'Brien C, Hoyt RW, Buller MJ, Castellani JW, Young AJ. Telemetry pill measurement of core temperature in humans during active heating and cooling. Med Sci Sports Exerc. 1998;30(3):468-472.
  11. Güler NF, Ubeyli ED. Theory and applications of biotelemetry. J Med Syst. 2002;26(2):159-178.
  12. Edwards RJ, Belyavin AJ, Harrison MH. Core temperature measurement in man. Aviati Space Environ Med. 1978;49(11):1289-1294.
  13. Ronneberg K, Roberts WO, McBean AD, Center BA. Temporal artery temperature measurements do not detect hyperthermic marathon runners. Med Sci Sports Exerc. 2008;40(8):1373-1375.


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