JAGS 38~1-9, 1990

CLINICAL

INVESTIGATION

Aging and Postural Control A Comparison of Spontaneous- and Induced-Sway Balance Tests B. E. Maki, PhD, P. J. Holliday, MSc, and G. R Femie, PhD

Two different balance testing methods were compared:

measure of postural stability, the data from this test were fitted with a model that was then used to predict the response to sudden (transient) perturbations, thereby simsway in response to an applied postural perturbation. ulating the response in actual falls. Although both Eyes-open tests were petformed in 64 healthy young and induced- and spontaneous-sway measures demonstrated elderly adults and in five elderly subjects with a history significant aging-related decreases in stability, the of falling. In both balance tests, the sway was defined in differences were more pronounced for the induced-sway terms of the displacement of the center of pressure on the data. Conversely, some of the spontaneous-sway measures feet. Spontaneous sway was quantified using a number of were much more successful in distinguishing the fallers different amplitude- and frequency-based parameters. from the nonfallers. There was a significant correlation lnduced sway was measured in response to anteriorbetween induced-sway and certain spontaneous-sway posterior acceleration of a platform on which the subject measures in the normal young adults; however, in the elstood. The induced-sway test was specially designed to be derly normals and fallers, the data from the two types of safe and nonthreatening for elderly subjects; thus, the balance tests either showed no correlation or, for certain platform perturbation was confined to small accelerations spontaneous-sway measures, tended to show an inverse and a gentle pseudorandom motion was used. To derive a relationship. J Am Geriatr SOC38:l-9, 1990 (1) measurement of spontaneous postural sway during quiet standing, and (2)measurement of induced postural

F

alling is known to be one of the most serious problems facing older persons. It is the leading cause of accidental death in the 65+ age group.' For every fall that leads to death, there are almost 20 falls that result in fracture of the proximal - femur. An estimated 200,000 hip fractures occur in the United States each year, 84% of which are in the 654age group, and the associated health-care costs have been estimated at $2 billion per year.2 Perhaps even more sigdicant than the acute injuries are the psycho-

From the Centre for Studies in Aging at S u n n y b m k Medical Centre, Department of Surgery, University of Toronto, Toronto, Ontario, Canada. Supported by Medical Research Council of Canada, Grant MA-8025.B. E. Maki holds a Research Fellowship from the Ontario Ministry of Health. G. R. Femie holds a National Health Research Scholarship from Health and Welfare Canada. Address correspondence and reprint requests to B. E. Maki, PhD, Centre for Studies in Aging, S u n n y b m k Medical Centre, 2075 Bayview Avenue, Toronto, Ontario, Canada M4N 3M5.

0 1990 by the American Geriatrics Society

logical after-effects of experiencing a fall: the so-called postfall syndrome, a severe fear of falling that limits mobility and independence? Although falling is a complex and multifactorial problem, there is little doubt that deterioration in postural balance can be one of the major contributing factors. This may result in an impaired ability to recover from the many small postural perturbations that are experienced in normal daily activities (eg, slips, trips, missteps, and jostles), perturbations that are easily withstood by healthy young adults.' Postural balance can be assessed subjectively by skilled clinicians or can be quantified through the grading of performance during various maneuvers, such as walking, standing up, reaching, bending, and tumingY Although these approaches may be useful in identifying gross balance deficits, posturographic measurements may enhance the ability to identify more subtle balance impairments. One of the simplest and most widely used posturographic balance tests involves the measurement 0002-8614/90/53.50

2 MAKIETAL

of spontaneous postural sway during quiet standing.6-" An alternate approach is to measure the response to an applied postural p e r t u r b a t i ~ n . ~ ~ * ' ~ - ~ ' In using a balance test to predict falling liability, it is important that the test characterizes the relative stability of the individual in actual falling circumstances. Spontaneous sway, although easily measured, may bear little relation to the postural performance in typical falling situations, which involve much larger amplitudes of swaying motion. Induced-sway balance tests, which use applied perturbations to elicit larger amplitudes of sway, might be expected to better simulate actual falling circumstances. The objective of this study was to compare measures of spontaneous sway with the results of an inducedsway balance test that was specially developed to be safe and nonthreatening for elderly subjects. The correlation between the spontaneous- and induced-sway measures was assessed, and the two types of measurements were further compared in terms of (1) ability to quantify the deterioration in postural control known to occur in normal aging, and (2) ability to identify elderly subjects who are at risk of falling. The ultimate goal is to find the simplest balance test that can provide the highest level of success in predicting falling liability. Such a test could then be used as a clinical screening tool to identify individuals who are at risk of falling before they experience a debilitating fall, thereby allowing preventive measures to be taken. MATERIALS AND METHODS Subjects Sixty-four healthy ("normal") subjects were tested: 32 between the ages of 20 and 40 (mean age, 25; SD = 5), and 32 between the ages of 61 and 79 (mean age, 69; SD = 5). There were equal numbers of males and females in each age group. The sample size was chosen to allow mean age- or sex-related differences of one standard deviation or more to be identified with a statistical power of 90%, at a 1% level of significance (two-way ANOVA). In selecting the normal subjects, a verbal questionnaire and simple clinical tests were used to screen for impairments or medications that might affect balance. In particular, subjects were excluded if they reported: (1) one or more falls during the previous year, (2) any diagnosed neuromotor disorders or vestibular deficits, (3) a history of balance or coordination problems, or dizziness, (4) any disorders, illnesses or injuries that the subject judged might interfere significantly with his/her activities, or (5)current use of any sedatives, tranquilizers, or muscle relaxants. In addition, subjects were excluded if the clinical testing showed: (1) a Snellen visual-acuity score poorer than 20/200 (with corrective lenses, if normally worn), (2) severely impaired range of motion at the hip, knee, or ankle, or (3) severely impaired kinaesthesis at the toes and ankle. The tests were

