THE EFFECT OF WHOLE-BODY VIBRATION ON HUMAN PERFORMANCE AND PHYSIOLOGICAL FUNCTIONS

Transcription

1 Industrial Health, 1977, 15, 13. THE EFFECT OF WHOLE-BODY VIBRATION ON HUMAN PERFORMANCE AND PHYSIOLOGICAL FUNCTIONS PART 1. EXPERIMENTAL STUDY OF THE EFFECT OF VERTICAL VIBRATION ON HUMAN SLEEP Kazuhide YAMAZAKI Railway Labour Science Research Institute, Japanese National Railways, Izumi-cho, Kokubunji-shi, Tokyo 185 Japan (Received March 14, 1977) There have been many reports of the effects of whole-body vibration on physiological function of human body, but there have been no experimental studies on the effect on sleep. Subjects were exposed to very low level of vibration (vertical direction) and the effects on sleep were studied. The subjects were 4 healthy male students and a vibration exposure was done to the subject sleeping on a vibration table once per 30 min for 30 sec. Vibration levels measured on bedclothes were 60, 65, 69, 74, 79 dbvl. Effects of vibration on sleep were investigated by comparing sleep stages before and after the vibration exposure. If the maximum permissible level of vibration on sleep is determined on the basis of the result of this experiment, 65 dbvl will be considered to be appropriate as vibration level not affecting to sleep in the Stage 2 so remarkably. If the maximum permissible level of vibration which does not affect to sleep is necessary to be determined, 60 dbvl will be appropriate. Sleep is one of important functions in physiological function of human body. Recently physiological studies on sleep have developed rapidly and quantitative assessment of sleep has become possible, because judgement of sleep stages by EEG became possible and it was demonstrated that paradoxical sleep exists. Therefore, in the field of environmental hygiene, it became possible to study experimentally the degree of disturbance of sleep due to variable environmental factors using such assessment method. There have already been some reports on sleep disturbance due to noises, but there has no reports on whole-body vibration. While the fact that whole-body vibration disturbs sleep is clear from our daily experiences or the results by questionnaire, there is no experimental report. The following reasons can be considered: 1) There is no good vibration testing system (experimental apparatus) for the ex- 13

2 K. YAMAZAKI periment of the effect of whole-body vibration on sleep. Especially as the subjects must sleep on the vibration table, the table must not generate machine noise, and in addition the size of vibration table is large enough to lie on it. 2) Many days for the sleep experiment are required to test many subjects, in which the days that the subjects can not be taken into sleep on the vibration table are included. Without settling the above-mentioned problem, it is difficult to elucidate the effect of the whole-body vibration on sleep. For this purpose, a vibration testing system to be possible to take lying posture and, moreover, not to generate machine noise, and techniques of long-term experiments must be developed. The authors studied experimentally the effect of whole-body vibration using a comparatively calm vibration testing system for human study1). METHOD 1) Testing system used in the experiment The shaker newly made is an electrodynamic type (AKASHI Co. Japan. Model ASE- 512 L) of which the size of the vibration table was 820 ~ 2000 mm. A plywood board with thickness of 20 mm was fixed on the vibration table by bolts. Two sheets of blankets were spread on the plywood board, on which subjects were asked to sleep. Noise of the shaker measured was db (A) around the head of the subjects who were sleeping. The noise was continuously Neared although sound level was constant even when the vibration as the stimulation was given. 2) Samples of vibration and method of the exposure The level of vibration exposed to the subjects was the same as the vibration in the vertical direction on the ground on which the hammer with power of 1/4 ton was dropped repeatedly. The vibration acceleration of the ground hit by the hammer was recorded on a data-recorder (SONY Co. Model PFM-15). Some appropriate parts for 30 sec were picked out from this tape. The vibration signal for 30 sec was exposed to the subject at the rate of 1 per 30 min while the subject was sleeping. Therefore, the subject was exposed by the vibration 16 times in sleep intermittently for 8 hr. Fig. 1 shows the pattern of vibration waves which appeared on the vibration table. 80 impulses of vibration are observed in 30 sec. (1 impulse per about 0.4 sec). In order to determine the exposed vibration level on the two sheets of blankets, the vibration pick-up set in a box made by aluminum (300 ~ 300 ~ 30 mm) was used2). The pick-up was put on the blanckets, on which a subject was laid. A vibration level meter (RION Co. Model VM-12A) was employed and its vibration level (dbvl) was read out by the meter having the frequency characteristic stipulated by the standard of ISO-2631 which showed the vibration sensation response on the whole-body3). 14

