Standing Balance After Vestibular Stimulation in Tai Chi–Practicing and Nonpracticing Healthy Older Adults

Article Outline

• Abstract
• Methods
  • Participants
  • Standing Balance Control Test
  • Procedure for Vestibular Stimulation
  • Data Recording and Analysis
  • Statistical Analysis
• Results
  • Participants
  • Test-Retest Reliability of Standing Balance
  • Effects of Vestibular Stimulation on Standing Balance Control
    • Older control subjects
    • Tai Chi practitioners
  • Comparisons of Standing Balance Control Between Tai Chi Practitioners and Control Subjects
    • Standing balance control before vestibular stimulation
    • Standing balance control after vestibular stimulation
• Discussion
  • Effects of Vestibular Stimulation on Standing Balance Control of Healthy Control Subjects
  • Comparison of Changes in Body Sway During Stance Before and After Vestibular Stimulation: Tai Chi Practitioners Versus Control Subjects
  • Limitations and Considerations for Future Studies
• Conclusions
• Acknowledgment
• References
• Copyright

Abstract 

Tsang WW, Hui-Chan CW. Standing balance after vestibular stimulation in Tai Chi–practicing and nonpracticing healthy older adults.

Objective

To compare the effects of vestibular stimulation on standing balance control between Tai Chi practitioners and older subjects.

Design

Cross-sectional study.

Setting

University-based rehabilitation center.

Participants

Tai Chi practitioners (n=24; age ± standard deviation, 69.3±5.0y) and control subjects (n=24; age, 71.6±6.1y) were recruited.

Interventions

Not applicable.

Main Outcome Measures

Subjects stood on a force platform with eyes closed before and after stimulation of their horizontal semicircular canals, applied by means of whole head-and-body rotation at 80°/s for 60 seconds, with subjects seated in a rotational chair. Body sway during stance was measured as total sway path, peak amplitudes, and mean velocities of sway in both anteroposterior (AP) and mediolateral (ML) directions.

Results

After head-and-body rotation, significant within-group increases were found in all measures in both AP and ML directions during stance with eyes closed in older control subjects but not in Tai Chi practitioners along the AP direction. In fact, significantly smaller increases in total sway path, peak amplitude, and mean velocity of body sway in the AP direction were found in the Tai Chi practitioners when compared with those of control subjects.

Conclusions

Our results show that long-term Tai Chi practitioners had better AP standing balance control after vestibular stimulation than older control subjects.

Key Words:  Accidental falls , Aging , Rehabilitation , Rotation , Tai Chi

ONE APPROACH TO INVESTIGATING how the central nervous system (CNS) synthesizes the 3 sensory (somatosensory, visual, vestibular) inputs involved in postural control was developed by Nashner,¹ who termed it a sensory organization test (SOT). In this protocol, subjects’ postural control is tested while standing under 6 reduced or conflicting sensory conditions, including combinations of 3 visual conditions (eyes open, eyes closed, sway referenced) with 2 support surface conditions (fixed, sway referenced). By using the SOT, research from our laboratory² and others³ has shown that older subjects sway significantly more during stance than young subjects when there is an increased reliance on visual and vestibular inputs because of reduction and/or distortion of other sensory inputs. This increase in body sway is thought to be attributable to the degeneration in the somatosensory, visual, and vestibular systems that can occur with aging.⁴ Inappropriate organization and weighting of sensory inputs at the level of the CNS may also contribute to the increased sway observed in older subjects.⁴, ⁵ The increase in body sway during stance under reduced and/or conflicting sensory conditions has been related to falls in these subjects. Wallmann⁶ used the SOT to compare 10 older fallers and 15 nonfallers (both aged >60y) and found that the fallers swayed significantly more when there was a reliance on visual inputs in the presence of distorted somatosensory inputs.

