FALLS occur commonly among older people, even

Transcription

1 Journal of Gerontology: MEDICAL SCIENCES 2008, Vol. 63A, No. 2, Copyright 2008 by The Gerontological Society of America Force Platform Balance Measures as Predictors of Indoor and Outdoor Falls in Community-Dwelling Women Aged Years Satu Pajala, 1 Pertti Era, 1,2 Markku Koskenvuo, 3 Jaakko Kaprio, 3,4 Timo Törmäkangas, 1 and Taina Rantanen 1 1 Department of Health Sciences, The Finnish Centre for Interdisciplinary Gerontology, University of Jyväskylä, Finland. 2 Finnish Brain Research and Rehabilitation Center Neuron, Kuopio, Finland. 3 Department of Public Health, University of Helsinki, Finland. 4 Department of Mental Health and Alcohol Research, National Public Health Institute, Helsinki, Finland. Background. Inability to maintain balance while standing increases risk of falls in older people. The present study assessed whether center of pressure (COP) movement measured with force platform technology predicts risk for falls among older people with no manifest deficiency in standing balance. Methods. Participants were 434 community-dwelling women, aged years. COP was measured in six stances on a force platform. Following balance tests, participants reported their falls with 12 monthly calendars. Incidence rate ratios (IRR) with 95% confidence intervals (CI) were computed from negative binomial regression models. For the analysis, those with 1 fall indoors were coded indoor fallers, those with 1 fall outdoors, but no indoor falls, were coded outdoor fallers. Outcome in the models was number of falls. Analyses were repeated including only participants without fall history prior to follow-up. Results. Among 198 fallers, there were 57 indoor and 132 outdoor fallers. The participants in the highest COP movement tertile, irrespective of the balance test, had a two- to fourfold risk for indoor falls compared to participants in the lowest COP tertile of the test. Inability to complete the tandem stance was also a significant predictor of the fall risk. The trend for increased risk for indoor falls was found also for participants in the highest COP movement tertile and without fall history. The COP movement in balance tests was not associated with outdoor falls. Conclusion. Force platform balance tests provide valid information of postural control that can be used to predict fall risk even among older people without apparent balance problems or fall history. When the force platform is not available, tandem stance provides a screening tool to show increased fall risk in community-dwelling older people. Key Words: Postural balance Falls Prospective study Fall prediction Balance assessment Fall risk. FALLS occur commonly among older people, even among those with good health who are living independently in their homes and having no apparent balance problems (1,2). It is important to identify balance problems at an early phase as the first fall can predispose older persons to more falls with possible injury and fear of falling, which further may lead to limited activity and disability. For this purpose, balance measurements sensitive enough to reveal incipient deterioration in balance control are needed. Functional balance measures usually lack the ability to capture balance impairment at its early phase when no manifest balance problem yet exists. Thus, these tests are prone to a ceiling effect (3,4). Postural control is a product of the central nervous system, the musculoskeletal system, and the sensory system. With force platform based measures, it is possible to obtain itemized information about integrated functioning of the balance control systems and to identify those individuals who still can successfully perform functional balance tests regardless of the incipient deficiency in balance control (5 8). Among older people, greater center of pressure (COP) displacement or velocity measured with a force platform is typically used to indicate poor balance (7 9). As a recent review (5) revealed, most of the previous information comes from case control designs. Only a few prospective studies on platform balance measures as predictors of future falls among older people exist. Only some of these existing prospective studies showed that increased COP movement correlates with fall risk (4,7,8,10 12) and, in one of them, the correlation was stronger for indoor than outdoor falls (11). In case control studies, a greater deterioration of balance control while dual tasking has been observed among older people with a history of falls than among nonfallers (13 15). Dual tasking refers to a situation where another task (for instance, solving an intellectual problem) is done while standing or walking. It has been suggested that deterioration in the maintenance of an upright posture in a dual-task situation indicates limited central processing capacity and/or difficulty in dividing attention between competing activities and may serve as an early sign of an increased fall risk (13 15). However, only one previous study has found that increased COP while doing a math task predicted increased risk for falls among older women (11). 171