3 EFFECT OF VERTICAL VIBRATION ON SLEEP Fig. 1. Pattern of vibration wave. In this case, the vibration acceleration level (dbal) which meant not passing the frequency weighting circuit of ISO-2631 was defined as 20 logio a/aref, where a; rms vibration acceleration value (m/s2) and aref; 10-5 m/s2. The frequency range observed was from 1 to 90 Hz. The exposed vibration level was determined as the peak value on the vibration level meter. The dynamic characteristics of the meter was "slow" which meant the building up time of 1 sec for switching on the continuous sinusoidal electric signals. The background vibration level was 55 dbvl. Five levels of the vibration such as 60, 65, 69, 74, 79 dbvl were selected for this experiment. 3) Experimental procedures The subjects were 4 healthy male students. The subjects slept in the laboratory at least more than one evening without vibration, and from the next evening they were exposed by vibration. The subjects came to the laboratory around 8 o'clock in the evening. Electrodes for measurement of electroencephalograph (EEG), electrooculograph (EOG), electromyograph (EMG), and electrocardiograph (ECG) were set to them and they put on usual cotton nightwares. They lay on the bed for the vibration testing system at 10 o'clock in the evening, and slept until 6 o'clock in the morning (sleeping hour: 8 hr). Room temperature was maintained at 22 Ž by air conditioner. EEG, EOG, and EMG were measured and recorded according to the method defined by "Association for the Psychophysiological Study of Sleep (APSS)"4) 4) Judgement of sleep stages and assessment of vibrations' influence The above-mentioned APSS method was used for the determination of sleep stages as follows. Stage W: The EEG consisted of a-waves an the waves with low voltage and various frequencies (awakening). 15

4 K. YAMAZAKI Stage 1 : The EEG consisted of the waves with various frequencies and relatively low voltage, but rapid eye-movements were not seen. Stage 2: The EEG consisted of the waves with various frequencies and relatively low voltage, and spindle waves of Hz and K-complex were observed. Stage 3: Slow waves with high amplitude were observed moderately. Stage 4 : Slow waves with high amplitude were observed frequently. Stage REM: The EEG consisted of the waves with various frequencies and relatively low voltage, and rapid eye-momements were accidentally observed. In the electromyogram, low amplitude was observed. In this report, Stage 3 and Stage 4 were graded together as Stage 3. According to the above-mentioned criteria, sleep stages of every 30 sec were judged. When various sleep stages existed in recording for 30 sec, the sleep stage which was observed mainly in the period was judged. Change of sleep stages on the subjects before and after the vibration exposure, clearly showed the influence of vibration of sleep. RESULTS 1) Changes in sleep stages by vibration exposure In the case that the vibration is exposed to the subject in sleep, if he is affected, he will have a poor sleep, but no changes will be observed in sleep stages if he is not affected. In Fig. 2, sleep stages for 30 sec before vibration exposure and sleep stages during Fig. 2. Changes of sleep stages by vibration exposure. 16