IAGS-JANUARY 1990-VOL 38, NO. 1

rescheduled if, on the day of testing, the subjects had consumed any alcohol or if they had engaged in strenuous physical activity within the previous three hours. The elderly normals were all independent and ambulatory community-dwellers. Over 78% of the elderly subjects and 88% of the young subjects reported that they participated in some form of physical exercise of at least 15 minutes duration one or more times per week. Only 3%of the elderly subjects reported that they spent most of their day sitting, in comparison to 25qb of the young subjects. Also tested were five elderly subjects with a history of falling. The fallers were residents of a residential-care complex for the elderly and ranged in age from 75 to 86. All were ambulatory and had experienced one or more falls during the previous year (but none within the previous month). The Balance Tests A "balance platform" was used to perform the spontaneous- and induced-sway balance tests. The platform is controlled to move back and forth during the induced-sway tests; during the spontaneoussway tests, the platform remains stationary. The subject stands on two force plates that are mounted on the balance platform. Safety handrails and foam padding are mounted to the front and sides of the force plates; the experimenter stands directly behind the subject during the tests. The force plates record the location of the center of pressure of the feet. During the induced-sway tests, an accelerometer records the platform acceleration. Details about the platform design and performance are provided e l s e ~ h e r e . ~ ~ , ~ ~ The perturbation used in the induced-sway test was an anterior - posterior platform acceleration. This type of perturbation simulates actual falling in that it creates a relative acceleration between the feet and the upper body. Essentially, the moving-platform test was designed to simulate falling relative to the platform frame of reference; thus, if the subject is able to remain perfectly erect and not sway relative to the platform, then visual cues must "tell" him or her that he or she is not swaying. This is achieved by coupling the visual surround to move with the platform, producing a testing situation equivalent to standing in a windowless, moving room. The rationale for these test conditions is discussed in more detail e l s e ~ h e r e . ~ ~ * ~ ' Although a sudden (transient) platform motion would provide the best simulation of actual falls, such a perturbation was not used in the balance test in order to avoid potential safety problems and the possibility of inducing anticipatory adaptations that might not occur in actual falls, eg, leaning forward.2sInstead of moving the platform suddenly, the platform was controlled to move gently back and forth continuously during the test. The resulting acceleration and center-of-pressure data were fitted with an input-output model, and this

IAGS-IANUARY 1990-VOL 38, NO. I

model was used to predict the response to a sudden platform motion, thereby simulating balance recovery during an actual fall. To simulate the unpredictable nature of fall-provoking perturbations, a pseudorandom perturbation was used. Pilot tests were performed to select appropriate perturbation characteristics.26(The pseudorandom signal was constructed as a s u m of sine waves with frequencies ranging from 0.1 to 5.0 Hz; the root-mean-square acceleration and velocity were 0.1 m/sz and 4 cm/s, respectively, and the total range of motion was 15 cm.23*24)

COMPARISON OF BALANCE TESTS

3

ration amplitude is determined by calculating the amplitude of transient platform motion at which the resulting center-of-pressure displacement would reach its maximum value, ie, the length of the base of support (in engineering parlance, this is known as “saturation”). The length of the base of support was defined as the anterior-posterior length of the feet. The analytical methodology and underlying rationale of this approach are presented in detail el~ewhere.~~*~’,~’