2 172 PAJALA ET AL. In fall studies, case control designs are prone to methodological problems such as recall bias or reverse causality (16,17). The predictive validity of force platform measurements may best be studied prospectively with fall reporting carried out with short intervals. In addition, there is some evidence to show that different risk factors underlie indoor and outdoor falls. Relatively healthy and active older people fall more often outdoors than indoors, and their falls are usually related to demands of the activity they are engaged in or the environment (2,18), whereas indoor falls are more closely related to impaired balance control, poor walking ability, lower physical activity level, and poor health (2,10,18). Consequently, it is important to include information about the fall scene in the fall surveillance studies. The aim of this prospective study was to examine force platform measurements in single-task and dual-task situations as predictors of falls in community-dwelling women aged years. In this study, our specific interest was to assess the association between COP movement and subsequent risk for falls according to scene of the fall, that is, falling indoors and outdoors. METHODS Participants The present study is part of the Finnish Twin Study on Aging (FITSA), for which the recruitment of the participants is described in detail elsewhere (19 21). Briefly, the participants were drawn from the Finnish Twin Cohort, which comprises a total of 13,888 twin pairs. We invited 828 women (414 female twin pairs) aged years to participate in the FITSA, of whom 434 (206 monozygotic and 228 dizygotic twin individuals from 103 and 114 twin pairs, respectively) eventually took part. Inclusion required that the participant was able to walk 2 km and independently travel to the research laboratory, and that both co-twins were able to participate. Nonparticipation was mainly due to refusal, poor health status, or the death of one or both twin sisters after the last update of the vital status of the members of the Finnish Twin Cohort. Postural balance was measured at the laboratory, and falls were monitored during the following 12 months among 428 participants (including 213 twin pairs and two individual co-twins). We excluded three persons (two who were blind and one who had a dementia diagnosis) from the fall follow-up, and three participants dropped out during the 1st month. Data of three participants who died and the four who dropped out during the followup year (mean follow-up 91 days, standard deviation [SD] ¼ 119 days) were included in the analysis up to the month their participation ceased. Force Platform Balance Tests The force platform balance test data were obtained using the Good Balance system (Metitur Ltd, Jyväskylä, Finland; In this system, a triangular force platform is connected to a computer through a three-channel amplifier with an analog-to-digital (A/D) converter. The sampling frequency used was 50 Hz, and the system registers vertical forces. Four outcome variables were: (i) mediolateral (ML) and (ii) anteroposterior (AP) velocity (mm/s) of the movement of COP was calculated on the basis of the displacement of the COP during each second of the test; (iii) the mean moment of velocity (VEL) (mm 2 /s) of the movement of COP was calculated as the mean of the areas covered by the movement of the COP during each second of the test; (iv) the length of the ML bandwidth (ML-BW) (mm) represents the smallest distance in the ML direction from the center point of the test to include 90% of the registered measurement data. To obtain comparable COP movement values across the tests with different recording time, the measured values were divided by the total extent of the recording time. Among the taller participants, a higher location of the center of mass may cause them to have greater COP movement than shorter people. Therefore, we adjusted COP measurement data for