Somatosensory cues from the support surface and visual inputs from the external surrounding can provide the correct information needed to maintain postural control only when they can serve as a correct and/or fixed reference. The information about the position and movement of the head with respect to gravity and inertial forces provided by the vestibular system is not similarly influenced by changes in surface and visual conditions. Vestibular data thus provide a reference that can be invoked to suppress conflicting somatosensory and visual inputs.⁵, ⁷ However, research has shown that age-related structural degeneration of the vestibular receptors,⁸ often superimposed on some degree of vestibulopathy (or inner ear disease), may mean that older subjects can no longer rely on accurate vestibular inputs to maintain their postural control. This may dispose them toward falls, especially in situations when vestibular cues conflict with somatosensory inputs in the absence of vision.

Under what situations would vestibular signal conflict with somatosensory input? The vestibular apparatus has 2 main components: the otolithic organs, which sense linear accelerations, and the semicircular canals, which detect angular accelerations.⁹ Relatively selective stimulation of the horizontal semicircular canals can be applied to a subject seated in a rotational chair with their head fixed at a 30° angle below the horizontal plane (figs 1A, 1B). At the onset of rotation, the inertia of the endolymph fluid in the horizontal canals bends the hair cells (the canal receptors) in a direction opposite to that of head rotation. When the chair is stopped suddenly, the same inertial force will continue to bend the hair cells, this time in the same direction of head rotation even though there is no longer any head rotation. If subjects are made to stand with eyes closed, then they have to suppress these erroneous vestibular inputs and rely on correct somatosensory inputs to maintain standing balance. However, somatosensory receptors are known to degenerate with aging.⁴ Furthermore, little is known about how older subjects maintain standing balance after vestibular stimulation, especially when the latter input conflicts with somatosensory signals in the absence of vision. Therefore, the first objective of this study was to investigate the balance control of older subjects under conflicting vestibular and somatosensory inputs while standing on a firm support surface in the absence of visual cues after excessive vestibular stimulation was applied by means of whole head-and-body rotation with subjects seated in a rotational chair.

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• Fig 1. 

  (A) A subject seated on the rotational chair with her head stabilized at 30° from the horizontal plane. (B) Spatial orientation of the 3 semicircular canals. The horizontal semicircular canals are located 30° above that of the horizontal plane.

In a previous study,² we found that experienced older Tai Chi practitioners exhibited better balance control than control subjects similar in age, sex, and physical activity level when they were asked to stand under sensory conditions that demanded an increased reliance on visual and vestibular systems. In a subsequent prospective study,¹⁰ we showed that older subjects (mean age, 67.6y) who had received 4 weeks of intensive Tai Chi training improved their balance control significantly, in terms of less body sway when standing under conditions requiring an increased reliance on the vestibular system. In view of the age-related degeneration of the vestibular and somatosensory receptors and/or inappropriate sensory organization that may dispose older persons to falls, it is important to know whether experienced Tai Chi practitioners exhibit better balance control when forced to rely on somatosensory inputs because of excessive vestibular and absent visual inputs. A second objective of this study was, therefore, to investigate whether older Tai Chi practitioners had developed better control of body sway during stance after vestibular stimulation in the absence of vision when compared with that of healthy older control subjects. The reason for testing the participants with their eyes closed was to minimize the influence of visual inputs to postural control that could occur in the dark.

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Methods 

Participants 

Seventeen subjects were recruited in a pilot study conducted to examine test-retest reliability, with 7 men and 10 women (age ± standard deviation [SD], 70.7±5.6y), including 3 men and 4 women who were Tai Chi practitioners. Another 48 community-dwelling older subjects (age, ≥60y) participated in the main study. Of these, 24 Tai Chi practitioners (12 men, 12 women; mean age, 69.3±5.0y) were recruited from local Tai Chi clubs. All of them had practiced Tai Chi for a minimum of 1.5 hours a week for at least 3 years (mean Tai Chi experience, 8.5±7.6y). Twenty-four older control subjects (12 men, 12 women; mean age, 71.6±6.1y) were recruited from several community older-adult centers. They had no previous experience in practicing Tai Chi, although some took morning walks or did stretching exercises. All the subjects were independent in their activities of daily living, and none required walking aids. They were able to communicate and follow the testing procedures. Subjects with any known vestibulopathy or complaint of dizziness were excluded. Other exclusion criteria included poorly controlled hypertension, severe cognitive impairment, diagnosis of metastatic cancer, Parkinson’s disease, stroke, or any other neurologic disorder. Also excluded were subjects showing symptomatic cardiovascular diseases when subjected to moderate exertion, symptomatic orthostatic hypotension, peripheral neuropathy of the lower extremities, and disabling arthritis. Subjects who reported 1 or more falls in the previous 12 months were also excluded.