Quantification of Spontaneous-Sway Performance Spontaneous anterior- posterior center-of-pressure Protocol The protocol included a spontaneous-sway fluctuations measured during the static tests were quantest and two induced-sway tests, the h t of which was tified by means of amplitude-based and frequencytreated as a learning trial. All of these tests were per- based measures. The amplitude-based measures informed with eyes open. Spontaneous sway was mea- cluded: (1) root-mean-square center-of-pressure sured for 77 seconds; the duration of each induced-sway displacement relative to the mean center-of-pressure test was 185 seconds. There was a two-minute seated location (RMS), (2) peak-to-peak range of center-ofrest between tests, to minimize fatigue. pressure displacement (RANGE), and (3) average speed For each test, the subjects were instructed to ”stand of center-of-pressure displacement (SPEED). These relaxed as if waiting in a line-up,” with feet comfortably three variables were analyzed both before and after norspaced (unshod), and arms at sides. The heels were malization. The normalization was performed by dividaligned against a vertical panel placed at right angles to ing each subject’s RMS, RANGE, and SPEED scores by the direction of platform motion (which was then re- the length of his or her base of support (BOS), ie, the moved before starting the test), but the subjects were anterior-posterior length of the feet. This normalizaotherwise free to select the angulation and medial- tion serves to express the measured center-of-pressure lateral spacing of their feet so as to simulate the base of sway amplitude as a fraction of the maximum possible support that they would use in everyday life. Foot trac- center-of-pressure displacement, and thereby allows ings were used to allow identical repositioning in subse- relative stability to be compared for subjects of differing quent tests. To ensure consistent visual and vestibular body dimensions. input, the subjects were instructed to look straight The data were used to estimate the mean frequencyof ahead, using a poster as a visual target (eye-object dis- sway (MFREQ), using the formulae presented by tance of 70 cm). During the perturbation tests, Hufschmidt et a1.28In addition, frequency-based mea“Muzak”” (ie, bland, monotonous music) was played sures were calculated from the power spectrum of the through headphones to mask auditory cues from the anterior- posterior center-of-pressure displacement. (The power spectrum was estimated by dividing the platform motor. For the induced-sway tests, the platform motion was data into segments, computing the Fast Fourier Transcontrolled to start and end gradually, with no sudden form for each segment, and averaging the resulting The power spectrum was characterchanges in acceleration. For the spontaneous-sway test, “period~grams.”~~) ized by: (1) the centroidal frequency (CFREQ), which is the platform remained stationary. a measure of the “average frequency” at which the sway Quantification of Induced-Sway Performance The energy tends to be concentrated; and (2) the dispersion balance score for the induced-sway test is called the (DISP), which quantifies the degree to which the sway “saturation amplitude” (SA). The saturation amplitude energy “spreads out” to include other frequencies.30 is a prediction of the maximum fall-provoking perturbation that the subject can withstand without ”losing Statistical Analyses The correlation between the balance” (ie, having to take a step or grab a handrail); spontaneous-sway and induced-sway scores was ashence, larger values are predictive of greater relative sessed by means of linear regression analysis. For the stability. normal subjects, a two-way ANOVA was used to test for The saturation amplitudeis estimated from the exper- age- and sex-related differences in the induced- and imental data as follows: (1) an input-output model is spontaneous-sway scores. For each faller, percentile fitted to the data, treating the platform acceleration as scores were calculated by comparing the individual’s the input and anterior-posterior center-of-pressure dis- scores with the distributions estimated for the elderly placement as the output; (2) the model is used to predict normals; subjects with scores lying beyond a selected the center-of-pressure displacement in response to a percentile level were classified as balance-impaired. sudden (transient) platform motion (so as to simulate Thus, at a 25% false-positivelevel, subjects were classibalance recovery during an actual fall); and (3)the satu- fied as balance-impaired if their SA score was below the

4 MAKIETAL

IAGS-IANUARY 1990-VOL 38, NO. I

25th percentile or if their RMS, SPEED, RANGE, RMS/ greater stability) in the young subjects. No significant BOS, SPEED/BOS, or RANGE/BOS score was above sex-related difference was found (P> .1). For the spontaneous-sway measures, significant agethe 75th percentile. For MFREQ, CFREQ, and DISP, the criterion was two-tailed, so that subjects with scores related differences were found for SPEED and SPEED/ either above the 87.5th percentile or below the 12.5th BOS (P < .01) and for RANGE, RANGE/BOS, CFREQ, and DISP (P < .05). MFREQ, RMS, and RMS/BOS percentile were classified as balance-impaired. In order to apply the regression and ANOVA statisti- showed no significant age-related differences (P > .05). cal methods, it was necessary to apply "variance-stabi- For all of the measures, the estimated mean (ie, lizing" transf~rmations~lto the spontaneous-sway the untransformed value) was higher for the elderly data. This was true for all the spontaneous-sway mea- subjects. Sex-related differences were significant for sures except DISP. (CFREQ was transformed by a recip- RMS (P < .05), for SPEED (P < .Ol), and for CFREQ rocal cubic function, and the other measures were trans- and DISP (P < .0001), with the higher mean values (unformed using the common logarithm function.) In the transformed) occumng in the males. For RMS/BOS and remainder of this paper, all references to the spontane- SPEED/BOS, the normalization with respect to base of ous-sway measures (other than DISP) apply to the support eliminated the sex-related differences (P > .1). transformed versions, unless specified otherwise. Identification of the Elderly Fallers The percentile For each analysis, the assumptions of the statistical scores of the fallers are listed in Table 3. When the elmodel (ie, normality, uniform variance, and independerly normals and fallers were classified using a 25% dence of the residuals) were assessed by plotting the false-positive criterion (a= 0.25), none of the fallers residuals and performing standard statistical t e ~ t s . ~ l * ~ * was identified as balance-impaired using the inducedThe statistical analyses were performed using a comsway score as the predictor variable. However, using mercial software package, Minitab (Pennsylvania State either SPEED, SPEED/BOS, or MFREQ, four of the five University, University Park, PA). fallers were identified correctly (at the a = 0.25 criterion or better). The remaining spontaneous-sway measures RESULTS each identified only one or two of the five fallers as Age- and Sex-Related Differences in Normal Subbeing balance-impaired. jects Descriptive statistics and the results of the ANOVAs are listed in Tables 1 and 2, respectively. The Correlation Between Spontaneous- and InducedANOVAs showed no significant age - sex interaction in Sway Responses Table 4 summarizes the results of the any of the analyses (P > .l);therefore, separate age and linear regression of the spontaneous-sway measures sex effects can be assessed. versus SA. For the normal subjects, the correlations beFor the induced-sway score (ie, SA), the ANOVA tween the spontaneous-sway measures and the SA were showed highly significant differences between the two relatively weak, as evidenced by the small correlation age groups (P < .0001), with a larger mean value (ie, coefficients. Nonetheless, in the young normals, the