the participant s height [(balance variable/participant height (cm)) 3 180] (9). Adequate reliability of this force platform method used is presented by Hoffman (22) and Sihvonen and Era (23). We conducted the balance tests at the research laboratory as part of a set of examinations of health and functional ability. Each participant did the balance tests during afternoon hours. A 30-minute rest interval preceded the tests. We carried out six different balance tests. During the side-byside eyes open (side-by-side EO) test, side-by-side eyes closed (side-by-side EC) test, and dual-tasking tests with a hand motor task (side-by-side DT hand motor) or an arithmetic task (side-by-side DT arithmetic) the participant stood on the force platform in stocking feet in a self-selected, natural, and comfortable stance with the feet side by side cm apart. In each of these four tests, the participant held her arms in a relaxed position in front of the body with the hands clasped together, and COP movement was recorded over 30 seconds. For the semitandem test with eyes open (semitandem EO), the participant placed the heel of one foot along the side of the big toe of the other foot, whereas for the tandem test with eyes open (tandem EO), the feet were positioned heel-to-toe along the midline of the platform. The participant s arms hung by her sides and could be used for balance correction. COP movement was recorded over 20 seconds under the semitandem and tandem conditions. During the DT hand motor test the participant held a button, which she pressed by moving only her thumb as rapidly as possible whenever she saw a red light at the gaze fixation point. The test used eight signals with varying intervals between them. In the DT arithmetic test the participant counted backward in threes, starting with a given number (selected at random from between 90 and 100); the number was told to the participant a moment before the test commenced. The participant calculated the result mentally and said aloud the result of each subtraction. The participants were encouraged to perform the test as fast and error free as possible. The participants performed one trial of each test in the following order: side-by-side EO, side-by-side EC, semitandem EO, tandem EO, side-by-side DT hand motor, and side-by-side DT arithmetic. We instructed the participants to gaze at a point marked at eye level at a distance of 2 meters and to stand as motionless as possible during all

3 FORCE PLATFORM BALANCE AND FALL RISK 173 the tests. Between tests, the participants were allowed to sit and rest for a few minutes, if they so wished. Two trained physiotherapists, working on alternate days, administered the tests. Fall Surveillance In the present study we defined a fall as unintentionally coming to rest on the ground, floor, or other lower level for reasons other than sudden onset of acute illness or overwhelming external force (24). We gathered the fall data using the calendar method (25). At the end of each month, the participant mailed the calendar page, with each day marked according to whether a fall happened or not, to the research center. If the participant forgot to mail the calendar, we reminded her via a telephone call. When a fall was reported, a research assistant called the participant and asked about the occurrence, location, circumstances, causes, and consequences of the fall. Participants reporting multiple falls during a 1-month period were not always able to recall each fall in detail. Therefore, complete interview data were obtained for 89% of all falls reported. For the analysis, participants who did not have falls during the follow-up were coded as non-fallers, those who had 1 fall indoors were coded indoor fallers, and those who had 1 fall outdoors, but no indoor falls, were coded outdoor fallers. In the regression analysis, number of falls was used as an outcome. Descriptive Variables We computed body mass index (BMI) from body weight and height, measured at the laboratory, by dividing weight in kilograms by height squared in meters. The Mini-Mental State Examination (MMSE), a test of maximal walking speed (m/s) over 10 meters, and a maximal isometric knee extension strength test were administered at the laboratory using standardized procedures described elsewhere (20, 26 28). A physician confirmed the participants self-reported chronic conditions and current medication during a clinical examination. Functional limitations, physical activity level, and occurrence of falls within the previous 12 months were