This study was conducted as a randomized, controlled, parallel-group experiment. The local ethics committee declared that the trial was in accordance with the ethical standards laid down in the Declaration of Helsinki, including its amendments [20]. An informed consent was signed by each participant prior to enrollment in the study.
Adult volunteers were recruited through public tenders in a university setting. All participants were healthy, not acclimated to heat and reported regular physical activity (>150 minutes per week). Exclusion criteria included functionally restrictive metabolic or acute diseases. Chronic diseases affecting the cardiopulmonary system, infections or drug abuse are also excluded from participation.
A total of 34 participants were included (age = 25; 4 years, height = 1.73; 0.09 m, body weight = 70.3; 13.3 kg).
The trial included a baseline examination on day 1, including documentation of medical history, questionnaire-based recording of participants’ health and fitness status, as well as anthropometric assessments and a cardiopulmonary exercise test to voluntary exhaustion.
The main study on day 2 (2-7 day washout between days 1 and 2) was a fixed intensity endurance exercise test for a maximum period of 45 minutes. During both exams, participants trained on a bicycle ergometer (Excalibur-Sport, Lode, Groningen, The Netherlands). The workload measured in Watts was automatically recorded. Heart rate (HR) was measured continuously via the chest strap and recorded as a 5-second average value on a corresponding watch (RS800/CX, S810i, S610i, Polar Electro). Breathing gas parameters were recorded using a breath-by-breath analyzer (Oxycon Mobile, Viasys Healthcare GmbH, Würzburg, Germany). Again, the five-second average values were analyzed. Patients wore a rubber face mask through which the inhaled air was transferred to a ventilation turbine and further directed to the portable device with O2 and co2 gas analyzers. Relative oxygen consumption (VO2) and carbon dioxide emissions (VCO2) data was sent telemetrically to a computer. Before each test, the mobile gas analyzer is calibrated with reference gases (ambient air, 5% CO216% O2) and automated standard volume. The breath-by-breath analyzer was successfully tested for reliability (coefficient of variation for VO2 = 3.4, and for VCO2= 4.3) and was compared to the gold standard method to assess validity (difference of -4.1, 3.1% and -2.8, 3.5% compared to the Douglas Bag method) [21]. According to Perret and Mueller’s recommendation, the same spirometry system was used in all studies [22]. In addition, in both studies the degree of perceived exertion was assessed using the Borg Scale (RPE; 6 [no exertion] up to 20 [maximal exertion]) [23].
Two types of short-sleeved shirts and a cooling vest were chosen for the experiment. One of the short-sleeved shirts was made of 100% cotton, while the other was made of 100% polyester with moisture-wicking finish (Decathlon, France). Participants were instructed to wear a shirt with a close-fitting but comfortable cut and chose the shirt size ad libitum (ranging from XXS to XL).
The third experimental garment was a sleeveless cooling vest (Idenixx, Germany) that provided a tight fit to the torso and integrated cooling elements at the front and back. The vest’s upper material was a polyester (83%) elastane (17%) blend and the cooling elements were made of a polyester fleece. Cooling elements were activated by immersion in water. The evaporation of the vest is intended to enhance the endogenous evaporative cooling of the body.
Volunteers were required to undergo a spirometer-based cardiopulmonary exercise test on a cycle ergometer to determine individual performance. A ramp-shaped protocol, adapted to an individual’s fitness level, was applied to reach voluntary exhaustion within 10-12 minutes. The initial workload was set at 50 W and was individually increased by 10, 15, 20, or 25 W every minute based on participants’ questionnaire-based report of fitness status. The testing protocol was in line with ACSM guidelines for exercise testing and prescribing [24]. Participants were introduced to the bicycle ergometer and the test protocol.
Criteria defining maximum exhaustion are: (1) Respiratory Exchange Ratio (RER) > 1.10, (2) Reaching an age-related maximum heart rate, (3) Rate of Perceived Exertion (RPE) via Borg scale ≥ 17 [17,18,19,20](4) maximum O2 respiratory equivalent (< 30) [25].
Maximum oxygen uptake (VO2max) was determined by the software by identifying the highest thirty second floating average of oxygen uptake throughout the test [26]. Verification was done manually by the researcher. The parameter was used to ensure homogeneous assignment of test conditions. The participants were ranked based on their VO2maximum Groups of three are formed from above. These groups of three participants were used as stratification grouping for the subsequent block randomization in the three test conditions.