TABLE 1. AGE- AND SEX-RELATED DIFFERENCES IN NORMALS DESCRIPTIVE STATISTICS Mean f Standard Deviation* Sway Measure

Induced sway SA (m/s) Spontaneous sway RMS (mm) RMS/BOS (%) SPEED (mm/s) SPEED/BOS ('Y/s) RANGE (cm) RANGE/BOS ('Yo) MFREQ (Hz/lO) CFREQ (Hz/100) DISP (/loo)

Youna

Elderly

Males

Females

0.367 k 0.051

0.299f 0.040

0.336 f 0.051

0.330 f 0.062

4.08f 1.88 1.68 f 0.790 5.08 f 3.17 2.08 f 1.27 2.28 f 1.15 9.42f 4.90 2.22 f 0.673 8.30 f 3.46 4.34 f 1.93

4.56f 1.97 1.86f 0.785 6.20 k 2.12 2.52 f 0.816 3.00 f 1.93 12.1 f 7.19 2.59 f 0.838 8.82 f 2.53 5.31 k 1.64

4.74 f 2.03 1.83 f 0.781 6.42f 3.17 2.48f 1.24 2.98f 1.97 11.5k 7.41 2.44f 0.665 9.60f 3.84 5.70 f 1.74

3.91 k 1.74 1.71 k 0.800 4.86f 1.97 2.12 k 0.890 2.30 f 1.09 10.0 k 4.85 2.37f 0.883 7.52 f 1.23 3.94f 1.51

SA = saturation amplitude; R M S , SPEED, RANGE, and MFREQ = root-mean-square value, average speed, peak-to-peak range, and mean frequency of center-of-pressure displacement; CFREQ and DISP = centroidal frequency and dispersion of center-of-pressure displacement power spectrum; and BOS = length of base-of-support. Sixteen males and 16 females in each age group.

IAGS-IANUARY 1990-VOL 38, NO. I

COMPARISON OF BALANCE TESTS

TABLE 2. AGE- AND SEX-RELATED DIFFERENCES IN NORMALS RESULTS OF ANOVAs F-Values' Sway Measure

Age

Sex

Interaction

35. Ej

0.29

Induced sway SA

Spontaneous sway log [RMS]

4.4t 0.73 7.4$ 2.0 3.9 0.81 0.39 21. Ej 20.6

1.4 1.3 8.0$ 7.5$ 4.4t 4.3t 3.8 5.3t 6.lt

log [RMS/BOS] log [SPEED] log [SPEED/BOS] log [RANGE] log [RANGE/BOS] log [MFREQ] 1/[CFREQI3 DISP

0.18 0.35 0.24 0.50 0.34 1.3 1.o 0.0066 1.4 0.70

SA = saturation amplitude; RMS, SPEED, RANGE, and MFREQ = root-mean-square value, average speed, peak-to-peak range, and mean frequency of center-of-pressure displacement; CFREQ and DISP = centroidal frequency and dispersion of center-of-pressure displacement power spectrum; and BOS = length of base of support. F-Values: test for significant differences between means due to age, sex, or age-sex interaction; 16 males and 16 females in each age group; degrees offreedom = 1.60. t P < .05; $ P < .01; 0 P < .OO01; P > .05 in all other cases.

correlation was significant (ie, the correlation coefficient was significantly different from zero) in some cases: namely, for RMS, RMS/BOS, RANGE, and RANGE/ BOS (P < .05).For SPEED/BOS, the analysis showed a possible trend toward a significant correlation (P< .1). None of SPEED, MFREQ, CFREQ, or DISP showed any significant correlation with SA (P > .1). For the elderly normals, there was some evidence of a possible trend toward a significant correlation between RANGE and SA (P < .1). For this regression, the sign of the correlation coefficient was opposite to that seen in

