Scientific Papers

A two-week exercise intervention improves cold symptoms and sleep condition in cold-sensitive women | Journal of Physiological Anthropology


Subjects

The subjects were 16 healthy females aged 20–21 who had sensitivity to cold. The subjects had not regularly engaged in exercise or sports activities in their daily lives for at least the past year. The presence or absence of exercise habits was confirmed by interviewing the subjects. The inclusion criteria were answering five or more of a 10-item questionnaire about body coldness and being aware of sensitivity to cold [19]. The reliability and validity of this questionnaire in screening women with cold sensitivity were confirmed in a previous study [20]. According to its inclusion criteria, 35.7% of young women were judged to have sensitivity to cold, which was similar to previously reported rates [20]. The subjects were randomly divided into an exercise group (EXE) (n = 8) who received exercise intervention and a control group (CON) (n = 8) who received no intervention in the order of their application. Height (EXE, 161.1 ± 4.5; CON, 156.3 ± 4.0 cm), weight (52.8 ± 6.2; 49.6 ± 3.8 kg, respectively), and body mass index (20.3 ± 1.5; 20.3 ± 1.0, respectively) were similar between the groups. There were no differences between the groups in sleep status in the month before the intervention using the Japanese version of the Pittsburgh Sleep Quality Index (PSQI-J) (EXE, 5.6 ± 1.5 and CON, 4.9 ± 2.1). None of the subjects had been diagnosed with insomnia; however, three in EXE and three in CON had PSQI scores of 6 or higher, which is considered a possible sleep problem. The daily sleep habits were also confirmed by interviewing the subjects.

This study was implemented after the approval by the Bioethics Committee of Yamaguchi Prefectural University (Approval No. 2021–26). Subjects were informed in advance of the purpose, methods, and risks of the experiment, and written consent was obtained. The experimental results are presented as numbers and have been managed under confidentiality.

Exercise intervention

EXE received a 2-week exercise intervention. The EXE group exercised at least 4 days per week during the intervention period, walking at least 5000 steps per day more than their average number of steps before the intervention. The subjects in EXE were also instructed to perform moderate exercise consisting of at least 15 min of fast walking or jogging to warm up the body in the walking-based aerobic exercise program. The time period and place for the exercise were decided by the subjects. Except for the exercise, the subjects were asked to maintain the same lifestyle during the exercise intervention as before the intervention. In CON, the subjects maintained the same physical activity level as before the intervention.

Experimental conditions and procedures

EEG, subjective sleep state, thermal sensation, thermal comfort, Tsk, core body temperature, and room temperature were measured on the 2 days immediately before and 2 days immediately after the intervention period at the subject’s home. The variables were measured twice (2 days before and after the intervention) and averaged over the 2 days. Thermal sensation, thermal comfort, and temperature variables were measured daily during the experimental period. An air conditioner was used to adjust the room temperature in the bedroom to 18 °C at the time of measurement. Subjects were asked to go to bed between 23:00 and 1:00, to take a bath at least 1 h before bedtime and to soak for no more than 10 min, to eat at least 3 h before bedtime, not to drink alcohol, and to use the same clothing and bedding on the days examined before and after the intervention period. All experiments were conducted in winter (December to February) when cold complaints are likely to occur.

Measurements

Physical activity

All subjects wore a physical activity meter with a built-in three-axis accelerometer (MTN-221, ACOS Inc., Nagano, Japan) except for bathing time, and their physical activity was continuously recorded from 3 days before to 14 days during the intervention period and 2 days after the intervention period (19 days in total). The number of steps and amount of physical activity were analyzed using a dedicated program. On the days when they exercised, the content of the exercise was recorded on a recording form.

Tsk, core body temperature, and room temperature

An infrared thermometer (NIR-10, CUSTOM Inc., Tokyo, Japan) was used for temperature measurement. By switching the measurement mode, this device can instantaneously estimate core body temperature (setting mode: oral temperature) from forehead surface temperature, Tsk, and room temperature with a minimum display temperature of 0.1 °C. The accuracy of measurements was ± 0.3 °C or less. The thermometer was left in a room at 18 °C for at least 10 min for room temperature calibration. Throughout the experimental period, Tsk of the left and right toes and the dorsum of the feet, room temperature, and core body temperature were measured before entering the bed. Foot Tsk was obtained from the average of the left and right Tsk of the toes and dorsum of the feet.