self-reported using a structured questionnaire. Statistical Methods We used the Kolmogorov Smirnoff test to assess the normality of the distributions of the balance test variables; a natural logarithmic transformation was used to correct for deviations from normality. We tested the differences in the means and distributions of the balance test, and differences in the descriptive variables between fallers and nonfallers using a design-corrected t test (adjusted Wald test). This was done to account for the lack of statistical independence of the twin observations. That is, we sampled the twin pairs and needed to account for the statistical interdependence for twins within pairs to derive correct standard errors and p values. We used STATA statistical software for these analyses (29). We assessed the association between the balance variables and falls using negative binomial regression models and indicated the strength of the association with Incidence Rate Ratios (IRR). Negative binomial regression modeling takes into consideration that fall events are nonindependent observations, because falls tend to be recurring events and the occurrence of a single fall makes a subsequent incident more likely. With this approach, it is possible to enter the Poisson-distributed count variable for number of falls in the models. This method also allows for the analysis of incomplete data, that is, including those relatively rare women who were withdrawn before the end of the surveillance period. IRRs are interpreted as relative risk estimates and represent the risk for persons in the categories of the predictor variable compared to those in the reference category. For the modeling, we formed four categories for each COP movement variable: those who were not able to perform the tests were assigned the value 0 ( unable ), and the values obtained for each test variable were divided into tertiles. First, we estimated risk values (IRRs) and their 95% confidence intervals (CIs) for those unable to complete the balance test compared to those who had a test result. Second, we calculated IRRs for the occurrence of falls between each COP tertile, using the lowest tertile as the reference group. IRRs were calculated separately for falling indoors, falling outdoors, and for falling irrespective of location. In addition, models were computed separately for those without fall history prior to our follow-up. The significance level of the risk estimates was set at,.05 after Bonferroni correction. For modeling, we used the STATA software (29), because this program can compute robust estimators of variance in clustered data and in this way accounts for the statistical interdependence between twins (30). RESULTS During the 12-month (4854 person-month) follow-up, 198 participants reported 434 falls, that is, the mean incidence was 8.9 falls per 100 person-months, which is equivalent to 0.75 falls per year per person. Mean follow-up time was 344 (SD ¼ 41) days, and median time to the first fall was 106 days. At least one fall was sustained by 198 (46.5%) participants and 91 (21.4%) participants fell more than once. The majority of the falls (79%) occurred outdoors and during daytime (80%), and the participant most often fell in her backyard or in the street or walkway. Fifty-seven participants had 82 indoor falls, and almost half of them (49%) also had at least one outdoor fall. In all, 160 participants experienced 310 outdoor falls. In the present estimation of fall risk for outdoor falls, we used the data of those 132 persons who had 1 outdoor fall but no indoor falls. Thirty-eight (63%) and 103 (78%) of the indoor and outdoor fallers, respectively, had no fall history within 12 months prior to the fall surveillance of the present study. The mean monthly number of all falls in the winter, when there is frequently lying snow or ice in Finland (from November through April, mean 37.5, SD ¼ 6.8), did not differ from the mean monthly number of falls in the summer (from May through October, mean 34.8, SD ¼ 9.2). Compared to outdoor fallers or nonfallers, indoor fallers had higher BMIs and more prescribed medications, reported more difficulties in mobility, and more often had a history of falls (Table 1). An MMSE score,24, a cut-off value commonly used to indicate cognitive impairment, was observed