5

the young normals. None of the other variables showed any significant correlation with SA (P > .1). Although the fallers tended to show higher estimates for the correlation coefficients, these estimates have a large bias error as they were based on a very small number of subjects,31and none of the correlationswas sigruficant (P > .1). Validation of the Statistical Models In all of the above analyses, the residual plots and statistical tests gave no evidence to suggest that the residuals were not normally and independently distributed or that the variances were not uniform (a= 0.05). DISCUSSION Biomechanical Considerations The different balance testing methodologies and measures may provide different types of information about postural performance and stability. As postural adjustments require displacement of the center of pressure, instability will result when the center of pressure cannot be displaced the required distance, ie, when it reaches the perimeter of the base of support. Whereas spontaneous RMS, RANGE, and, particularly, RMS/BOS and RANGE/BOS reflect the degree to which this limit of stability is approached during quiet standing, the induced-sway test provides this information at much larger amplitudes of sway and is therefore expected to give better predictions of large-amplitude postural performance during actual falls. Small-amplitude spontaneous sway may fail to correlate with largeamplitude performance because of nonlinearities in the posture control system (eg, sensory thresholds) and/or amplitude-dependentchangesin balancing strategy (eg, reliance on passive muscle stiffness versus coordinated muscle activation). The induced-swaytest has a further advantage in that it allows for the identificationof an input- output model

TABLE 3. CLASSIFICATION OF ELDERLY FALLERS Percentile Score Based on Balance Test Score Spontaneous-Sway Measures

Subject F1 F2 F3 F4

F5

Log RMS

Log SPEED

Log RANGE

Log RMS/ BOS

66 63 1 50 91'

91' 96' 16 99'

70 59 3 52 86*

70 74 2 52 92*

99'

Log

Induced-Sway Measure

SPEED/ BOS

Log RANGE/ BOS

Log MFREQ

1/ CFREQ'

DISP

SA

94' 99' 20 99' 99'

74 69 3 54 88.

80 92* 9 7' 99' 98'

39 16 85 13 4'

63 84 15 91' 9 7'

58 83 32 90 58

SA = saturation amplitude; RMS, SPEED, RANGE, and MFREQ = root-mean-square value, average speed, peak-to-peak range, and mean frequency of center-of-pressure displacement; CFREQ and DlSP = centroidal frequency and dispersion of center-of-pressure displacement power spectrum; m d BOS = length of base of support. Balance-impaired at a = 0.25 criterion (two-tailed for MFREQ, CFREQ, and DISP; lower one-tail for SA; upper one-tail for other measures).

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IAGS-JANUARY 1990-VOL 38, NO. 1

MAKIETAL

TABLE 4. CORRELATION BETWEEN SPONTANEOUS- AND INDUCED-SWAY MEASURES Correlation Between Spontaneous-Sway Measure and Induced-Sway Measure (Saturation Amplitude)* SpontaneousSway Measure

Young Normals

Elderly Normals

Elderly Fa11e rs

log (RMS] log [RMS/BOX] log [SPEED] log [SPEED/BOX] log [RANGE] log [RANGE/BOS] log [MFREQ] 1/[CFREQl3 DISP

-0.35t -0.42t -0.24 -0.31 -0.37t -0.43t 0.13 0.12 -0.11

0.22 0.15 0.08 -0.02 0.32 0.26 -0.21 -0.23 0.21

0.49 0.51 0.61 0.64 0.49 0.51 0.35 -0.65 0.65

RMS, SPEED, RANGE, and MFREQ = root-mean-square value, average speed, peak-to-peak range, and mean frequency of center-of-pressure displacement; CFREQ and DlSP = centroidal frequency and dispersion of center-of-pressure displacement power spectrum; and BOS = length of base of support. Estimated correlation coefficient (Pearson product moment). t Correlation significant (correlation coefficient # 0)at P < .05;P > .05 in all other cases CT-test for significance of correlation; degrees of freedom = 30 for each normal group, 3 for fallers).

(or increased SPEED, in the absence of a proportionate increase in RMS) may be indicative of a compensation for some underlying neural or sensorimotor dysfunction. Age-Related Differences in Normal Subjects Because of the physiological changes that are known to occur in aging (eg, decrease in nerve conduction velocity, deterioration of visual, vestibular and somatosensory function, reduction in muscle strength, and degeneration of motoneurons), one would expect to find age-related differences in postural stability. The following results have been demonstrated in elderly subjects: (1) deteriorated balancing synergies in which muscle contractions are delayed and occur in a reversed seq u e n ~ e , ' ~ (2) J ~ ,more ~ ~ extreme and less effective responses to impulsive force perturbations,2' (3)increased postural sway in response to platform tilt perturbations,zO and (4) increased spontaneous postural sway~6-10.14.16

