The protective equipment worn during American football has been shown to increase thermal strain; however, the perception of this increased heat has not been examined.

To evaluate perceptual responses of American football players while wearing different 7V7 Uniforms during exercise in the heat and to evaluate how these responses may be used to monitor athlete safety.

Patients or Other Participants:

Ten men with more than 3 years of competitive experience as football linemen (age = 23.8 ± 1.3 years, height = 183.9 ± 1.8 cm, mass = 117.4 ± 3.5 kg, body fat = 30.1% ± 1.7%) participated.

On 3 occasions in hot, humid (33°C, 48%–49% relative humidity) environmental conditions, participants completed 10 minutes of strenuous repetitive box lifting (RBL), 10 minutes of seated rest, and up to 60 minutes of treadmill walking. At each trial, they wore a different uniform condition: control (CON) clothing comprising shorts, socks, and sneakers; partial (PART) National Football League (NFL) uniform comprising the uniform without helmet or shoulder pads; or full (FULL) NFL uniform. Exercise, meals, and hydration status were controlled.

Rectal temperature (Tre), skin temperature (Tsk), rating of perceived exertion (RPE), thermal perception (THM), perception of thirst (TST), and perception of muscle pain (MPN) were obtained for time points matched across trials.

Nineteen of the 30 trials ended before 60 minutes of treadmill walking as a result of participant exhaustion. Mean treadmill time was longer for the CON condition (51.7 ± 13.4 minutes) than for the PART (43.1 ± 15.6 minutes; t9 = 3.092, P = .01) or the FULL (36.2 ± 13.2 minutes; t9 = 4.393, P = .002) conditions. Neck and forearm Tsk increased between the initial time point and the end of exercise in the PART (33.6 ± 0.9°C and 35.0 ± 0.6°C, respectively; F2,18 = 9.034, P < .001) and the FULL (33.4 ± 0.9°C and 35.2 ± 0.6°C, respectively; F2,18 = 21.011, P = .002) conditions. Rate of Tre rise was greater in the FULL (0.042 ± 0.010°C/min) than in the PART (0.034 ± 0.006°C/min) condition (F2,27 = 10.69, P = .04). We found a relationship at the post-RBL and final time points between RPE and THM (r = 0.75, P < .001 and r = 0.59, P < .001, respectively), RPE and TST (r = 0.76, P < .001 and r = 0.61, P < .001, respectively), and RPE and MPN (r = 0.63, P < .001 and r = 0.64, P < .001, respectively). The RPE was greater at the end of exercise in the PART (17 ± 2) and FULL (18 ± 1) conditions than in the CON (15 ± 3) condition (F2,18 = 7.403, P = .005).

Although no differences in perceptual scales existed between the PART and FULL conditions, the Tsk and rate of Tre increase differed, indicating that football athletes find it difficult to perceptually rate exercise conditions as potentially dangerous hyperthermia develops. In addition, correlations between the perceptual scales further defined perceptual responses during exercise in the heat.

Athletes push to the physical limits of the human body in the sport of American football, especially when exercising in the heat. Between 1995 and 2001, 21 athletes died as a result of exertional heat stroke that they sustained while practicing or playing football.1 The high incidence of heat-related deaths in football is explained in part by the heat balance equation2:

where S is heat storage, M is metabolic heat production, W is external work, K is conduction, R is radiation, C is convection, and E is evaporation. In this case, heat storage is a function of the endogenously created body heat minus any work that is done plus or minus the 4 methods of heat dissipation. Factors such as convection are commonly thought to allow an athlete to dissipate heat; however, when the air moving over the athlete's skin is warmer than the skin temperature (Tsk), convection will increase heat storage. Athletes who play football typically have large body and muscle masses, which result in great metabolic heat production that must be dissipated to maintain a safe body temperature. The uniform used by football players, including the shoulder pads and helmet, hinders heat loss via all avenues (conduction, radiation, convection, and evaporation) and increases metabolic heat production because of the weight of the equipment and clothing. Although an athlete may sweat at a greater rate while wearing a uniform,3,4 a large percentage of the sweat drips off the body and, thus, does not contribute to evaporative heat loss.5 Because of their large body sizes and intense levels of effort6 and the environment (both the ambient and the microenvironment created between the skin and uniform), football players encounter a high risk of hyperthermia, which may lead to heat illness or death due to heat stroke.

Heat stress has physiologic consequences that are associated with a decrease in exercise performance and an increase in the rating of perceived exertion (RPE).7–10 Similarly, this has been observed3,4,11–14 with exercise while wearing a football uniform. The RPE also increases in hypohydrated individuals who exercise in the heat.15 However, other psychophysiologic or perceptual scales, such as perception of thirst (TST),16 thermal perception (THM),17 and perception of muscle pain (MPN),18 or the modified Environmental Symptoms Questionnaire (ESQ)19–22 have not been examined in reference to uniformed athletes exercising in the heat. Greater understanding of the perceptual responses of uniformed athletes exercising in the heat may offer coaches and athletic trainers a field-applicable method by which to evaluate the health of their players so they can challenge their athletes while minimizing the risk of heat illness or death. Therefore, the purpose of our investigation was to evaluate the perceptual responses of American football players wearing different football uniforms during exercise in a hot environment and to determine if these responses may be used to monitor athlete safety. We hypothesized that (1) the addition of shoulder pads and a helmet would inhibit body heat loss, (2) THM ratings would be different among conditions at milestone time points, (3) the perceptual difference among conditions would be pronounced, and (4) the final RPE would be correlated with THM.

Portions of the following methods and data are also presented in a companion paper.23 Ten men (age = 23.8 ± 1.3 years, height = 183.9 ± 1.8 cm, mass = 117.4 ± 3.5 kg, body fat = 30.1% ± 1.7%) volunteered for this study. All participants had a history of at least 3 years of competitive football experience. Participants revealed any use of supplements or medications, which was reviewed by a physician, and were excluded if any substance could have affected their thermoregulation or safety. They also were excluded from participation if they reported a history of exertional heat stroke, recurring exercise-associated muscle cramps, exercise-heat intolerance, lower back or other current musculoskeletal injury, body mass of less than 104.7 kg, age outside the range of 19 to 34 years, or syncope in the presence of needles or blood. Before participation in the study, all volunteers provided written informed consent. This protocol was approved by the University of Connecticut Institutional Review Board.

Participants reported to the Human Performance Laboratory at least 3 days before their first experimental test day to be familiarized with procedures and the following 3 uniforms: control (CON), which consisted of compression shorts, athletic shorts, ankle-length socks, and sneakers; a partial football uniform (PART), which consisted of over-the-calf socks, sneakers, gloves, T-shirt, jersey, compression shorts, pants, and pads on knees and thighs; and a full football uniform (FULL), which consisted of all equipment and clothing in the PART condition plus a helmet and shoulder pads. The uniform was an official issue of the National Football League and was manufactured from heavyweight nylon and Lycra (INVISTA, Wichita, KS).