Subjects
Ten healthy male students (22 ± 1.2 years, 172 ± 6.20 cm, 62 ± 8.6 kg) participated in this study. Participants were instructed to refrain from any intense muscle activity on the day prior to the experiment. They were asked to refrain from drinking caffeinated beverages, and smoking cigarettes during the 2 h period immediately preceding the experiment. Informed consent for participation in the study, approved by the bioethics committee of the Graduate School of Engineering, Chiba University, was obtained from all subjects.
Procedure
The experiment was conducted from May to July 2010. The air temperature and relative humidity of the pre-room were controlled at 27°C and 50%, respectively. In this study, four different bating methods were investigated: full immersion bath, showering, mist sauna, and no bathing as the control. For each condition, the subjects were controlled as follows. In the full immersion bath condition the subject was immersed in hot water (40°C) to the neck. In the mist sauna condition the subject was splashed with mist of fine drops of warm water (40°C). During showering the subjects were drenched in a sitting position outside the bathtub. Four experiments were conducted at the same time of day on separate days. The protocol consisted of four experimental sessions on four different days which were 1 week apart from session to session. The order of the four conditions was counterbalanced between the subjects. They were directed to wear only swimming trunks. After the subjects entered the pre-room, they were asked to relax for at least 30 min. Each subject conducted the muscle fatigue task for 10 min in the pre-room and bathed for 10 min. Then, subjects rested for 20 min in the pre-room. The procedure of the experiment is shown in Figure 1.
Task
Maximal voluntary contraction (%)
A maximal voluntary isometric elbow flexion of the right arm was performed three times during 5 s. The maximum value of the rest trial was taken as the maximal voluntary contraction (MVC) reference value for that individual. To exclude the influence of cumulative fatigue, we measured MVC two days before the experiments.
The muscle fatigue-inducing task and the rest contraction task
Each subject was asked to sit down on the chair. To adjust to a fixed posture and elbow level against varying height of the subjects, scaffolding was furnished. In the muscle fatigue-inducing task, targets were set on the visual feedback display at 30% of the MVC reference value (30% MVC). The subject was required to maintain a constant 30% MVC level with constant posture of the elbow flexion (90°) as long as he could maintain the contraction level and the posture. In the test contraction task, to elevate fatigue recovery in the electromyogram (EMG), the subjects performed test contractions of 10% MVC for 15 s in the bathtub.
Measurements
Rectal temperature (Tre) and skin temperature (Tsk) at four sites (thigh, foot, forearm, chest) were measured with thermistors. The thermistor probe for Tre was inserted 10 cm beyond the anal sphincter. These temperature data were recorded every 2 s by a data logger (LT-8; Gram Corporation). Mean skin temperature (Tsk) was calculated using the formula of Ramanathan [20]. Surface EMG signals were recorded using surface electrodes (TSD150, BIOPAC Systems) which were placed on the right biceps brachii abraded etanol-cleaned skin.
Near-infrared spectroscopy (NIRS) was performed with a NIRO-300 (Hamamatsu Photonics, Japan) at a wavelength of approximately 700 to 1000 nm with probes placed on the right biceps brachii and monitored continuously. Near infrared spectroscopy (NIRS) is a non-invasive diagnostic tool that facilitates the direct monitoring of oxygen saturation and changes in oxyhemoglobin (O2Hb), deoxyhemoglobin (HHb), and total hemoglobin (tHb) concentration in tissues such as the brain and muscle. NIRS determines hemoglobin concentration and saturation, allowing us to determine changes in tissue oxygenation and blood volume from detecting fluctuations in concentration of O2Hb, HHb, and tHb. NIRS offers unambiguous quantification (by separating absorption from scattering) of tissue oxygenation and provides accurate and immediate information on tissue [21, 22]. We used O2Hb as the index of muscle blood flow.
The skin blood flow on the forearm (ventral) was measured by a laser Doppler flowmeter (ALF21, Advance Co., Ltd.). Subjects were instructed to keep their arm as stable as possible to minimize artifacts due to the movement of the laser Doppler probe. All signals were converted from analog to digital at a 1 kHz sampling rate (A/D converter: BIOPAC MP150), and were stored in a computer (Dynabook EX/56 L, Toshiba). The subjective evaluation of fatigue was measured by the VAS method.
Statistical analysis
Changes (△) of the physiological parameters were calculated by subtracting from the value of the muscle fatigue-inducing task immediately after. In the MNF, the change rate (◇) was calculated by dividing with the value of the muscle fatigue-inducing task immediately after. For the physiological parameter, a two-way repeated measures ANOVA (bathing method factor × time factor) was conducted. When a significant F value was found, we performed a Bonferroni as a post-hoc test. In addition, Pearson’s correlation was used to examine the relationship among the bathroom temperature and Tre in the mist sauna condition as well as among the MNF of the EMG and Tre. All statistical analyses were performed using SPSS 18.0 J (SPSS, Japan). The probability level of 0.05 was taken as indicative of statistical significance. Data are shown as mean ± SD of the mean unless otherwise stated.