4 174 PAJALA ET AL. Table 1. Baseline Characteristics of Nonfallers, Indoor Fallers, and Outdoor Fallers During a 12-Month Follow-Up Among Community-Dwelling Women Aged Years Indoor Fallers N ¼ 57 Outdoor Fallers* N ¼ 132 Nonfallers N ¼ 230 Indoor Fallers vs Nonfallers Outdoor Fallers vs Nonfallers Indoor Fallers vs Outdoor Fallers Variable Mean (SD) Mean (SD) Mean (SD) p Value y p Value y p Value y Age (y) 69.2 (3.6) 68.4 (3.1) 68.5 (3.5) BMI (kg/m 2 ) 30.7 (5.8) 27.7 (3.9) 27.5 (4.7) MMSE (score) 26.9 (2.3) 27.1 (2.2) 27.0 (2.2) Maximal walking speed (m/s) 1.6 (0.4) 1.7 (0.3) 1.7 (0.3) Maximal isometric knee extension strength (N) 292 (86) 298 (87) 289 (80) Number of prescribed medications 2.9 (3.5) 1.9 (1.9) 1.9 (1.8) Number of chronic conditions 2.4 (1.8) 1.8 (1.6) 2.0 (1.4) Variable N (%) N (%) N (%) p Value y p Value y p Value y Physical activity level Sedentary 22 (38%) 33 (25%) 63 (26%) Moderately active 25 (43%) 69 (52%) 126 (53%) Active 11 (19%) 30 (23%) 50 (21%) Difficulties In walking 2 km 32 (50%) 32 (24%) 71 (30%) In climbing up one flight of stairs 26 (28%) 31 (23%) 58 (24%) Fallen within 12 mo before the follow-up 20 (34%) 28 (21%) 35 (15%) Fear of falling 22 (37%) 47 (36%) 101 (42%) Notes: *Outdoor fallers are those with one or more outdoor falls, but no indoor falls during the follow-up. y p values were calculated using the Wald test. SD ¼ standard deviation; BMI ¼ body mass index; MMSE ¼ Mini-Mental State Examination. in 6 (10.3%) indoor fallers, in 6 (4.6%) outdoor fallers, and in 14 (5.9%) nonfallers. None of the participants had an MMSE score,17, which is used to indicate dementia. Outdoor fallers did not differ from the nonfallers in their physical functioning and health characteristics. In the balance test variables, a significantly higher AP velocity and velocity moment in the side-by-side EO and EC tests were observed for indoor fallers than outdoor fallers or nonfallers. Additionally, indoor fallers had greater COP bandwidth in the side-by-side EC test and greater AP velocity in the semitandem EO test, compared to outdoor fallers or nonfallers (Table 2). As the hand motor task used in the dual-tasking test proved to be quite easy for the participants and did not produce a significant change in balance performance compared to performance in the side-by-side EO test, we did not analyze the results of DT hand motor test further. Compared to those who did the tandem EO tests in an acceptable manner, those who were not able to complete tandem EO test (n ¼ 82) had a fourfold risk for indoor falls (IRR for falling indoors 4.33 [95% CI, ; p,.05 after Bonferroni correction], for falling outdoors 1.01 [95% CI, ; p..05], and for falling irrespective of location 1.96 [95% CI, ; p..05]. Meaningful analyses could not be performed for the other force platform balance tests, as practically all participants were able to complete them. IRRs for falling indoors or outdoors according to the COP movement tertiles for each balance test are presented in Table 3. Compared to those in the lowest balance test COP tertile, those in the highest tertile for balance test COP movement had higher IRRs for indoor falls. After Bonferroni correction for multiple tests, particularly IRRs for AP velocity in side-by-side EO and EC tests and semitandem EO test remained significant (at a ¼ 0.05 level). The models computed separately for participants without fall history showed the increased risk for indoor falls for those in the highest AP COP tertile, especially in side-by-side EO (IRR 2.00; 95% CI, ) and semitandem EO (IRR 1.35; 95% CI, ) tests. However, these estimates did not remain significant after Bonferroni correction. The association between the COP movement variables and risk for falling outdoors did not reach significance. When considering all falls as outcome, the present data showed no systematic predictive effects of the COP variables. DISCUSSION The main finding of the present study was that increased COP movement in force platform balance tests identified those older people who showed no obvious balance impairment but had an increased risk for indoor falls, regardless of whether they had a history of recent falls. This finding expands existing evidence (5 7,31) that force platform based COP measures can be validly used in the assessment of postural balance in older people. Among the present sample of rather healthy, active, 63- to 76-year-old community-dwelling women, however, we could not show an association between the force platform measures and falling outdoors or falling irrespective of location. Our results showing that increased AP COP movement in particular predicted falls are novel. However, Maki and