The respiratory compensation point (RCP) was detected for each participant using the 9 Panels Board and identifying (1) non-linear increase in ventilation (VE ) compared to linearly increasing or non-increasing carbon dioxide emissions (VCO2); (2) non-linearly decreasing end-tidal CO2partial pressure (PANDCO2) as well as an increase in the respiratory equivalent for CO2 [27, 28]. Interpretation of graphics, as described above, is a well-established approach [27, 28] and was executed by two independent investigators.
Before the main study, all participants were instructed to prepare for exercise in the heat by providing adequate hydration (minimum 1.5 L/day; pretest 0.5 L). During the test, volunteers were not allowed to drink water. After a 5-minute rest phase, Bioimpedance Analysis (BIA) was performed using a tetrapolar device (Nutriguard-MS, Data Input, Darmstadt, Germany) with single frequency (50 kHz). Resistance (R) and reactance (Xc) in Ohms (Ω) were processed by Nutriplus software (Data Input, Darmstadt, Germany). Body weight in kg was then determined using a conventional digital scale. Probands were weighed only while wearing underwear and socks. Sports shorts and the randomly assigned upper body clothing option were weighed separately.
The endurance exercise test on day 2 was performed in an air-conditioned and humidified room. We applied standardized warm environmental conditions, defined by a temperature of 30.5 °C (acceptable range of 1 °C) and a relative humidity of 43% (acceptable range of 13%). Humidity and temperature were monitored using a thermometer and a hygrometer. During the endurance test, the upper body was covered by one of three experimental garments. Due to its decisive feel and weight, the test garment could not be blind to the participant and the experimenter. Participants performed on the same cycle ergometer as at the baseline study with identical bike settings as documented during the initial study. They attempted to complete a 45-minute ride with a workload of 80% of the RCP. Volunteers are instructed to keep the cadence above 60 rpm. If this limit was permanently undershot, the test had to be classified as terminated due to voluntary exhaustion. The corresponding termination time was recorded as the outcome (exercise performance in minutes). The time limit to a maximum of 45 minutes of practice was imposed for safety reasons.
In addition to heart rate (beats per minute [bpm]) and oxygen uptake (milliliter per kg body weight per minute). [ml/kg/min]) Inner ear temperature was measured using a digital infrared ear thermometer (Braun ThermoScan, Mexico) to display the core temperature outcome (degrees Celsius) [°C]). All measurements at all time points were performed by the same researcher using the same thermometer. As self-reported data results, we recorded the level of perceived exertion via the Borg scale (6 [no exertion] up to 20 [maximal exertion]) [23] and feelings scale (+ 5 [very good] to -5 [very bad]). In addition, there are sensations related to temperature (0 [unbearably cold] to 8 [unbearably hot]), sweating (0 [not at all] to 3 [heavily sweating]), clothing moisture (0 [no sensation] to 3 [wet]) and skin moisture (0 [dry] to 3 [too wet]) [14] were assessed. All outcomes, except exercise performance, were documented at rest before testing, at 5-min intervals during cycling, and at trial termination. To create a realistic scenario (outdoor exercise simulating cycling speed), airflow was simulated using a fan, located 49 cm in front of the ergometer, which directed an airflow of 20 km/h to the upper body. [29]. The air flow was controlled using a wind sensor.
Statistical analysis was performed using Prism (version 9.1.0, GraphPad Software, LLC) and Jamovi (version 1.6.23.0). A survival time analysis was implemented using a 3-group Kaplan-Meier estimator. A Log-Rank test was requested between the groups. For both analyses, the dependent variable was the duration of the individual test termination. Basic data (cardiopulmonary exercise test, anthropometric measurements), pre- and post-exercise data for objective variables (heart rate, inner ear temperature, VO2) as well as self-reported parameters (RPE, feeling scale, thermal, sweating, clothing wetness and skin wetness feeling) were analyzed using Kruskal Wallis tests (non-parametric analysis of variance due to non-normal distribution of residuals) and Dwass-Steel-Critchlow pairwise comparisons -Fligner (post hoc test). Time series analysis for objective and self-reported outcomes during exercise was performed based on 95% confidence interval comparisons for up to nine time points (5, 10, 15, 20, 25, 30, 35, 40, and 45 minutes). ) [30]. The differences in body and clothing weight before and after training were analyzed using Student’s t-test. A p-value cutoff of 0.05 was set for significance testing.