In general agreement with these findings, the present results did in fact demonstrate significant differences between young and elderly normals, with reduced stability (ie, smaller SA in the induced sway test, larger spontaneous RANGE and RANGE/BOS) and increased balancing "activity" (ie, larger spontaneous SPEED and SPEED/BOS) predicted for the elderly. Note, however, that no significant age-related differences were found in that characterizes the "cause-and-effect" relationship spontaneous anterior - posterior RMS or RMS/BOS. between perturbation (input) and postural response This is in agreement with the results of Hayes et a1,I6 (output). Spontaneous-sway tests fail to characterize although these authors did find some small but signifithis input-output behavior and hence can provide no cant age-related increases in the medial-lateral direcdirect information about the latency, magnitude, or tion. speed of the stabilizing response to a fall-provoking The present results also demonstrated significant perturbation. Thus, the common assumption that sponage-related differences in frequency-based spontanetaneous RMS and RANGE provide a measure of posous-sway measures, ie, increase in CFREQ and DISP in tural instability should be used with some caution. the elderly subjects. This appears to agree with Hayes et Certain spontaneous-sway measures may be indicaa1,I6 who reported increased high-frequency sway in tive of underlying neural or sensorimotor dysfunction. some elderly subjects. In spite of the age-related differFor example, frequency-based measures such as ences in CFREQ and DISP, the power spectrum was MFREQ, CFREQ, and DISP may reflect the changes in found to be similar in shape for all subjects, and in no the frequency content of sway that have been observed cases showed the pronounced peaks that have been rein patients with cerebellar lesions33or disturbances of ported in certain disorder^.^^-^^ muscle aff e r e n t a t i ~ n . ~ ' , ~ ~ Spontaneous SPEED represents the length of the Sex-Related Differences in Normal Subjects The "sway path" that is traced out by the center-of-pressure present study showed no significant sex-related differexcursions during a given time interval, and thus may ences in those variables that can be interpreted most reflect the amount of regulatory balancing "activity" directly in terms of postural stability, ie, SA, RMS/BOS, that is needed to maintain an upright posture.28Subjects and RANGE/BOS. Sex-related differences in nonnorwith high values of SPEED may be quite stable, in the malized RMS and SPEED were apparently due to sexsense that the center of pressure does not approach the related differences in body size, as they disappeared limits of the base of support, but may require frequent when the center-of-pressure displacement was related postural corrections to achieve this degree of stability. to the length of the base of support. The sex-related MFREQ is proportional to SPEED/RMS and thus may differencesin CFREQ and DISP may also be due largely relate the amount of balancing activity that occurs to anthropometric factors. Most postural-sway studies (SPEED) to the degree of stability that is achieved have failed to demonstrate any gender effects. The sig(RMS).If this interpretation is correct, increased MFREQ nificant sex-related differences reported by Overstall et

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a19 could be due to a failure to normalize for the influence of anthropometric differences. Identification of Balance-Impaired Individuals In using data from eyes-open tests, the greatest success in identifying the five fallers was achieved using spontaneous SPEED, SPEED/BOS, or MFREQ. It is not clear why these variables should yield better predictions of falling than the induced-sway score (SA)or spontaneous RMS/BOS and RANGE/BOS, variables that can be more readily interpreted in terms of biomechanical stability. One possible explanation relates to the suggestion presented earlier that increased MFREQ (or SPEED, without a proportionate increase in RMS) might be indicative of a subject who has compensated for some neural or sensorimotor dysfunction. It may be that the underlying dysfunction contributes to the falls of these subjects in a manner that is unrelated to postural stability, eg, a sensory defiat that results in a reduced ability to detect and avoid environmental hazards. In contrast to the present results, Bartlett et all7 attained a success rate of only 574b in using speed of sway to identify individual fallers. The disparity may lie in methodological differences (these authors measured low-frequency hip displacement). Using a combination of eyes-closed anterior-posterior and medial -lateral root-mean-square sway and range of sway, Kirshen et all3 were only able to identify nine of 17 fallers (53%) and 37 of 49 controls (76%) correctly. They did not present the results obtained using eyes-open data. The present results showed relatively normal (or better) values for eyes-open RMS and RANGE in four of five fallers. Correlation Between Spontaneous- and InducedSway Responses For spontaneous RMS, RANGE, RMS/BOS, and RANGE/BOS, larger values are generally expected to indicate greater instability. Because the SA is instead a measure of stability, one would expect a negative correlation between these measures of spontaneous sway and the SA.Only in the young normal subjects was this found to be the case. The elderly normals and fallers either failed to show this trend or tended to show the opposite trend. For the other sway measures, the correlations were found to lack statistical sigruficance. Nonetheless, it is interesting to note that, almost without exception, the sign of the estimated correlation coefficient for the elderly normals and fallers was opposite to that seen in the young normals. Conceivably, in the elderly normal and faller groups, the induced- and spontaneous-swayresults fail to correlate as expected because of the varying extent to which the different subjects modulate their balancing strategies in the two testing situations. In particular, the more unstable subjects may show a greater tendency to increase the “stiffness” of their posture control system during the induced-sway test, in order to prevent them-

