Subjects
The subjects were 10 healthy male volunteers, who were accustomed to weekly sauna baths. All subjects were physical education students and were involved in recreational physical activity during the previous months. Before the actual experiments, their health-status was checked by a health questionnaire. The subjects had a mean age of 25.3 ± 8.4 years, mean height of 1.78 ± 0.07 m, mean body mass of 79.6 ± 7.5 kg, mean hemoglobin of 160 ± 10 g/l, and mean hematocrit of 0.46 ± 0.02.
Experimental procedure
The subjects participated in six sauna bath experiments each one separated by 1 week. On the morning, at 08–09 AM, of the experimental sauna bath days (in March–April), the subjects arrived to the lab after 10–12 h fasting. First a fasting blood sample was obtained (basic health parameters not shown here) and then they had a light breakfast. At least 1 h after eating all sauna measurements were performed between 09–12 AM. No food, drink (except 5 dl water in the sauna) or smoking was allowed until the end of the experiment. No other own sauna baths were allowed during the whole study period. Furthermore, strenuous exercise was not allowed for 3 days and alcohol for 2 days before the experimental sessions.
In the first experiment, the subjects had 30 min FIRS bathing and then were sat for 30 min at room temperature of 21°C and humidity of 25–30%. Thereafter, the subjects were familiarized with treadmill running, counter movement jump (CMJ), and their one repetition maximum (1RM) in bench press and in bilateral leg press were measured. The next four experiments (strength training session plus FIRS 30 min, strength training session plus no sauna 30 min, endurance training session plus FIRS 30 min, endurance training session plus no sauna 30 min; Figure 1) were performed in a randomized order. At the end of each experiment, the subjects sat for 30 min at room temperature. In the last experiment the subjects had 30 min traditional sauna bathing (TRAD) and then sat for 30 min at room temperature.
FIRS and TRAD sauna bath
Far-infrared sauna bathing occurred sitting in a special FIRS sauna (Radiant FIRS SGC1210BR, Harvia Ltd, Muurame, Finland). Its width is 1.20 m, depth 1.05 m, height 1.91 m, and voltage 230-V. The type of radiator is carbon fibre and the emitted wavelength is 4–17 µm. The temperature in FIRS sauna was set at 35–50°C (35°C at the level of legs and 50°C at the bather’s face) and relative humidity of 25–35%, respectively. In TRAD, the electrically heated sauna temperature was similar 35–50°C, but relative humidity was increased to 60–70% by throwing water on the hot rocks of the sauna heater. In both saunas the subjects were sitting wearing only shorts and they had to drink 5 dl water during 30 min bathing.
Strength training session (STS), isometric strength tests (ISTs), and counter movement jump (CMJ)
A 10 min warm-up consisted both 5 min riding a bicycle with heart rate between 120 and 140 beats/min and 5 min dynamic stretching exercises for whole body. After that, the STS lasted 60 min including dynamic hypertrophic training in bilateral bench press and in bilateral leg press and also IST performances in bench press and in leg press. ISTs were performed before and after the training session, as well as following 30 min of FIRS bath or sitting. The bilateral leg IST was performed on an electromechanical dynamometer (David 210, David Health Solutions Ltd., Helsinki, Finland) using three single maximal trials with 2 min recovery between each test. Similarly the maximal bench press performance was measured on an electromechanical dynamometer (Bench Press, Department of Biology of Physical Activity, Jyväskylä, Finland). The arm angle was 90° in bench press and knee angle was 110° in leg press. Maximal CMJ was performed on a contact mat (Newtest Ltd., Oulu, Finland) three times at each measurement point with a recovery of 3 min. The vertical rise of center of gravity was calculated from flight time (Komi and Bosco 1978). The best result out of three trials was selected in CMJ and in ISTs.
The dynamic bench press and dynamic leg press training included 5 × 10RM with a load, which was evaluated according to the 1RM test in pretest. If the subject could not succeed in the last repetitions in a set the researcher assisted slightly to enable the subject to complete all 10 reps. Recovery was 2 min between sets. Each subject carried out the STS at the same time in the morning in both STS experiments and used similar absolute weights in the dynamic training.
Endurance training session (ETS), VO2 and heart rate
A 10 min warm-up consisted both 5 min riding a bicycle with heart rate between 120 and 140 beats/min and 5 min dynamic stretching exercises for whole body. After that, the ETS lasted 34–40 min including 10 min light aerobic work on a bicycle and then running on a treadmill until exhaustion (range 24–30 min). The treadmill exercise began with an 8 km/h speed and 1° angle of the treadmill. Thereafter, the speed was increased by 1 km/h after every 3 min. All subjects were voluntarily exhausted between 24 and 30 min. During running, gaseous exchange was measured using Sensor Medics Breath Gas Analyzer (Vmax series 229, California, USA). The device was calibrated before every measurement and VO2 was determined as a mean from the final 30 s of every stage. Heart rate was measured by a Polar heart rate monitor (Polar Electro Oy, Kempele, Finland). The same ISTs and CMJ tests were performed before and after exercise and following 30 min of FIRS bath or sitting.
Blood pressure
Brachial systolic and diastolic blood pressures were measured from the arm with an electronic blood pressure monitor (Omron M1, Normomedical Ltd, Helsinki, Finland).
Blood collection and analysis
Blood samples were drawn from the antecubital vein in a sitting position. Analysis included hemoglobin (Hb), serum total testosterone, cortisol, growth hormone, lactate, and pH. Serum samples were kept frozen at −80°C until analyzed. Two milliliters of blood were taken in K2 EDTA tubes (Terumo Medical Co., Leuven, Belgium) for measurements of Hb concentration with a Sysmex KX 21N Analyzer (Sysmex Co., Kobe, Japan). For the determination of serum hormone concentrations, five milliliters of blood were taken into serum separator tubes and the concentrations were analyzed by an immunometric chemiluminescence method with Immulite® 1000 (DPC, Los Angeles, USA). The sensitivities of the assays were 0.5 nmol/l for testosterone, 5.5 nmol/l for cortisol, and 2.6 µg/l for growth hormone. The intra-assay coefficient of variation (CV) was 5.7% for testosterone, 4.6% for cortisol, and 4.2% for growth hormone. Blood samples for lactate were obtained from the fingertip and collected into capillary tubes (20 µl), which were placed in a 1 ml hemolyzing solution and analysed automatically after the completion of testing according to the manufacturer’s instructions (EKF diagnostic, C-line system, Biosen, Germany). pH was analyzed with IL GEM Premier 3000 Blood Gas System (Instrumentation Laboratory, Lexington, MA, USA). The intra-assay CV was 0.1% for pH.
Statistics
Before applying statistical methods, the data was checked for normality by Shapiro–Wilk’s test and the homogeneity of variances by Levene’s test. Then statistical analyses were performed with PAWS Statistics version 20.0 for Windows (SPSS, Inc, Chicago, IL, USA). Differences between conditions were determined through one-way ANOVA. Bonferroni correction was used as a post hoc test. Data are presented as mean ± SD. The statistical difference was considered to be significant at the p < 0.05 level.