This study was conducted as a randomized, controlled, parallel-group experiment. The local ethics committee stated the trial as to be in accordance with the ethical standards set by the Declaration of Helsinki including its modifications [20]. Informed consent was signed by every participant prior to study enrollment.
Adult volunteers were recruited via public tendering in a university setting. All participants were healthy, non-heat-acclimated and reported regular engagement in physical activity (> 150 min per week). Exclusion criteria included functionally restrictive metabolic or acute illnesses. Chronic disease affecting the cardiopulmonary system, infections or drug abuse excluded from participation as well.
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 encompassed a baseline examination at day 1 including documentation of medical history, questionnaire-based recording of participants` health and fitness status as well as anthropometric assessments and a cardio-pulmonary exercise test until volitional exhaustion.
Main examination at day 2 (2–7 days washout in between day 1 and 2) was an endurance exercise test with fixed intensity for a maximum period of 45 min. During both examinations, participants exercised on a bicycle ergometer (Excalibur-Sport, Lode, Groningen, Netherlands). Workload measured in Watt was recorded automatically. Heart rate (HR) was continually measured via chest strap and registered as a 5-second mean value on a corresponding watch (RS800/CX, S810i, S610i, Polar Electro). Respiratory gas parameters were recorded by using a breath-by-breath analyzer (Oxycon Mobile, Viasys Healthcare GmbH, Würzburg, Germany). Again, five second mean values were analyzed. Patients wore a rubber face mask through which the respired air was transferred into a ventilation turbine and further directed to the portable device containing O2 and CO2 gas analyzers. Relative oxygen consumption (VO2) and carbon dioxide output (VCO2) data were telemetrically transmitted to a computer. Prior to every testing, the mobile gas analyzing device has been calibrated using reference gases (ambient air, 5% CO2, 16% O2) as well as automatized 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 Douglas Bag method) [21]. According to Perret and Mueller`s recommendation the same spirometry system has been used across all examinations [22]. In addition to that Rate of perceived exertion was assessed in both examinations by using Borg- Scale (RPE; 6 [no exertion] to 20 [maximal exertion]) [23].
Two kind of short-sleeve shirts and a cooling vest have been chosen for the experiment. One of the short-sleeve shirts consisted of 100% cotton whereas the other one was made of 100% polyester with wicking finish (Decathlon, France). Participants were instructed to wear a shirt with a close but comfortable cut and chose the shirt size (ranging from XXS to XL) ad libitum.
The third experimental garment was a sleeveless cooling vest (Idenixx, Germany) providing a tight fit to the torso and integrating front and backside cooling elements. The upper material of the vest was a polyester (83%) elastane (17%) mix and the cooling elements were made of a polyester fleece. Cooling elements were activated by water immersion. Vest evaporization is intended to add to body´s endogenous evaporative cooling.
Volunteers had to undergo a spirometer-based cardio-pulmonary exercise test on a bicycle ergometer to determine individual performance capacity. A ramp-shaped protocol, adjusted to one´s fitness level, was applied to reach volitional exhaustion within 10–12 min. The initial workload was set at 50 W and was increased individually by 10, 15, 20 or 25 W every minute based on participants questionnaire-based report regarding fitness status. The test protocol was in line ACSM´s guidelines for exercise testing and prescription [24]. Participants were allowed to familiarize themselves with the bicycle ergometer and the test protocol.
Criteria defining maximum exhaustion have been: (1) respiratory exchange ratio (RER) > 1.10, (2) achieving age-dependent maximum heart rate, (3) rate of perceived exertion (RPE) via Borg-Scale ≥ 17 [17,18,19,20], (4) maximum O2 breathing equivalent (< 30) [25].
Maximal oxygen uptake (VO2max) was determined by the software by identifying the highest thirty seconds floating mean of oxygen uptake during the whole test [26]. Verification took place manually by the investigator. The parameter was used to ensure homogenous assignment of testing conditions. Participants were ranked according to their VO2 max. Groups of three have been formed top down. These groups of three participants were used as stratification grouping for the subsequent block randomization into the three testing conditions.