Clinical evaluation of the subjects included (1) a general health questionnaire, (2) Mini-Mental State Examination (MMSE), and (3) a physical activity level questionnaire. The general health questionnaire was used to screen out subjects according to the exclusion criteria.¹¹ The validated Chinese version of the MMSE of Folstein et al was then administered.¹² The scale ranges from 0 to 30. A score below 24 was considered indicative of cognitive dysfunction, and such subjects were excluded from this study. A modified version of the Minnesota Leisure Time Physical Activity Questionnaire¹³ was used to evaluate the energy expended in leisure-time physical activities and household tasks. They were categorized into 3 levels according to their metabolic equivalent (METS) status: light (intensity ≤4.0 METS), moderate (intensity range >4.0 to ≤5.5 METS), and heavy activities (intensity >5.5 METS). This modified measuring tool has been used in our previous studies², ¹¹, ¹⁴, ¹⁵ to compare the physical levels of the Tai Chi practitioners with those of the control subjects. The protocol was approved by the Ethics Committee of Hong Kong Polytechnic University, and written informed consent was obtained from all subjects.

Standing Balance Control Test 

The balance test involved subjects standing quietly with their arms at their sides and feet together on a force platform^a for 30 seconds with eyes closed. The trajectory of their center of pressure (COP) during the 30-second stance was recorded. A total of 3 trials were conducted with a 1-minute rest in between. The total sway path of the COP and the peak amplitudes and mean velocities of body sway in the anteroposterior (AP) and mediolateral (ML) directions were recorded. These values were used to compare balance performance before and after vestibular stimulation as well as between the 2 groups. The rationale of choosing the control of body sway in our study is because researchers have linked greater postural sway to increased risk of falling in older adults. More specifically, Fernie et al¹⁶ found that the average speed of sway was significantly greater for institutionalized older subjects (mean age, 81.8y) who had fallen 1 or more times a year when compared with those who had not fallen. In their 1-year prospective study on the older adults (age range, 62−96y) who had a fall, Maki et al¹⁷ also observed significantly larger amplitude of AP body sway in them.

Procedure for Vestibular Stimulation 

After the quiet standing balance test, a rotational chair^b was used to stimulate the vestibular system. Subjects sat on the chair with their head fixed at 30° of flexion (see fig 1A). This position placed the horizontal semicircular canals in the plane of rotation to stimulate their receptors selectively (see fig 1B). Subjects experienced clockwise whole head-and-body rotation at 80°/s for 60 seconds with their eyes closed. The reason for choosing such speed and duration of whole head-and-body rotation was based on a study conducted by Goebel and Paige¹⁸ in which young healthy subjects (age range, 20−35y) were rotated at approximately 180°/s for 30 seconds while seated. The slower speed and longer duration adopted in the present study were found to be more appropriate for older adults and produced reliable data (see the reliability test results). After rotation, subjects were asked to open their eyes and were guided to stand on a force platform with their feet together and arms by their sides as soon as possible. They were then asked to close their eyes again and to maintain their balance without moving their arms or feet.

The COP trajectory was recorded during the subsequent 30-second stance. Only 1 trial was conducted with each subject because some subjects had reported dizziness after vestibular stimulation in our pilot study. The time required to get up from the rotational chair until the subject stood on the force platform with eyes closed was recorded by using a stopwatch and served as an outcome measure and a covariate when comparing the 2 older groups in the statistical analysis. Changes in 3 body sway measures were used to evaluate the effects of vestibular stimulation on standing balance control. They were (1) total sway path of the COP, (2) peak amplitudes, and (3) mean velocities of body sway in both AP and ML directions.

