Sixteen physically active adults participated in this study (8 males and 8 females) (18–30 years old). All were enrolled in strength training in the last 2 months and were classified as apparently healthy subjects, with negative responses to all PAR-Q questions,17 and classified as having a below-average coronary risk.18 Throughout the data collection protocol, the Brazilian ethical standards for research with human beings were followed according to Law 466/12, which is in line with procedures and methods used for the ethical standards of experiments with humans proposed by the Declaration of Helsinki.19

The following exclusion criteria were considered: smoking subjects, musculoskeletal injuries in the last two months, skin burns (in the analyzed body regions), pain symptoms in the evaluated areas, skin allergies, sleep disorders, fever in the last seven days, medical or physical therapy with creams, ointments or lotions, as well as the use of antipyretics and/or diuretics, any nutritional supplement that could change the water body homeostasis or body temperature in the last two weeks, or eating disorders (bulimia and anorexia).20Table 1 shows the anthropometric characteristics of the participants.

Fig. 1 shows the four phases in which the study was developed.

Figure 1.

Scheme of the experimental design of the study.

(0.15MB).

In PHASE 1, an anthropometric assessment was performed to characterize the participants. Moreover, the 1RM test was carried out following the recommendations proposed by Marins and Giannichi,22 in three exercises: dumbbell biceps arm curl, lat pulldown, and leg press. First, a 10-min organic warm-up was performed on a cycle ergometer. Subsequently, participants were familiarized with the strength exercises. Finally, participants underwent the 1RM test; they were asked to perform one repetition with adequate technique throughout the range of motion, in the cadency 2:2 (2s in the concentric phase and 2s in the eccentric phase) until reaching the maximum load.23

At least 48h after the 1RM test, participants came back to the lab for the load calibration test (PHASE 2). On this day, the warm-up period included an organic phase on a cycle ergometer for 5min and then a local phase with a circuit of 1 set of 15 repetitions in each exercise at 40% 1RM. The calibration test consisted of performing 1 set of 12 repetitions in each exercise with an intensity between 70% and 80% 1RM.

After a minimum period of 24h from PHASE 2, the main phase of data collection was carried out (PHASE 3). Before starting the execution of the exercise session, participants underwent the same warm-up as in the previous phase. All the exercises were performed for 3 sets of 12 repetitions in a circuit format with the load obtained in the confirmation of the 1RM performed in PHASE 2 (70–80% 1RM). The passive rest interval between exercises was of 30–45s and between sets was 90–120s. Participants were asked to follow the cadence 2:2 during the exercise execution. Finally, PHASE 4 consisted of returning to the laboratory 24 after the end of the training for the acquisition of new thermal images. To standardize the effect of the menstrual cycle on thermoregulatory responses, the experimental protocol was carried out using the first day of menstruation as a baseline. Therefore, data collection started up to 72h after the start of menstruation.

The thermography protocol to register the body regions of interest (ROIs) was performed with a FLIR® (T420) imager, with a measuring range of −20 to +120°C, 2% accuracy, sensitivity ≤0.05°C, an infrared spectral band of 7.5–13μm, autofocus and a refresh rate of 60Hz and resolution of 320×240 pixels, setting the emissivity at 0.98. The first thermogram was taken at rest before starting the exercise session and the second thermogram was obtained approximately 20min after the end of the training. The thermograms were obtained in a room with controlled temperature (21.3±0.9°C) and humidity (59.2±2.4%), as recommended by Fernández-Cuevas et al.20 An anti-reflective black background was used behind the participant to minimize any reflection from infrared radiation.24 To obtain the thermograms, an acclimation period longer than 10min was considered.25

Two thermograms of the anterior and posterior views were obtained (Fig. 2). Through Flir Tools® 4.1 software and following the recommendations of Marins et al.,26 the ROIs considered in this study were delimitated in the thermograms by selecting the corresponding areas of the biceps brachii (between the axillary line and 5cm above the elbow), the upper back (over both scapulae) and quadriceps (between a point 5cm above the upper edge of the patella and occupying the greatest length of quadriceps).

Figure 2.

Examples of thermograms collected in men and women: at rest, after exercise, and after a 24-h recovery period. The considered body regions of interest can be viewed.

(0.81MB).

Since the Shapiro–Wilk test showed that the data were normally distributed, a descriptive statistic was used with mean, standard deviation, and maximum and minimum values. To analyze the temporal effect of exercise, ANOVA with post hoc Tukey's test was performed. The unpaired t-test was used to compare the thermal responses between men and women. Statistical analyses were performed in SPSS Statistics Subscription® version 22, setting the level of significance at α=0.05.