Thermal sensation and thermal comfort

Thermal sensation and thermal comfort were measured immediately before bedding throughout the experimental period using a visual analogue scale (VAS) [19]. Subjects were asked to report thermal sensation for the feet and whole body by marking on a 15-cm line rating scale, which was labeled “cold” 2.5 cm from the left end and “warm” 2.5 cm from the right end. We instructed the subjects to mark on the scale how strongly they experienced the sensation of coldness or warmth. In addition, the subjects were allowed to mark the level of thermal sensation beyond the cold or warm point, if necessary. The scales were labeled “discomfort” and “comfort” instead of “cold” and “warm”. Then, the length from the point of discomfort to the marked point was measured as the rating score of thermal sensation or thermal comfort, and presented as a positive value.

A questionnaire survey concerning thermal sensation in 11 regions of the body (arm, abdomen, hand, fingertip, foot, toe tip, neck, shoulder, back, lower back, and buttocks) was performed immediately before sleep, and before and after the intervention. Thermal sensation was scored using the following scale: “3. feel strong coldness”, “2. feel coldness”, “1. feel coldness a little”, “0. not at all”, “ − 1. feel hotness a little”, “ − 2. feel hotness”, and “ − 3. feel strong hotness”.

Sleep questionnaires

Subjective sleep quality was evaluated using the Oguri-Shirakawa-Azumi (OSA) sleep inventory (middle-aged and aged version) immediately after waking up [21]. This is a self-reported questionnaire composed of 16 items each with a 4-point scale. The items are consolidated into five subscales: factor I “sleepiness on rising”, factor II “initiation and maintenance of sleep”, factor III “frequent dreaming”, factor IV “refreshing”, and factor V “sleep length”. The Zi value was calculated with higher values indicating better sleep quality. Subjects were also asked to report subjective sleep quality for the exercise intervention using VAS. A 15-cm line rating scale, which was labeled “did not sleep at all” 2.5 cm from the left end and “slept very well” 2.5 cm from the right end was used. In both groups, sleep surveys using the OSA sleep questionnaire and VAS were performed twice before and after the intervention.

Sleep EEG

In order to objectively evaluate the state of nocturnal sleep before and after intervention, EEG was recorded from just before lights out to the time of waking using a portable recording system (Sensor ZA-X, Proassist Ltd., Osaka, Japan) [22]. The EEG sensor consists of a telemeter and receiver of a bioamplifier (sampling rate 128 Hz, frequency response 0.5 to 40 Hz). One channel is for monitoring EEG with electrodes on the right forehead and left neck (Fp2-M1), and the second channel is for monitoring electro-oculogram (EOG) and electromyogram with electrodes on the orbit and submental muscles. In all conditions of the experiment, sleep EEG was performed four times, twice on 2 days before and after the intervention.

EEG data was automatically analyzed using a sleep analysis research program (SleepSign-Lite, KISSEI COMTEC Ltd., Nagano, Japan) and the waveforms were evaluated with EOG by visual inspection, which has been reported to be highly reliable [22, 23]. Sleep stages were analyzed by dividing them into waking, rapid eye movement (REM), light sleep (stages N1 and N2), and deep sleep (stage N3) according to the AASM manual for scoring rules Ver. 2.4 [24]. Each sleep stage was carefully checked and corrected by the authors after automatic analysis. The REM sleep phase was determined from eye movements and beta waves. Stages N1 and N2 were grouped together as the light sleep stage: stage N1 was determined as the point at which alpha waves were less than 50% occupied and theta waves became prominent and stage N2 was determined using spindle waves or K-complex as indicators. Stage N3 was determined as a high amplitude delta wave occupancy of 20% or more.

In addition to the analysis of sleep stages, power spectral values of alpha waves (8–12 Hz) and beta waves (16–25 Hz) were analyzed using the sleep analysis research program. The power spectral values were averaged during wakefulness for 2–3 min immediately before sleep and throughout sleeping time. The values were presented as percentages of spectral power of frequency band of 1–25 Hz. In addition, intervention-induced changes in power spectral values were expressed as a percentage change from the pre-intervention baseline.

Statistical analysis

Means ± SD of all measurement items were calculated. A repeated two-way analysis of variance was performed to test the difference between the experimental conditions in each measurement item, and Bonferroni multiple comparison test was performed as a post hoc test when the interaction was significant. An unpaired t test was used to test the difference in pre-intervention means. A level of less than 5% (p < 0.05) was considered significant.



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