5 EFFECT OF VERTICAL VIBRATION ON SLEEP and immediately after the exposure were shown by each exposed vibration level. Stage 1. No subjects experienced awakening in 60 db of exposure, but the majority experienced it in 65 db. All subjects experienced it in 69 db, 74 db and 79 db. Stage 2: No subjects experienced awakening in 60 db, but some subjects experienced it in 65 db. The higher the vibration level is the greater the rate of awakening All subjects experienced it in 79 db. became. Stage 3: No subjects experienced awakening in 60 db and 69 db, but some subjects experienced it in 74 db. Stage REM: No subjects experienced awakening in 60 db and 65 db, but some of them did in 69 db. The response to the vibration of stage REM might be the middle degree between Stage 2 and Stage 3. The rae of awakening by vibration exposure was stated by sleep stages and exposure vibration levels, as shown in Fig. 3. Fig. 3. Rate of awakening by sleep stages at vibration exposure. 17

6 K. YAMAZAKI The poorer the sleep was, the greater the exposed vibration level was. Analysis of variance was made for studying whether the sleep stages immediately before vibration exposure and exposed vibration levels induced some difference to the rate of awakening of the subjects. As the result, sleep stages and exposure vibration levels showed significant effects upon the rate of awakening (p < 0.01). Moreover, the interaction of them was also significant (p < 0.01). 2) Changes in number of heart beat by vibration exposure in sleep Number of heart beat was calculated from the R-R interval of ECG measured in the subjects in sleep, and the change of it by vibration exposure was studied. The rate of quantitative change of heart beat immediately after vibration exposure to the heart beat before vibration exposure was shown in Fig. 4 by exposure vibration levels and sleep stages. As shown in the figure, the higher the exposed vibration level was, the greater than change in the number of heart beat was. In general, the number of heart beat increases when the sleep of the subjects becomes poor or he is waked. Therefore, it is suggested that the higher the exposed vibration levels are, the severer the sleep is disturbed, which is in accordance with the result of studying effects of vibration on sleep from the findings of EEG etc. 3) Perception of vibration of 60 db As the result of experiment in the item (1), it was found that the vibration of 60 db (lid not affect the sleep. From this fact, it is obscure whether the subject does not feel Fig. 4. Changes of number of heart beat by vibration exposure. 18

7 EFFECT OF VERTICAL VIBRATION ON SLEEP the vibration of 60 db or it is not enough to affect his sleep, although the subject does feel the vibration of 60 db. Eleven male students were laid on two sheets of blankets in the same way for the experiment exposed. of sleep, and the vibration of 60 db of a hammer impact on the ground was The exposure times were 5, 10, 15, 30 sec, and the vibration was exposed at random order and intervals. The subjects had to show a signal when he felt the vibration. As the result of exposure of 44 vibrations (in total), 29 vibrations were perceived (65.9%), 15 vibrations were not perceived (34.1%), and about two third of the subjects felt the vibrations. In the exposure times of 5, 10, 30 sec, respectively, 7 of 11 subjects felt the vibrations, and in the exposure time of 15 sec, 8 of 11 felt them. were scarcely any differences between the exposure times. There Two of 11 subjects felt all 4 vibration exposures correctly, 5 of them felt 3 of 4 exposures, 3 felt 2 of 4 exposures, and 1 subject did not feel any vibration in 4 vibration exposures. As the result, the vibration of 60 db of a hammer impact seems to be almost noticed, but sometimes it will not be perceived, and there are a few subjects which feel nothing. It is considered that the vibration of this degree does not affect enough to disturb their sleep even if most of the subjects feel the vibration. do DISCUSSION Since there have been no experimental studies on the effect of whole-body vibration on sleep, the effect on sleep was studied in the normal subjects by exposing the vibration of the ground on which a hammer is working. In order to evaluate the degree of disturbance of sleep due to variable environmental factors, there are two methods, the method to compare sleep stages before and after the vibration exposure and the other method to examine the change in mean value of sleep stages for one night due to exposed vibration levels. Although the latter method is applied on the assumption that the subjects indicate constant pattern of sleep even when vibration is not given, such a thing does not occur in fact. Therefore, the effects of vibration must be studied after grasping the tendency of pattern of sleep thoroughly by repeatedly conducted experiments on the same subject. As discussed before, the experiment for studying the effects of vibration on sleep is inefficient, and in fact, it is impossible to repeat experiments many times. Therefore, in this report, the authors employed the former method (by comparing sleep stages before and after the vibration exposure) for the assessment of the effects of vibration on sleep. As the result, the poorer the subjects sleep was the greater the exposed vibration level was. The higher the exposed vibration level was, the greater the change in the number of heart beat was. 19