COMPARISON OF BALANCE TESTS

7

selves from falling, but may show relatively normal stiffness during the spontaneous-sway test, in the absence of a serious postural threat. Conversely, for the young normals, the two testing situationsmay represent equally low levels of postural threat; therefore, these subjects may tend to show similar stiffnesslevels in both types of tests. One mechanism by which stiffnessmodulation could occur would be co-contractionof antagonist muscles. Age-related increases in co-contraction in the postural responses to transient perturbations have, in fact, been reported.3‘j Comparison of the Balance Tests The clinicalutility of the induced-sway test can be assessed by comparing its sensitivity in identifying impaired postural control to that of the simpler spontaneous-swaymeasurements. In using the eyes-open induced-sway data to identify fallers, the results were poor and greater success was achieved using certain spontaneous-sway measures. However, as reported much greater success in identifying the fivefallers was achieved using the results from blindfolded induced-sway tests (in contrast to the elderly normals, the fallers were relatively unaffected by the change in visual conditions). Blindfolded spontaneous sway was not measured; therefore the two testing methodologies cannot be compared with regard to vision-deprived balancing performance. Although Kirshen et all3found eyes-closed measurements of spontaneous sway to provide the best discrimination of fallers from controls, Bartlett et all7 obtained the same misclassification rates using either eyes-open or eyes-closed data. In contrast to Bartlett‘s results, the induced-sway misclassification rates seem to be highly dependent on the visual conditions. It may be that the induced-sway test is a more sensitive measure of the influence of visual factors on postural performance. A comparative evaluation of the two testing methods under both visual conditions is currently underway. For the normal subjects, the age-related differences in the spontaneous-sway measures were not as pronounced as those seen in the induced-sway results (ie, P < .01 or P < .05 for spontaneous sway compared to P < .0001 for induced sway). This suggests that the induced-sway test may be a more sensitive measure of the deterioration in balance expected to occur in normal aging. Limitations The induced-sway test is based on the premise that data obtained using a small, continuous, pseudorandom perturbation can be used to predict the response to the large, sudden (transient) perturbations that actually cause falls. Although the test may be useful clinically even if it fails to yield completely accurate predictions of transient response, one would expect the ability to identify fallers to improve with the accuracy of the transient-responsepredictions. Direct assessmentof the theoretical premise for the induced-sway test was

8 MAKIETAL

performed by comparing the predictions derived from pseudorandom tests with actual measurements of transient response in young adult subject^.^^**^ The results showed the pseudorandom predictions to overestimate the center-of-pressure displacement at larger transient amplitudes; however, since the transient perturbations were predictable in these tests, the results may have been confounded by adaptive effects. The experiments are currently being repeated under randomized conditions. Diener et a13' have suggested that continuous and transient platform perturbations activate different modes of postural control, based on experiments in which they varied the visual and vestibular inputs; however, it is difficult to extrapolate from their experimental paradigm. Their continuous perturbation was a sinusoidal tilting motion; it is well known that subjects can learn to predict this type of motion and thereby learn to "ride" the platform with minimal In addition, a platform tilt elicits a much different response than horizontal platform translation, as the stretch-reflex response to the tilt acts to destabilize the body.39The authors are currently performing similar experiments using pseudorandom platform translations instead of sinusoidal platform tilts. For both induced and spontaneous sway, the analyses were limited to the anterior-posterior direction. It seems likely that greater success in identifying fallers will be achieved by also including measures of medial lateral stability. In the present study, subjects were allowed to choose their foot angulation and mediallateral spacing so as to simulate their everyday standing posture. In medial-lateral tests, it will be particularily important to replicate the base of support that the subject would normally choose, as it is conceivable that fallers could compensate substantially for deteriorated balance by increasing their medial - lateral foot spacing. Other potential limitations of the study relate to the subject selection. In screening the normal subjects, it was necessary to rely on self-reports and simple clinical tests in order to identify medical disorders; thus, some balance-impaired individuals may have inadvertently been included in the normal subject groups. Conversely, the fallers were not necessarily balance-impaired, as their falls could have resulted from other causes, eg, transient physiological disturbances, impaired ability to avoid environmental hazards, or exposure to extraordinarily severe perturbations. Ultimately, in order to achieve the most accurate predictions of falling liability, it may be necessary to combine the balance measures Some of with tests that assess these other risk fa~tors.~,'~ the differences that were detected could have been due to the more advanced age of the fallers. In addition, although individuals with very recent falls were screened from the study, poor test results in the fallers could still have been due, at least in part, to the debilitat-

JAGS-JANUARY 1990-VOL 38, NO. 1

ing effects of their falls. Subjects who are at risk but have yet to experience a fall may present more subtle balance impairments. It should be emphasized that the results for the five fallers presented here are intended to provide no more than a preliminary evaluation of the predictive capabilities of the balance tests, as the small sample size severely limits the accuracy with which the true misclassi6cation rates can be e~timated.'~ Much larger numbers of subjects are currently being tested in order to provide a more definitive assessment of the balance tests.