Data Recording and Analysis 

The COP data from the force platform were sampled at 100Hz and were smoothed by using a second-order Butterworth low-pass filter with a cutoff frequency of .85Hz. The total sway path was calculated as the cumulative distance covered by the COP over the sampled time period. The peak amplitude of body sway was expressed as a sway angle, a variable commonly used to measure the control of postural sway, including that of our previous studies.², ³, ¹⁰, ¹⁴, ¹⁵ As was performed before, the maximum sway angles in the AP and ML directions were determined by using the COP trajectory with respect to the subject’s height.³, ¹⁵ The mean velocity was calculated by dividing the sum of the displacement trajectory, after resolving into its AP and ML directions, by the sampling interval of 30 seconds.

Statistical Analysis 

Age, weight, and height were compared between the 2 groups by using independent t tests. Because of the categoric nature of the variables, a chi-square test was considered more appropriate for between-group comparison of the sex distribution and physical activity levels. An intraclass correlation coefficient (ICC) was applied to assess the test-retest reliability of the measures. The notation ICC_3,x signifies that model 3 was used for assessing intrarater reliability, with x denoting the number of trials used in the different tests. Paired t tests with Bonferroni adjustments were performed to compare the control of body sway before and after vestibular stimulation in healthy control subjects and in Tai Chi practitioners. Multivariate analysis of variance (ANOVA) was used to compare the measures of body sway recorded during quiet stance between the Tai Chi and control subjects. If statistically significant differences were found in the multivariate tests, univariate tests were conducted for each of the measures. Multivariate ANOVA was also used to analyze the percentage changes in the measures before and after vestibular stimulation between the 2 groups. The time required to get up from the rotational chair until the subject stood with their eyes closed was compared between the 2 groups by using an independent t test. It was also used as a covariate in the statistical analysis because the effect of vestibular stimulation on balance control could become minimal if subjects waited too long to get up from the rotational chair. Thus, any significant difference in this time interval could introduce a confounding variable for between-group comparisons. A significance level (α) of .05 was chosen for statistical comparisons.

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Results 

Participants 

Fifty-five community-dwelling subjects volunteered to take part in this project. Two Tai Chi subjects were excluded because of regular tennis practice (n=1) and history of heart surgery (n=1). Five control subjects were excluded because of history of falls in the previous 1 year (n=2), minor stroke (n=1), regular badminton practice (n=1), and knee pain (n=1). Altogether, 24 Tai Chi practitioners and 24 controls, all aged 60 or older, underwent the testing procedures. Independent t tests showed no statistically significant difference in age, height, or weight between Tai Chi practitioners and control subjects (P>.05; table 1). Chi-square tests also found no statistically significant difference between the 2 groups in either sex distribution or physical activity level (P>.05; see table 1). Tai Chi practitioners and control subjects were thus similar with respect to age, height, weight, sex, and physical activity levels. All the subjects had scored at least 24 on the MMSE, indicating an absence of cognitive dysfunction.

Table 1. Comparison of Age, Height, Body Weight, Sex, and Physical Activity Level Between Older Control and Tai Chi Subjects

Characteristics	Control Subjects (n=24)	Tai Chi Subjects (n=24)	P
Age (y)	71.6±6.1	69.3±5.0	0.148
Height (cm)	154.5±8.7	156.5±8.8	0.432
Body weight (kg)	58.0±9.0	58.2±8.5	0.963
Sex (male/female)	12/12	12/12	1.000
Physical activity levels (n)			0.232
Light ≤4 METS	21	17
Moderate ≤5.5 METS	3	5
Heavy >5.5 METS	0	2

NOTE. Values are mean±SD or as otherwise indicated.

