I. Introduction

Vertigo, dizziness and giddiness are one of frequent complaints met in home doctors, and the sensation of vertigo occurs so frequent especially for aged people. In home doctors, most of the patients with vertiginous complaints were consulted to a nearest otologist or neurologist, or were to start medical therapy without an established diagnosis. Major vertigious patients were far from serious pathology like as acoustic tumor, cerebellar infarction or bleeding^［1］, and minor patients were diagnosed as Ménière disease^［2］, vestibular neuronitis, or benign paroxysmal positional nystagmus^［3］. Most vertigious patients have smooth prognosis with the conservative therapy of medication. Aged people are known to show declines in postural control during not only walking but also standing^［4-5］. And accidental falls sometime cause aged people serious outcomes^［6-7］. Many studies about postural control for aged people had reported a comparison between aged and young groups^［8-11］. And, most of the reports had discussed about the postural control only on a horizontal plane. We studied postural control for aged people in a vertical direction and on a horizontal plane, by testing the coefficient correlations among age, body weight （BW）, body height （BH） and the postural control.

II. Materials and Methods

Subjects

Eighty volunteers, non vertigious patients who had been medicated for hypertension or hypercholesterolemia, were participated in this study. Informed consent was obtained from all subjects after fully explanation of this study. Mean age was 70 years and standard deviation was 11.5 （62 females and 18 males）. They had no past history of vestibular diseases such as Ménière’s syndrome, vestibulitis syndrome, nor benign paroxysmal positioning vertigo. Mean BW was 56 Kg （standard deviation; ±10.4）, and mean BH was 155 cm （±8.0cm）.

Measurements

Center of foot pressure （COP） was studied as for postural control, which was measured on a foot plate with 3 load cells^［12］. Three signals from the cells were sampled at 20 Hz into 12 bit, and they were transformed into 3 signals; 2 were on a horizontal plane and one was in a vertical direction. COP-oscillations in the medial-lateral （M-L） and anterior-posterior （A-P） directions were studied on a horizontal plane, and those in the vertical direction were studied as a signal summed with the 3 load cells. Path length and path area of COP were studied on the horizontal plane. The path area was calculated with 2 shifts and a mean of COP-shifts on the horizontal plane.

Tasks

Simple 2 tasks were asked to the subjects; one was to stand upright on the plate quietly with eyes open for 20 sec, and another was to do with eyes closed for 20 sec.

Analysis

Power spectra of the oscillatory signals for each task were obtained with the fast Fourier transform for the M-L, A-P and vertical directions. Total powers between zero and 10 Hz were tested with the analysis of variance for the effects of the 3 directions and the 2 tasks, and the Bonferroni test was done for a post-hoc test.
The coefficient of correlations was obtained each other among COP-path length/sec, COP-path area/sec, age, BW, and BH. And the Spearman’s test was used to study significant correlations for further discussion.

III. Results

Figure 1^{(Fig. 1)} showed COP-dynamics on the horizontal plane for 2 cases of 50 and 86 years old females. Both subjects showed COP-oscillations increased with eyes closed （thin traces）. The subject of 86 years showed marked oscillation with eye closed （thin trace at the right graph）.

Figure 2^{(Fig. 2)} showed the time course of the signals for the same subjects as in Fig. 1. The oscillations with eyes open showed fairly stable but those with eyes closed showed increased, which were noted for the signals from the 3 load cells, M-L direction, A-P direction, and the vertical direction. Mean path length was 114 mm/s （standard deviation; ±42.5） with eyes open, 227 mm/s （±90.3） with eyes closed, for all subjects. Mean area was 1555 mm2/s （standard deviation; ±981.6） with eyes open, 5078 mm2/s （±3608.9） with eyes closed, for all subjects. The correlation coefficient was 0.39 （P＜0.05） between age and the path length with eyes open, was 0.37 （P＜0.05） between age and path length with eyes closed, was 0.36 （P＜0.05） between age and the path area with eyes open, and was 0.32 （P＜0.05） between age and path area with eyes closed. Figure 3^{(Fig. 3)} showed all data with age and their regression lines. The correlation coefficient was 0.34 （P＜0.05） between the total power （0-10 Hz） of M-L shift and age with eyes open, and was 0.25 （P＜0.05） between the total power of M-L shift and age with eyes closed, was not significant between the total power （0-10 Hz） of A-P shift and age with eyes open, and was 0.26（P＜0.05） between the total power of A-P shift and age with eyes closed.

