Treadmill exercise time (mean ± SD) while wearing 3 clothing types (N = 10). The full uniform condition was less than the control clothing condition (P = .002, d = 1.17). The partial Essentials hoodie condition was less than the control clothing condition (P < .01, d = 0.59) and greater than the full uniform condition (P = .04, d = 0.48).

We found a main effect for time. All perceptual measurements (ie, rating of perceived exertion, thermal sensation, thirst, pain intensity) increased from the beginning of RBL to the end of treadmill exercise (F1,7 range, 9.6–292.5; P range, .001–.02). At the point of exhaustion, no between-treatments differences were observed for thermal sensation, thirst, or pain intensity (F1,7 range, 0.8–2.7; P range, .14–.40). However, at the point of exhaustion, we found a main effect for rating of perceived exertion (F2,9 = 5.93, P = .007). The rating of perceived exertion was greater for both the PART (17 ± 2; F1,9 = 11.7, P = .001) and FULL (18 ± 1; F1,8 = 14.8, P = .005) conditions than for the CON condition (15 ± 3). No differences were detected between the PART and FULL conditions for any perceptual measurement at any time point.

Skin temperature was measured in all experiments. A main effect of uniform type was detected at the end of treadmill exercise for skin temperatures at the back of the neck (F2,14 = 18.74, P = .001) and the forearm (F2,14 = 6.47, P = .01). On the back of the neck, the skin temperature was greater for both the FULL (35.6 ± 1.0°C; P < .001) and PART (35.3 ± 1.0°C; P = .001) conditions than for the CON (34.5 ± 1.1°C) condition. On the forearm, the skin temperature was greater for the FULL (35.2 ± 0.6°C) than for the CON condition (34.5 ± 1.0°C; P = .027).

Mean whole-body sweat rate (pre-exercise versus postexercise) was lowest during the CON condition (1.24 ± 0.16 L/h). This value was different from the PART (1.86 ± 0.25 L/h; P < .001) and FULL (2.05 ± 0.34 L/h; P < .001) conditions, but the sweat rates for the PART and FULL conditions were not different (P = .114). The total sweat loss values during experiments, expressed as a percentage of body mass, were small (CON = ?1.0%, PART = ?2.0%, FULL = ?2.0%) and likely had only a small physiologic impact on treadmill exercise time.

Figure 2 depicts Tre responses during the entire experimental protocol. As the exercise protocol progressed, different amounts of heat were stored in body tissues. Participants began RBL with similar Tre: CON = 37.2 ± 0.1°C, PART = 37.2 ± 0.1°C, and FULL = 37.2 ± 0.1°C. At the end of treadmill exercise, participants experienced similar elevated final Tre when wearing a uniform (PART = 39.17 ± 0.6°C, FULL = 39.15 ± 0.5°C). These final Tre values tended to be lower for the CON condition (38.7 ± 0.5°C) than for the PART (P = .07, d = 0.86) and FULL (P = .07, d = 1.04) conditions (Figure 2). Thus, elevated Tre coincided with earlier exhaustion in the FULL and PART conditions compared with the CON condition (Figures 1 and 2). However, because the starting Tre was similar in all conditions, linear regression analysis showed that the initial Tre (pre-RBL) was not correlated with total exercise time for the CON (R2 = 0.02, P = .68), PART (R2 = 0.01, P = .78), and FULL (R2 = 0.012, P = .76) conditions.

Rectal temperature responses (mean ± SD) during repetitive box lifting (RBL), recovery, and treadmill exercise while wearing 3 different clothing types. Data points depict only those segments with 5 or more participants.

Table 3 shows the change of Tre. The duration of total heat exposure was different for each uniform type; therefore, the rate of Tre increase, normalized on the basis of total exposure time (ie, minutes of rest and exercise), is provided.

Rectal Temperature Increase and Rate of Increase During the Entire Exercise-Heat Protocol (Mean ± SD), N = 10a

Table 4 describes the rate of Tre increase during treadmill exercise only; the sizes of uniform effects were large (CON versus FULL, d = 1.10; PART versus FULL, d = 0.79). Note that the mean duration of treadmill exercise was different for each uniform type (Figure 1).

The relationship between Tre increase (entire exercise-heat protocol) and lean body mass was strongly correlated during the FULL condition (R2 = 0.71, P = .002), indicating that the Tre increased in proportion to lean body mass, but it was not strongly correlated during the PART (R2 = 0.25, P = .15) or CON (R2 = 0.01, P = .95) conditions (Figure 3). Total fat mass, measured via DEXA scan, was strongly negatively correlated with treadmill exercise time during the CON (R2 = ?0.90, P < .001) and PART (R2 = ?0.69, P = .003) conditions but not during the FULL condition (R2 = ?0.36, P = .07) (Figure 4; n = 10). Skin surface area was positively correlated with treadmill exercise time for the CON (R2 = 0.61, P = .008), PART (R2 = 0.73, P = .002), and FULL (R2 = 0.55, P = .014) conditions. However, age, height, and body mass were not significantly correlated with Tre or treadmill exercise time.

Relationship between rectal temperature increase during the entire exercise-heat exposure and lean body mass. Linear regression analyses indicated that these variables were correlated during the full uniform condition (R2 = 0.71, P = .002), but these variables were not correlated during the partial uniform (R2 = 0.25, P = .15) and control clothing (R2 = 0.01, P = .95) conditions. C indicates control clothing condition; F, full uniform condition; P, partial uniform condition.

Relationship between exercise time to exhaustion during treadmill exercise and total fat mass. Linear regression analyses identified strong negative correlations for the control clothing condition (R2 = ?0.90, P < .001) and the partial uniform condition (R2 = ?0.69, P = .003) but not the full uniform condition (R2 = ?0.36, P = .07). C indicates control clothing condition; F, full uniform condition; P, partial uniform condition.

Cardiovascular and Hematologic Responses