Test-Retest Reliability of Standing Balance 

The total sway path, peak amplitudes, and mean velocities of sway during quiet standing were recorded before and after vestibular stimulation in the same subjects after 1 week. Table 2 shows the ICC values of the measures during quiet standing before and after vestibular stimulation, which ranged from .70 to .88 and from .61 to .97, respectively, thus showing fairly to very good data repeatability. The lower bound value of the 95% confidence interval of the ICC for the peak amplitude of AP body sway before and after vestibular rotation (.12, .10, respectively; see table 2) as well as the peak amplitude after stimulation (.13) and mean velocity of ML body sway before stimulation were considered low (.29). Although this might be because of the small sample examined in the reliability test, these results should still be interpreted with some caution.

Table 2. Test-Retest Reliability of Measures Before and After Vestibular Stimulation (n=17)

	Before Vestibular Stimulation		After Vestibular Stimulation
Measures	ICC3,3	95% CI	ICC3,1	95% CI
Total sway path (cm)	.88	.67–.96	.90	.71–.96
AP body sway
Peak amplitude (deg)	.70	.12–.89	.65	.10–.88
Mean velocity (cm/s)	.87	.63–.96	.86	.61–.95
ML body sway
Peak amplitude (deg)	.78	.36–.92	.61	.13–.86
Mean velocity (cm/s)	.74	.29–.91	.89	.68–.96
Time from sit to stand with eyes closed after whole head-body rotation	NA	NA	.97	.91–.99

Abbreviations: CI, confidence interval; NA, not applicable.

Effects of Vestibular Stimulation on Standing Balance Control 

Paired t tests showed that all body sway measures during stance significantly increased after vestibular stimulation was applied to the control subjects seated in a rotational chair (all P<.005; table 3). The percentage increase ranged from 31.3% to 37.1% regardless of the direction of body sway, whether AP (forward-backward) or ML (sideways) (figs 2A–C).

Table 3. Comparison of Body Sway of Control Subjects and Tai Chi Practitioners During Stance With Eyes Closed Before and After Vestibular Stimulation

Measures	Control Subjects			Tai Chi Subjects
	Prevestibular Stimulation (n=24)	Postvestibular Stimulation (n=24)	P	Prevestibular Stimulation (n=24)	Postvestibular Stimulation (n=24)	P
Total sway path (cm)	35.3±5.7	46.4±9.0	.000⁎	34.1±8.7	40.8±12.2	.000⁎
AP body sway
Peak amplitude (deg)	2.1±0.5	2.8±0.7	.000⁎	2.2±0.4	2.5±0.6	.30
Mean velocity (cm/s)	0.68±0.11	0.91±0.23	.000⁎	0.66±0.14	0.76±0.22	.018
ML body sway
Peak amplitude (deg)	2.3±0.4	3.1±0.6	.000⁎	2.3±0.4	2.9±0.8	.002⁎
Mean velocity (cm/s)	0.82±0.15	1.06±0.20	.000⁎	0.78±0.24	0.96±0.34	.000⁎

NOTE. Values are mean ± SD or as otherwise indicated.

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• Fig 2. 

  The mean values of body sway measures before and after vestibular stimulation between Tai Chi practitioners and control subjects. The parenthesis denotes the P values after statistical analysis of the percentage increases in (A) total sway path, (B) peak amplitude and mean velocity of AP body sway, and (C) peak amplitude and mean velocity of ML body sway between the 2 groups using univariate tests.

Statistical analysis showed that the increase in the body sway measures of Tai Chi practitioners ranged from 14.3% to 26.5% after vestibular stimulation (see fig 2). The increases of peak amplitudes and mean velocities of sway in the AP direction were insignificant in these subjects (P=.30, P=.018, respectively; see table 3).

Comparisons of Standing Balance Control Between Tai Chi Practitioners and Control Subjects 

Multivariate tests of the body sway measures during quiet standing results showed no significant difference between the 2 groups (P=.909). Subsequent univariate analysis showed that Tai Chi practitioners and control subjects performed equally well in controlling their body sway, as measured by the total sway path, the peak amplitudes, and the mean velocities of sway in both AP and ML directions (all P>.05; see figs 2A–C).