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TS,Hadley E, Suzman R (eds):Symposium on Falls in the Elderly: Biologic and Behavioural Aspects. Philadelphia, WB Saunders, 1985, p. 501 3. lsaacs B: Falls in old age, in Hinchcliffe R (ed): Hearing and Balance in the Elderly. New York, Churchill Livingstone, 1983, p. 373 4. Wild D, Nayak USL, Isaacs B: Description, classification and prevention of falls in old people at home. Rheumatol Rehab 20:153, 1981 5. Tinetti ME, Williams TF, Mayewski R: Fall risk index for elderly patients based on number of chronic disabilities. Am J Med 80:429, 1986 6. Sheldon JH: The effect of age on the control of sway. Gerontol Clin 5:129, 1963 7. Hasselkus BR, Shambes GM: Aging and postural sway in women. J Gerontol 6:661, 1975 8. Black FO, OLeary DP, Wall C, et al: The vestibulo-spinal stability test: normal limits.Trans Am Acad Ophthalmol Otol84:549, 1977 9. Overstall PW, Exton-Smith AN, Imms FJ, et al: Falls in the elderly related to postural imbalance. Br Med J 1261,1977 10. Femie GR, Holliday PJ: Postural sway in amputees and normal subjects. J Bone Joint Surg 60A895,1978 1 1 . Brocklehurst JC, Robertson D, James-Groom P: Clinical correlates of sway in old age-sensory modalities. Age Aging 11:1, 1982 12. Femie GR, Gryfe CI, Holliday PJ, et al: The relationship of postural sway in standing to the incidence of falls in geriatric subjects. Age Ageing 11:11,1982 13. Kirshen AJ, Cape RDT, Hayes KC, et al: Postural sway and cardiovascular parameters assodated with falls in the elderly, J Clin Exp Gerontol 6291, 1984 14. Era P, Heikkinen E: Postural sway during standing and unexpected disturbance of balance in random samples of men of different ages. J Gerontol8:287, 1985 15. Gabell A, Simons MA, Nayak U S L Falls in the healthy elderly: predisposing causes. Ergonomics 28:965, 1985 16. Hayes KC, Spencer JD, Lucy SD, et al: Age-related changes in postural sway, in Winter DA, Norman RW, Wells RP, et a1 (&): Biomechanics IX-A. Champaign, Ill., Human Kinetics Publishers, 1985, p. 383 17. Bartlett SA, Holliday PJ, Maki BE, et al: On the classificationof a geriatric subject as a faller or non-faller. Med Biol Eng Comput 24:219, 1986 18. Woollacott MP, Shumway-Cook AT, Nashner LM:Changes in the postural response system with aging. Soc Neurosci Abstr 8:838, 1982 19. Woollacott MP, Shumway-Cook AT, Nashner LM: Postural reflexes and aging, in Mortimer JA, Pirozzola FJ,Maletta GJ (&): The Aging Motor System. New York, Praeger, 1982, p. 98 20. Holliday PJ, Femie GR Postural sway during low frequency floor oscillation in young and elderly subjects, in lgarashi M, Black FO (eds): Vestibular and Visual Control in Posture and Locomotor Equilibrium. Basel, Switzerland, Karger, 1985, p. 66 21. Wolfson LI, Whipple R, Amerman P, et al: Stressing the postural

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32. Montgomery DG: Design and Analysis of Experiments, 2d ed. New York, John Wiley and Sons, 1984,p. 85 33. Mauritz KH, Dichgans J, Hufschmidt A: Quantitative analysis of stance in late cortical cerebellar atrophy of the anterior lobe and other forms of cerebellar ataxia. Brain 102:461, 1979 34. Aggashyan RV, Gurfinkel VS, Mamasakhlisov GV, et al: Changes in spectral and correlation characteristics of human stabilograms at muscle afferentation disturbance. Agressologie 14D5,1973 35. Dichgans J, Mauritz KH, AUum JHJ, et al: Postural sway in normals and atactic patients: analysis of the stabilizing and destabilizing effects of vision. Agressologie 17C:15,1975 36. Woollacott M, Inglin B, Manchester D: Response preparation and posture controk neuromuscular changes in the older adult. Ann NY Acad Sci 515:42,1988 37. Diener HC, Dichgans J, Guschlbauer 8, et a1 Role of visual and static vestibular influences on dynamic postural control. Hum Neurobiol 5:105, 1986 38. Andm R O Diagnostic implications of induced body sway, in Honrubia V, Brazier MAB (eds): Nystagmus and Vertigo: Clinical Approaches to the Patient with Dizziness. New York, Academic Press, 1982,p. 191 39. Diener HC, Dichgans J, Bootz F, et al: Early stabilization of human posture after a sudden disturbance: influence of rate and amplitude of displacement. Exp Brain Res 56:126,1984 40. Morse JM: Computerized evaluation of a scale to identify the fall-prone patient. Can J Public Health 77:21, 1986