Core Temperatures In NFL Players

Abstract

Heat stroke deaths in football (FB) players usually occur during the first 3 days of pre-season practices. Experimental studies show that maximal core temperatures (Tc max) decline in subjects in response to the physiological adaptations associated with acclimatization. Using ingestible sensors that allow accurate measurements of Tc during exercise, we recently recorded Tc in National Football League (NFL) players during pre-season practices and compared the Tc response of large interior linemen to those of smaller backs and receivers. However, we are not aware of field study comparing the Tc responses of the same NFL players during the first few days of pre-season practice versus practices on the 10th day. PURPOSE To measure Tc in professional FB players and compare the Tc max reached on the 2nd or 3rd day (day 2/3) of practice to those obtained on the 10th day. METHODS Eight NFL first or second team players, age = 26.1 ± 3.3 yr, height = 188.9 ± 5.9 cm, mass = 120.2 ± 24.4 kg, body surface area (BSA) = 2.45 ± 0.23 m2 and BSA/mass = 0.0197 ± 0.0014 m2 kg−1, representing the following positions; wide receiver, corner back, tight ends, linebacker, defensive and offensive linemen participated. Subjects ingested a temperature sensor at 11:00 PM prior to, or in the morning of the days of data collection during preseason training camp. Subjects' resting Tc were recorded in an air-conditioned area 1 h prior to and then approximately every 12 min during two h of practice in full football equipment. Tc was also measured during the afternoon practices with players dressed in a helmet, shorts and shell. Body weight was recorded before and after practices for determination of percent dehydration (%Dehy). ΔTc over time were analyzed using repeated measures ANOVA and differences in Tc max and %Dehy on days 2/3 versus day 10 were analyzed using correlated t-tests. RESULTS Wet bulb globe temperatures ranged from 23.7 to 29 °C on days 2/3, and 20.4 to 27.2 °C on day 10. A significant linear trend was found for Tc on days 2/3 and day 10, p<0.01. When the highest Tc recorded in each subject were compared, Tc max were higher on days 2/3 (39.1 ± 0.6 °C, range = 38.3 to 40.9 °C) compared to day 10 (38.3 ± 0.4 °C, range = 37.6 to 39.4 °C), p<0.001. There were no differences in %Dehy on days 2/3 (−1.26 ± 1.1%) versus day 10 (−1.25 ± 0.87). No correlations were found between the players' Tc max and %Dehy. CONCLUSIONS NFL players reached higher Tc during the first few days of preseason practices compared to day 10. The highest Tc in all FB was generally obtained during live scrimmaging. %Dehy on days 2 or 3 was not different from day 10 and was not associated with Tc max. Supported by a faculty development grant from West Chester University.