Both groups required similar time to get up from the rotational chair to stand on the force platform with their eyes closed, with 21.4 seconds for Tai Chi subjects and 22.7 seconds for control subjects (P=.355). Multivariate tests indicated an overall statistically significant difference across the percentage changes of all body sway measures recorded during standing after vestibular stimulation between Tai Chi practitioners and control subjects (P=.028). Univariate tests showed significantly less percentage increase in the total sway path among Tai Chi practitioners (mean, 19.5%) than that of control subjects (32.4%, P=.016; see fig 2A). There were also less percentage increases in the peak amplitude and mean velocity of Tai Chi practitioners’ AP body sway after vestibular stimulation (mean, 15.4% and 14.3%, respectively) when compared with those of the control subjects (34.7% and 34.6%, respectively; P<.05; see fig 2B). However, the differences in ML body sway measures were not significant between the 2 groups (see fig 2C).

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Discussion 

Effects of Vestibular Stimulation on Standing Balance Control of Healthy Control Subjects 

On average, older control subjects swayed significantly more after vestibular stimulation. This was reflected in all the 5 sway measures including total sway path and peak amplitude and mean velocity of body sway in both AP and ML directions (P<.005; see table 3) during stance after whole head-and-body rotation. In a previous study² on quiet stance with eyes closed in the absence of prior vestibular stimulation, we did not find that older subjects (mean age, 68y) showed any significant increase in the peak amplitude of AP body sway. That finding showed that older adults could maintain their balance during quiet stance on the basis of correct somatosensory and vestibular inputs and despite the absence of visual information. However, subjects in the present study showed significantly increased body sway during stance with eyes closed after they experienced vestibular stimulation by whole head-and-body rotation for 60 seconds and despite the presence of correct somatosensory cues.

Similar postural testing after vestibular stimulation has been conducted with younger subjects. Goebel and Paige¹⁸ studied 20 young healthy subjects (age range, 20−35y) after rotating them manually in a swivel chair at approximately 180°/s for 30 seconds with their eyes closed. After rotation, the young subjects swayed significantly more in sway-referenced visual with fixed support surface conditions (ie, conflicting visual but correct somatosensory inputs) and eyes open with sway-referenced support surface (correct visual but conflicting somatosensory inputs), with an increase of 13% and 21% AP body sway, respectively. These young participants had to suppress excessive vestibular inputs that conflicted with either their correct somatosensory or correct visual inputs. This experiment showed that even young subjects showed postural instability when they had to rely on only 1 correct sensory input while suppressing conflicting sensory inputs arising from the remaining 2 sensory systems. The protocol adopted for the older subjects in the present study was not as demanding as that of Goebel and Paige¹⁸ because the older subjects were asked to suppress only 1 conflicting (vestibular) input in the absence of visual information. However, the increase in AP body sway of the older control subjects (34.7%, see fig 2B; P<.005, see table 3) was comparatively more than those of the younger subjects (13% and 21%). Although different experimental paradigms were used in the 2 studies, this finding is consistent with those showing that older subjects generally have poorer balance control than younger subjects that may dispose them to falls.⁶

The significant increase in body sway after vestibular stimulation in the older subjects may be because of aging, which could have resulted in (1) deterioration of somatosensory receptors,⁴, ¹⁹ (2) delays in central processing,²⁰ and (3) muscle weakness.²¹ In the present study, the subjects had to rely on correct somatosensory information to maintain their standing balance to suppress conflicting vestibular inputs in the absence of visual inputs. However, their somatosensory receptors may have undergone degeneration because of aging,⁴ hence negatively affecting the balance control attributed to it. For instance, in a previous study, we¹⁴ found that older subjects aged 71 years showed a significantly greater percentage (255%) of the absolute knee joint repositioning error when compared with that of the younger subjects aged 20 years. Hence, the decreased joint proprioception could have contributed to the increased body sway seen after vestibular stimulation in the older control subjects.