5 FORCE PLATFORM BALANCE AND FALL RISK 175 Table 2. Baseline Balance Test Results in Nonfallers and Those With 1 Fall Indoors or Outdoors During a 12-Month Follow-Up of Community-Dwelling Women Aged Years Balance Test/COP Movement Variable Indoor Fallers N ¼ 57 Outdoor Fallers* N ¼ 132 colleagues (8) showed that AP sway amplitude was higher among fallers than nonfallers, although in their study ML COP movement was the strongest predictor of falls. Association between increased ML COP movement and higher fall risk has been shown in most of the previous studies (5,8,10 12). Compared to the present study, however, these previous studies have often been done among smaller samples and among older participants (mean age 80 years) including both men and women. In addition, the ML movement parameters and the type of force platform tests used have varied from study to study, as has the determination of fall outcome. Force platform stance tests are usually done with the predetermined position of the feet and the stance width. The required stance as such may result in an excess of movement in the ML direction, even in people would have only minor balance impairment. In the present study, the participants themselves were allowed to determine the stance width for the side-by-side tests. We consider that selfselected stance width better reflects actual performance of an individual. However, it is possible that some of our participants compensated for their balance deficits by increasing their support base with a wider stance position, and thus the tests of present study were unable to capture the deficiencies in ML COP control. Indirect support for this explanation Nonfallers N ¼ 230 Indoor Fallers vs Nonfallers Outdoor Fallers vs Nonfallers Indoor Fallers vs Outdoor Fallers Mean (SD) Mean (SD) Mean (SD) p Value y p Value y p Value y Side-by-side EO ML velocity (mm/s) 5.0 (2.1) 4.4 (1.6) 4.5 (1.7) AP velocity (mm/s) 9.6 (3.6) 7.8 (2.4) 7.8 (2.6) Velocity moment (mm 2 /s) 18.7 (13.2) 13.3 (7.5) 13.8 (7.7) Bandwidth (mm) 9.9 (4.7) 8.4 (3.6) 8.7 (3.8) Side-by-side EC ML velocity (mm/s) 6.9 (4.2) 5.7 (2.5) 5.8 (2.7) AP velocity (mm/s) 14.0 (5.5) 11.8 (4.9) 12.5 (6.8) Velocity moment (mm 2 /s) 33.4 (39.1) 23.9 (23.3) 24.0 (26.8) Bandwidth (mm) 12.0 (7.0) 9.8 (8.8) 9.6 (4.6) Side-by-side DT arithmetic ML velocity (mm/s) 6.8 (3.7) 6.8 (3.7) 6.6 (3.9) AP velocity (mm/s) 12.9 (7.5) 11.4 (6.7) 11.4 (6.9) Velocity moment (mm 2 /s) 33.4 (43.8) 29.0 (37.3) 28.2 (33.1) Bandwidth (mm) 11.5 (6.3) 11.9 (8.1) 11.5 (8.3) Semitandem EO ML velocity (mm/s) 15.6 (4.7) 14.9 (3.6) 15.3 (4.3) AP velocity (mm/s) 13.0 (4.1) 11.4 (10.7) 11.6 (4.0) Velocity moment (mm 2 /s) 55.0 (34.2) 45.0 (25.2) 47.2 (30.0) Bandwidth (mm) 24.2 (6.9) 22.6 (6.2) 22.9 (6.4) Tandem EO ML velocity (mm/s) 24.1 (5.6) 23.5 (6.4) 23.5 (6.5) AP velocity (mm/s) 17.9 (6.0) 17.9 (8.6) 17.1 (6.8) Velocity moment (mm 2 /s) 91.9 (40.4) 91.1 (66.7) 89.5 (58.2) Bandwidth (mm) 25.0 (6.7) 24.5 (5.6) 25.5 (7.6) Notes: *Outdoor fallers are those with 1 outdoor fall, but no indoor falls during the follow-up. y p values were calculated using the Wald test. COP ¼ center of pressure; EO ¼ eyes open; EC ¼ eyes closed; DT ¼ dual tasking; ML ¼ mediolateral; AP ¼ anteroposterior; SD ¼ standard deviation. comes from the observation that the present group of women who were unable to complete the tandem stance test, in which balance was compromised by a narrow support base, had a significantly increased risk for falling indoors. In addition, this finding confirms previous suggestions that inability to complete the force platform test predicts increased risk for falls (5,6) and is in agreement with studies showing an association between inability to complete the 10-second tandem stance test and fall risk (12,32,33). Consequently, in case the force platform is not available for detailed balance assessment and fall risk determination, the tandem stance test could serve as a useful clinical screening tool to assess fall risk. Bergland and Wyller (11) reported a 2.4-fold risk for injurious falls in women older than 75 years with poor performance in a force platform balance test while doing a calculation task. We expected that, among the relatively healthy participants of the present study, balance tests under dual-tasking conditions would identify prospective fallers. However, this did not happen. It is possible that, in the present study, the second tasks (done while balancing) were inadequate to challenge the central control of posture and thus reveal impaired balance due to deterioration of the central capacity or induced by prioritizing between two tasks.

