The body's center of gravity (COP) is located over the second sacral vertebra, and should remain inside the base of support to maintain static balance during quiet standing. Activities displacing the center of gravity, such as leaning, require dynamic balance, which returns the center of gravity to the second sacral vertebra over the base of support. This requires coordinated efforts involving the sensory systems (visual, vestibular, and proprioceptive) and motor systems (upper and lower extremity muscle strength and joint flexibility).1 Moreover, postural control and its adaptation to the environment are based on background postural tone and on postural reflexes or reactions, which are considered to originate from inputs from the visual and vestibular systems and from the somatosensory system, with inputs at the level of the different body segments. Postural tone is predominantly observed at the level of the limbs, back and neck extensor muscles, and the masseter muscle of the jaw. The main force vector of these muscles counteracts the effect of gravity when the subject is standing on a support surface.2

Postural sway should be minimal to maintain stable balance. According to a study by Baloh et al.,3 there is increased postural sway with aging under both static and dynamic conditions. Deficits of posture and balance control can severely limit activities of daily living. Such deficits can lead to falls, which are a major source of morbidity and mortality in elderly persons.4 Therefore, prevention of falling is important, and many programs for preventing falls have been developed and proven their effectiveness.4-6 Evaluation of balance in a standing position is one of the tests used to predict the risk of falling. Clinical tests for evaluation of balance include the Tinetti test, Berg balance scale, and tests using a force plate as a quantitative method for balance evaluation. Tests using a force plate are objective and quantitative, but there are various test methods according to each laboratory, and a standardized method has not yet been developed.7-10 There are several systems and parameters that use a force plate, and only a few clinical standard levels have been reported.11,12

This study aimed to investigate a possible standardized method by comparing results according to feet width, and to identify factors affecting the test results of a quiet standing balance evaluation conducted by posturography in a normal person.

The mean age of subjects in this study was 44.1±14.5 years, their mean shoulder width was 38.4±3.0 cm, mean height was 164.7±9.4 cm, and their mean weight was 61.8±10.7 kg. The mean percentage of feet width decided by the subjects' comfort was 56.5±15.7% of mean shoulder width (Table 1). There was no significant difference between results obtained from men and women (Table 2). The balance parameters evaluated in the eyes closed condition showed significantly lower values than in the eyes open condition (p<0.01).

Maintenance of balance relies on the harmonious integration and coordination of multiple body systems including the vestibular, visual, auditory, sensory, and autonomic systems.13 The information from these different end organs is integrated in the central nervous system (CNS), an appropriate response is formulated, and the musculoskeletal system is directed to perform the appropriate head, eye, trunk, and limb movements to maintain posture.14 Pajala et al.7 reported that wide feet width could compensate impaired balance by adding more support to the base, and Benvenuti et al.15 suggested that narrow feet width could increase instability by flexion of the hip joint to maintain balance. In this study, COP movement (ML-E and ML-S) was greater when the subject's feet width was as wide as the shoulder width than half of the shoulder width, or at a comfortable width. Indices of balance evaluation on a force plate are expressed by degrees of overall performance of multiple systems to provide balance of the subject. Several pathways of the CNS including the pyramidal, extrapyramidal, reticulospinal, medial vestibulospinal, and lateral vestibulospinal tracts influence balance by controlling voluntary movement and muscle tone. Also, the peripheral afferent system and cerebral cortex perform their roles in balance by providing sensory feedback, and receipt and transfer of information.14,16,17 The semicircular canal, utricular, and saccular maculae of the vestibular system, and the visual system also participate in balance control. All of these various systems should function normally to minimize COP movement during evaluation of quiet standing balance on a force plate. We believe that the reason the greatest COP movement was in test 1 is that the subjects contracted their distal lower muscles (peroneus longus, gastrocnemius, etc) rather than their antigravity axial muscles, which are the main muscles used in balance control, to maintain balance with their feet spread widely apart. This means that the indices for evaluation of quiet standing balance on the force plate will appear as if balance ability is impaired. Although excessively wide feet width can produce a stable position, it creates great COP movement by inappropriate contraction of the distal lower muscles. These results can produce a coveringeffect of balance impairment when the evaluation of multiple systems participating in balance control is made on a force plate.

This study shows that balance indices including distance and speed of COP movement were better when the feet width of a subject equaled half of the shoulder width, or was a comfortable width, rather than a full shoulder width. COP movement was smaller in a position with half a shoulder width than in a position with a comfortable width, but it is difficult to conclude that half of a shoulder width produces a more stable result because there was no statistical significance. Also, the mean percentage of feet width decided by the subject's comfort was 56.5±15.7% of the shoulder width. This means the comfortable width was not very different from the half shoulder width.

Therefore, we think that an appropriate method for evaluation of quiet standing balance includes making feet width the same as half the shoulder width, because that measurement can be made objectively and will produce relatively better results.

The results of many balance indices were poor as the ages of the subjects became older. Several studies have reported that balance ability decreases in old age.18-20 Hair cell loss in the ampula, neuronal loss in the vestibular nuclei, and visual impairment due to cataracts and glaucoma occurs with aging.21,22 In addition, balance impairment in old age can be induced by sensory impairment due to arthritis, diabetes, vitamin B12 deficiency, atherosclerosis, and musculoskeletal impairment due to sarcopenia, joint mobility restriction, and decreased postural response. Orthostatic hypo tension related to medications, and decreases of concentration and alertness also contribute to balance impairment in old age.23,24 There was no difference in balance indices between men and women. This result is consistent with those of previous studies which compared the balance abilities of men and women.25,26 The mean height of subjects was 171.3±7.3 cm for males and 158.1±5.9 cm for females.. However, we think that the difference did not affect the results of the evaluation because height and balance ability were not significantly correlated. Also we could verify that visual compensation clearly contributed to balance by the statistical difference observed in the results between positions with eyes open and eyes closed.

The Good Balance Evaluator® used in this study converts the results of a standing balance evaluation performed during 30 seconds to numerical values. For the best results, subjects are required to stand quietly and minimize movement with substantial concentration. The subjects should concentrate on static standing for 180 seconds during 6 evaluations to minimize COP movement. The results of an evaluation could be made worse by a momentary loss of posture due to a lack of concentration. This could also increase the range of statistical error among subjects. Without a subject's active cooperation and concentration during evaluation, the results of a test could be made worse, regardless of the subject's ability to balance. This could be one drawback of conducting a balance evaluation using equipment with a force plate. To compensate for this limitation, re-evaluation with inducing concentration could be considered in case that test results are suspicious of lack of concentration in comparison with results of clinical balance evaluation tools, such as, Tinetti test, Berg balance scale.

Not all age groups were included in this study, because we did not enroll subjects aged <20 years or >69 years. Also, this study evaluated only quiet standing balance on a fixed force plate and not clinical dynamic balance, which could be a limitation of the study. A further study including dynamic standing balance on a moving force plate and a broader range of age groups is needed to properly evaluate the role of posture in maintaining balance.

COP movements were increased with age and when subjects closed their eyes during evaluation of quiet standing balance conducted by posturography. Gender and height did not affect test results. We suggest that an appropriate method to test balance is by standing on a force plate with feet width the same as half the shoulder width, because many subjects felt comfortable in this posture and it presented a relatively accurate measurement of balance capacity. The results of this study are expected to be used as standard data in further evaluations of quiet standing balance in normal subjects.