The central processing for balance control is responsible for selecting an appropriate orientation reference based on information from the somatosensory, visual, and vestibular systems that sometimes may be conflicting. Aging may also render the processing of sensory information too slow in the CNS, which may lead to a delay in the arrival of motor commands at the muscles to prevent a fall. Alternatively, the central processing handling conflicting sensory information may result in inappropriate weighting of sensory inputs leading to postural instability.²⁰

Good balance control during functional activities requires sufficient muscle strength. However, muscle size and related neuromuscular functions are known to decrease with aging.²¹ In fact, decreased muscle strength is one of the intrinsic factors that causes falls in older subjects.²² In the present study, our older subjects might need better muscle strength to maintain the greater standing balance control required after head-and-body rotation. However, their decreased muscle strength might not have been able to cope with the more demanding task required, thus resulting in increased body sway.¹⁵

Comparison of Changes in Body Sway During Stance Before and After Vestibular Stimulation: Tai Chi Practitioners Versus Control Subjects 

Tai Chi practitioners and control subjects were similar in their standing balance control before vestibular stimulation (see figs 2A–C). However, Tai Chi practitioners showed a significantly smaller percentage increase in body sway after vestibular stimulation, in terms of total sway path (P=.016; see fig 2A), peak amplitude (P=.023; see fig 2B), and mean velocity (P=.003) in the AP direction when compared with those of control subjects. A similar trend appeared in the peak amplitude and mean velocity of ML sway, but the differences between the 2 groups did not reach statistical significance (see fig 2C).

The finding that Tai Chi practitioners had less increase in body sway after vestibular stimulation may be because of the effect of Tai Chi practice on joint proprioception receptors, central processing, and muscle strength. Our previous study¹⁴ has shown that older practitioners (mean age, 69.6y) with 8.4 years of Tai Chi experience achieved significantly better acuity in knee proprioceptive sense. They showed smaller knee angle errors in passive knee repositioning when compared with control subjects similar in age, sex, and physical activity level. In fact, their knee proprioceptive acuity was even comparable to that of young university students.¹⁴ More accurate joint position sense should provide the CNS with the needed information for better standing balance control, especially when subjects had to rely on somatosensory inputs to compensate for conflicting vestibular inputs and absence of visual cues as in the present paradigm.

A second possible explanation is the effect of Tai Chi practice on central processing. Our previous study² has shown that older Tai Chi practitioners (mean age, 70.7y) with 7.2 years of Tai Chi experience exhibited better balance control than control subjects when they stood under sensory conditions that demanded an increased reliance on the visual and vestibular systems. Of particular interest is that these practitioners attained the same level of balance performance as young, healthy subjects when standing under reduced or conflicting somatosensory and visual conditions.² In a prospective study,¹⁰ we further showed that older subjects who received only 4 weeks of intensive Tai Chi training had improved their balance control significantly, in terms of less body sway when standing under conditions requiring an increased reliance on the vestibular system and more smooth shifting of their weight in different directions within their base of support. These results showed that Tai Chi could improve sensory organization and weighting of sensory inputs under changing sensory conditions.

Tai Chi practice involves many head-and-body rotations, changes in the base of support from double- to single-leg standing, and fixing the gaze on the hands during head and/or trunk rotation. Such a demand on the balance system may explain the improved balance control achieved. Hu and Woollacott²³ had also studied the effect of multisensory training, designed to improve intersensory interaction among the somatosensory, visual, and vestibular systems. Their training program included standing quietly for 1 hour each day while sensory inputs relevant to postural stability were systematically manipulated by having the eyes open or closed and standing on a firm or foam support surface, with the head in a neutral or extended posture. After 4 weeks of training, these investigators found that healthy older participants (age range, 65−90y) fell less frequently when somatosensory inputs from their feet were minimized during stance on a sway-referenced support surface. Their results suggest that task-specific training can help older subjects to improve the organization of sensory information for better balance control, as Tai Chi did.²⁰

Training and repeated use have been shown to induce plastic changes in the brain. To elaborate, subjects were asked to practice a finger opposition task for about 20 minutes every day by touching the thumb to each finger tip in a specific repeating sequence.²⁴ Both the speed and accuracy of the movement increased and reached a plateau in about 3 weeks. Functional magnetic resonance imaging revealed that the area of cortex activated during the trained sequence was larger than that activated during a novel untrained sequence. In other words, more extensive representation in the motor cortex was found in subjects after repeated practice of a single motor task for only 3 weeks. If such finding could be generalized to Tai Chi practice, then the latter could have resulted in plastic changes in the CNS responsible for balance control. Obviously, further research is warranted.