Respiratory compensation point (RCP) has been detected for each participant by means of the 9 Panels Board and identifying (1) non-linear increase in ventilation (VE) compared to linear increasing or non-increasing carbon dioxide emission (VCO2); (2) non-linear decreasing end tidal CO2 partial pressure (PETCO2) as well as an increase in breathing equivalent for CO2 [27, 28]. Interpretation of graphic depictions, as described above, is an established approach [27, 28] and has been executed by two independent investigators.
Prior to main examination all participants were instructed to prepare for exercising in the heat via sufficient hydration (minimum of 1.5 L/day; 0.5-liter prior testing). During trial volunteers were not allowed to drink water. After 5-minute-resting phase Bioimpedance Analysis (BIA) was performed by 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). Thereupon body weight was determined by a customary digital scale in kg. Probands have been weighed only wearing underwear and socks. Sport shorts and the randomly assigned upper body clothing option have been weighed separately.
Endurance exercise-test at day 2 was conducted in a room with air conditioning and humidity regulation. We applied standardized hot ambient conditions defined by a temperature of 30.5 °C (tolerable range of 1 °C) and relative air humidity at 43% (tolerable range of 13%). Humidity and temperature were controlled using a thermometer and a hygrometer. During the endurance test the upper body was covered by either one of the three experimental garments. Due to the decisive feel and weight, the testing garment could not be blinded to the participant nor the investigator. Participants performed on the same bicycle ergometer as in baseline examination with identical bike settings as documented within the first examination. They tried to complete a 45-minute ride with the workload of 80% of RCP. Volunteers have been instructed to keep the cadence above 60 rpm. If this limit was permanently fallen below, the test had to be classified as terminated due to volitional exhaustion. Corresponding termination time was noted as outcome (exercise performance in minutes). The time limitation to maximal 45 min of exercise was set due to safety reasons.
In addition to heart rate (beats per minute [bpm]) and oxygen uptake (milliliters per kg bodyweight per minute [ml/kg/min]) inner ear temperature was measured by using a digital infrared ear thermometer (Braun ThermoScan, Mexico) to represent the outcome core temperature (degrees Celsius [°C]). All measurements during all timepoints were conducted by the same investigator using the same thermometer. As self-reported data outcomes we captured Rate of Perceived Exertion via Borg scale (6 [no exertion] to 20 [maximal exertion]) [23] and Feeling Scale (+ 5 [very good] to -5 [very bad]). Moreover sensations regarding temperature (0 [unbearably cold] to 8 [unbearably hot]), sweating (0 [not at all] to 3 [heavily sweating]), clothing humidity (0 [no sensation] to 3 [wet]) and skin wettedness (0 [dry] to 3 [too wet]) [14] were assessed. All outcomes, except Exercise Performance, were documented at rest before the test, at 5 min-intervals while cycling and when terminating the trial. To create a realistic (outdoor-exercise mimicking cycling speed) scenario, airflow was simulated using a fan, located 49 cm in front of the ergometer, directing 20 km/h airflow towards the upper body [29]. Air flow was controlled using a wind sensor.
Statistical analysis was executed by using Prism (Version 9.1.0, GraphPad Software, LLC) and Jamovi (Version 1.6.23.0). A survival time analysis has been implemented by using a 3-group Kaplan-Meier estimator. Log-Rank-test was applied for between groups examination. For both analyses the dependent variable was the duration for individual test termination. Baseline data (cardiopulmonary exercise test, anthropometric measures), 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 Humidity- and Skin Wettedness Sensation) have been analyzed by using Kruskal Wallis Tests (non-parametric analysis of variance due to non-normal distribution of residuals) and Dwass-Steel-Critchlow-Fligner pairwise comparisons (post hoc test). Time series analysis for objective and self-reported outcomes during exercise were conducted on the basis of 95%-confidence interval comparisons for a maximum of nine time points (5, 10, 15, 20, 25, 30, 35, 40 and 45 min) [30]. Pre to post exercise differences in body and clothing weight were analyzed using Student`s t-test. A p-value cutoff of 0.05 was set for significance testing.