8 K. YAMAZAKI From those results, how much is the permissible whole-body vibration level of vertical direction against normal sleep of man on the bedclothes? It is necessary to know the pattern of normal sleep of man for considering the problem. The authors let the subjects have natural sleep in the sound-proof chamber maintained at proper temperature by an air conditioner in August, 1973, and studied the state of normal sleep stages by the same criteria with those in this experiment. The subjects were 6 healthy male students. Since each of them had measurement at 4 nights, data of 24 nights in total were obtained. Sleep stages per 30 sec were judged on the data during 8 hr of each night and the total was obtained as shown in Table 1. The state of 45.4% of sleep (for 8 hr) belonged to the sleep of Stage 2. In summing up the sleep of Stage 3 to it, the state of 62.8% of sleep became a deep sleep of more than moderate. Table 1. Rate of Each Sleep Stage at Normal Sleep The state of 11.2% of sleep belonged to Stage 1, which was only seen in the beginning of sleep. Therefore, when the permissible maximum value of vibration is considered, it seems that the vibration level with the degree which scarcely disturbs the sleep of Stage 2 must be regarded as the permissible maximum value. From the result of this experiment, the vibration level was determined as 65 db. In vibration exposure of 65 db measured on the bedclothes, the following results were obtained. In Stage 1, 71.4% of the subjects experienced awakening, and the rest (28.6%) did not experience any change. In Stage 2, 4.2% of the subjects experienced awakening, and the sleep of 25.0% of them became Stage 1. The rest (70.8O) did not experience any change. No effects of 65 db of exposure were observed in Stage 3 and Stage REM, and in addition, no significant effects on the number of heart beat were obtained. The vibration level as the permissible maximum value more strictly, which does not influence to sleep, is determined as 60 db. Since this experiment was conducted using healthy male adults as the subjects, special consideration must be paid on the subjects such as sickly men, old men, and children. However, there are no data yet for elucidating such kind of problem. 20

9 EFFECT OF VERTICAL VIBRATION ON SLEEP CONCLUSION The effects of vibration of a hammer (vertical direction) on sleep were experimentally studied. ( 1 ) The poorer the subjects sleep and the higher the exposed vibration level was, the severer the disturbance of sleep and the higher the rate of awakening was. ( 2 ) The higher the exposed vibration level was, the more the number of heart beat increased. ( 3 ) The permissible maximum vibration level is considered to be 65 db measured on the bedclothes as vibration level which scarcely disturbs the sleep of Stage 2 in 8 hr-sleep. Moreover, it seems that the permissible value determined more strictly is 60 db. ACKNOWLEDGEMENT The authors thank Dr. T. Miwa (National Institute of Industrial Health) for his advice. This investigation was supported by research fund for prevention of public nuisance in Environmental Agency of Japan. REFERENCES 1) Yamazaki, K. (1975). Bull. Railway Labour Sci. Res. Inst., No. 29, 41. (in Japanese) 2) Miwa, T. and Yonekawa, Y. (1971). Ind. Health, 9, 81. 3) ISO. (1974). IS (E). 4) Rechtschaffen, A. and Kales, A. (ed.) (1968). A Manual of Standardized Terminology, Techniques and Scoring System for Sleep Stages of Human Subjects. Public Health Service, U.S. Government Printing Office, Washington. (Translated into Japanese by Dr. S. Kiyono, Japan) 21