A third possible explanation for the Tai Chi practitioners to have less increase in body sway might be their superior muscle strength. Our previous study has shown that these practitioners had stronger knee and flexor muscle strength in both concentric and eccentric contractions when compared with healthy control subjects.¹⁵ Furthermore, their knee muscle strength was negatively correlated with body sway in a single-leg stance test when they were subjected to AP platform perturbations. The greater muscle strength may enable our Tai Chi practitioners to maintain better balance control after whole head-and-body rotation, despite the need for them to suppress conflicting vestibular inputs.

Falls can lead to soft-tissue injuries, fractures, and mobility impairments. They may also lead to fear of falls, which can result in self-imposed activity restriction.²⁵ This reduced activity can cause further deterioration of balance control in older adults. In a previous study, we found that Tai Chi practitioners had significantly greater balance confidence in daily activities than control subjects, as measured by the Activities-Specific Balance Confidence scale.¹⁵ By using the same balance confidence scale, Sattin et al²⁶ found that a Tai Chi program, twice a week for 48 weeks, led to a significant improvement in the balance confidence of transitionally frail older adults when compared with a wellness education control group. Daily activities often involve head and/or trunk rotation that require accurate sensory inputs, effective CNS processing, and sufficient muscle strength for effective motor outputs. Our finding of better standing balance control after vestibular stimulation may explain the effect of Tai Chi on balance confidence found by other investigators in transitionally frail older adults.

Limitations and Considerations for Future Studies 

Because of the cross-sectional study design, we could not establish a causal relation between the practice of Tai Chi and the improvement of balance control when standing after vestibular stimulation in the absence of visual inputs. Because all participants were healthy older community-dwelling subjects, the present findings cannot be generalized to frail subjects or those who have a history of falls. Nevertheless, in our prospective study,¹⁰ we already showed that 4 weeks of daily Tai Chi practice were sufficient to significantly improve balance performance in the older healthy subjects. Findings from the present study lend further support to the idea of using Tai Chi as a fall-prevention program for older subjects.

Shumway-Cook et al²⁷ showed that older adults who suffered postfracture falls had lower balance control and slower gait speed when compared with those who did not fall after fracture. Mangione et al²⁸ used a home program of moderate to high intensity exercises to train older patients with a hip fracture and found significant improvement in the isometric strength of their lower limbs after a 12-week intervention. Tai Chi is regarded as a form of moderate exercise²⁹ and can be practiced outdoor or indoor on an individual or group basis. Because of its proven benefits on balance control and muscle strength, it has the potential to be an effective rehabilitation strategy for older adults who have suffered hip fracture. Further study is warranted.

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Conclusions 

Our study shows that older subjects showed significantly more increases in body sway measures during stance with eyes closed after vestibular stimulation by head-and-body rotation for 60 seconds. In contrast, Tai Chi subjects manifested better control of their body sway along the AP direction. These findings reveal that long-term practice of Tai Chi may have improved standing balance control when there were excessive vestibular inputs in the absence of visual cues that forced subjects to rely more on somatosensory inputs. Findings from our previous studies were drawn to argue that this could come about as a result of improved joint proprioception, better central processing, and/or better muscle strength. Such improved balance control may help older adults to achieve balance control when turning of their head-and-body is suddenly halted as they go about their daily activities. As such, Tai Chi could be an effective fall-prevention program for older persons.

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Acknowledgment 

We thank Bill Purves for his English editorial advice.

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References 

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• a Model 9286AA; Kistler AG, Ernetschwilerstr, 8737 Gommiswald, Switzerland.
• b System 2000; Micromedical Technologies Inc, 10 Kemp Dr, Chatham, IL 62629.