Participants were also familiarized with several psychophysiologic scales. We used a shortened and modified version of the ESQ.19 This 16-item version was modified to make it more applicable to the sensations associated with hot environments.20,22–24 The version contained statements, such as “I feel warm” and “I have a headache,” which the participants rated from 0 (not at all) to 5 (extreme) using pen and paper. Participants completed this questionnaire once during familiarization. We also administered the RPE,10 THM,9,17 TST,16 and MPN scales before exercise, during exercise, and postexercise during the familiarization session.25

The RPE scale is based on the theory that “man reacts to the world as he perceives it, and not as it ‘really is’”10; scores range from 6 (no exertion at all) to 20 (maximal exertion). The THM instrument subjectively rates an individual's thermal sensation; it ranges from 0 (unbearably cold) to 8 (unbearably hot), with the center point corresponding to a score of 4 (comfortable). The TST measures the thirst sensations that an exercising individual experiences; it ranges from 1 (not thirsty at all) to 9 (very, very thirsty). Each participant was instructed that TST is separate from THM, and an increase in THM does not assume an increase in TST. The MPN is used to rate muscle pain encountered during exercise; it is a 13-point incremental scale ranging from 0 (no pain at all) to 10 (extremely intense pain, almost unbearable), with additional values for 0.5 and an optional point placed outside point 10 to denote unbearable pain. Participants were informed that MPN is independent of RPE.25

At the familiarization session, we recorded each participant's age, height, and mass without the uniform Baseball Uniforms. To familiarize participants with the exercise protocol, they donned the FULL condition in an environmental chamber (model 2000; Minus-Eleven Inc, Malden, MA) at 23°C (73.4°F). Each participant was taught proper lifting form (ie, keeping feet close to the box, bending knees, and holding head up to limit the stress placed on the lower back). They completed 5 minutes of repetitive box lifting (RBL)26 at the experimental pace (10 lifts/min). The RBL involved lifting a 20.4-kg (45-lb) metal box with handles, walking 2.4 m, and placing it on a platform 1.32 m from the floor. An investigator (E.C.J.) slid a second identical box down a ramp from the platform to the starting position as the participant carried the first box to the platform. Another investigator (M.S.G.) provided constant pacing feedback by counting “3, 2, 1, lift” before each box lift. Participants were informed as each minute milestone passed. The participants recovered by sitting for 5 minutes, and then they performed 10 minutes of treadmill (model 356i; Precor Inc, Woodinville, WA) walking at 5.63 km/h on a 5% grade.

Three trials separated by at least 3 days and by no more than 7 days were completed in a randomized, counterbalanced order. Participants completed a 24-hour diet record before each trial and were instructed to consume the same foods at the same times before each trial. Upon arrival at the Human Performance Laboratory, participants consumed a small standardized breakfast (1 whole-wheat bagel, 1 oz [28 g] cream cheese, 2 medium bananas, and 250 mL of water) and were allowed 1 hour for digestion. Participants provided urine samples, which were analyzed using a refractometer (model A300CL; Spartan, Tokyo, Japan), to confirm euhydration via urine specific gravity (Usg).27 Participants were not tested if Usg was equal to or more than 1.028. Next, participants entered the environmental chamber that was maintained at 33°C (91.4°F) and 48% to 49% relative humidity and stood for 10 minutes to allow blood pressure to equilibrate. Participants completed 10 minutes of RBL, 10 minutes of seated rest, and up to 60 minutes of treadmill walking (5.63 km/h, 5% grade) (Figure 1).

Protocol schematic. a Repetitive box lifting consisted of lifting a 20.4-kg metal box with handles and walking 2.4 m to place it on a platform 1.32 m from the floor at a rate of 10 lifts per minute for 10 minutes. b Treadmill walking occurred at a pace of 5.63 km/h at 5% grade for 60 minutes or until volitional fatigue occurred.

Participants completed the modified ESQ before and after completion of each experimental trial. The RPE, THM, TST, and MPN were administered before and after the RBL task, each 20 minutes throughout treadmill walking, and at cessation of exercise.

Body mass was measured with the participants wearing shorts only before RBL and at cessation of exercise, after they had exited the chamber and removed all components of the football uniform. We used a digital scale (model BWB-800S; Tanita Corporation, Tokyo, Japan), measuring to the nearest 50 g. This measurement was used to determine sweat rate.We measured rectal temperature (Tre) via a rectal thermistor (model 401; Yellow Springs Instruments, Yellow Springs, OH) inserted 10 cm past the anal sphincter, forearm and posterior neck Tsk via an infrared temperature scanner placed against the skin (Ototemp HTTS-3000; Exergen Corporation, Watertown, MA), and heart rate via telemetry (Polar Electro, Port Washington, NY) (for participant safety) before RBL and every 5 minutes until exhaustion or completion of treadmill walking. Participants were not tested if their resting Tre was equal to or more than 37.8°C. Rate of Tre rise (RTre) was calculated by first determining the net change in Tre from the initial to the final time point and then dividing that value by the total time of the trial. To calculate the estimated time to reach the Tre limit of 40°C, we used the RTre values and the mean observed initial Tre in the following equation:

where T is time to reach 40°C (minutes) and iTre is the mean initial Tre for the respective trial.

Internal uniform humidity was measured with a wand stylus (Digi-Sense Humidity Meter; Cole-Parmer Instrument Company, Vernon Hills, IL), which has a resolution of 0.1% and an accuracy of ±2% relative humidity. This humidity meter was calibrated by the manufacturer before shipment and was used soon after purchase. Uniform humidity measurements were taken by inserting the humidity probe under the back of the participant's jersey and T-shirt in the lumbar region of the lower back (being careful not to rest the probe against the