Figure 4 ^{(Fig. 4)}showed power spectra for the 3 directions of the same subjects as in Fig. 1 and 2. The power spectra with eyes closed showed increased on the horizontal plane for both subjects. For the vertical direction, the power spectra with eyes closed had increased for the subject of 86 years. But power spectra with eyes closed did not increase for the subject of 50 years. The analysis of variance was done for all subjects to show significant differences among the total powers for the effects of the 3 directions （P＜0.0001） and of the 2 tasks （P＜0.0001）. The post-hoc test showed significant between the powers in M-L and A-P directions （P＜0.003）, between the powers in M-L and vertical directions （P＜0.0001）, and between the powers in A-P and vertical directions （P＜0.0001）.

Table 1 ^{(Table 1)} was a matrix of the coefficient of correlations for the total power of the oscillations. Significant correlations were noted between age and BH, between age and M-L power with eyes open, between age and M-L power with eyes closed, and between age and A-P power with eyes closed. Significant correlations were noted between BH and BW, between BH and A-P power with eyes closed, between BH and the vertical powers. Significant correlations were noted between BW and A-P power with eyes closed, between BW and the vertical powers. Significant correlations were noted between M-L powers with eyes open and those with eyes closed, between M-L powers with eyes open and A-P powers. Significant correlations were noted between M-L powers with eyes closed and A-P powers with eye closed, between M-L powers with eyes closed and the vertical powers with eyes closed. A significant correlation was noted between A-P powers with eyes closed and the vertical powers with eyes closed. A significant correlation was noted between the vertical powers with eyes open and the vertical powers with eyes closed.

Table 2 ^{(Table 2)} showed a matrix of coefficient of correlations for the shifts on the horizontal plane, among COP-path length/sec, COP-path area/sec, age, BH and BW. The length and the area were correlated to age significantly, but not correlated to BH and BW.

IV. Discussion

Age was correlated to the total powers of the oscillatory signals in M-L direction with eyes open and closed. Age was correlated to the total powers with eyes closed in A-P direction.
The total powers in the vertical direction correlated to BH and BW, but did not correlate to age. Previous our study had shown spectra powers around 6 Hz in the vertical direction increased in proportion to aging^［13］. In this study, the vertical oscillation showed no correlation to aging, but careful inspection of all individual data showed an inclination of the powers increased around 6Hz with aging. Therefore, the vertical oscillation shall be left for further study.
Age correlated to COP-path length and area on the horizontal plane. But BH and BW did not correlated to the length and area on the horizontal plane. Hue et al. had reported that body weight was a strong predictor of postural stability^［14］. But, BW and BH correlated to the total powers in the vertical direction, which suggested both factors might be risk factors for aged falls from our results.
Standing upright is a complex multisensory function^［15-16］, using sensory-motor feedback from vestibular^［17］, visual^［18-19］, auditory^［20］, proprioceptive^［18］and somatosensory systems ^［21］.
Aging is an ultimate problem for humans, and aging might be one of frequent explanations to vertigious patients. But precise diagnosis might provide us appropriate order-made therapy.
An occasional aged patient complained of lasting dizziness sensation with conservative medication, who had diagnosed as no marked equilibrium disease by otologists. For any cases, dizziness sensation should be considered as side effects from medicines for other disorders. When COP-path length and area were within normal range for his/her age, the vertical oscillation should be measured. And other specialists might be consulted, e. g., neurologists, ophthalmologists, endocrinologists, psychologists and so on. In conclusion, this simple non-invasive COP-measurement in the vertical and horizontal directions, will provide more information as objective and quantitative data to us and patients. Precise evaluation, chronological course, effects of medical therapy, and effects of rehabilitation will be provided for postural control.

Acknowledgments

We thank all the subjects and Yukinobu Kobayashi MD, PhD for clinical evaluation of patients, and for his constant support.