6 176 PAJALA ET AL. Table 3. Incidence Rate Ratios (IRRs) and 95% Confidence Intervals (CIs) From Negative Binomial Regression Models for Falls Indoors and Outdoors in the Middle and Highest Center of Pressure (COP) Movement Tertiles in Force Platform Balance Tests in Community-Dwelling Women Aged Years Indoor Falls Balance test/cop Highest COP vs Lowest COP Middle COP vs Lowest COP Highest COP vs Lowest COP Middle COP vs Lowest COP Movement Variable IRR (95% CI) p Value IRR (95% CI) p Value IRR (95% CI) p Value IRR (95% CI) p Value Side-by-side eyes open ML velocity 2.71 ( ) ( ) ( ) ( ).118 AP velocity 3.76 ( ).001* 0.92 ( ) ( ) ( ).132 Velocity moment 3.00 ( ).005* 0.99 ( ) ( ) ( ).304 Bandwidth 2.40 ( ) ( ) ( ) ( ).852 Side-by-side eyes closed ML velocity 2.18 ( ) ( ) ( ) ( ).595 AP velocity 4.33 ( ).001* 1.90 ( ) ( ) ( ).235 Velocity moment 3.59 ( ).002* 1.53 ( ) ( ) ( ).877 Bandwidth 2.96 ( ).003* 1.36 ( ) ( ) ( ).954 Side-by-side DT arithmetic task ML velocity 1.64 ( ) ( ) ( ) ( ).963 AP velocity 2.27 ( ) ( ) ( ) ( ).295 Velocity moment 1.76 ( ) ( ) ( ) ( ).930 Bandwidth 1.64 ( ) ( ) ( ) ( ).517 Semitandem eyes open ML velocity 1.50 ( ) ( ) ( ) ( ).315 AP velocity 4.08 ( ).001* 2.54 ( ) ( ) ( ).540 Velocity moment 2.65 ( ).006* 0.93 ( ) ( ) ( ).425 Bandwidth 1.69 ( ) ( ) ( ) ( ).495 Tandem eyes open ML velocity 1.07 ( ) ( ) ( ) ( ).324 AP velocity 1.06 ( ) ( ) ( ) ( ).901 Velocity moment 2.35 ( ) ( ) ( ) ( ).670 Bandwidth 1.20 ( ) ( ) ( ) ( ).378 Notes: The lowest tertile of COP movement (i.e., those with smallest COP movement) is the reference group. *Significant at the level a ¼ 0.05 after Bonferroni correction (false-positive error rate lowered to 0.05/5 ¼ for each test). ML ¼ mediolateral; AP ¼ anteroposterior; DT ¼ dual tasking. The balance behavior in dual-task tests is described in detail in our previous report (34). Associations between force platform measures and falls are found more often in studies with retrospective than prospective fall surveillance. In terms of efficient fall prevention, however, it is essential to identify the people at risk for falls before the first incident. Performance-based measures, such as the Berg Balance Scale, are rather good in identifying older people with risk for future falls (3). These measures are inexpensive and easy to carry out even at home but, especially among younger, still active older people, they often suffer from a ceiling effect. The present, rather extensive force platform data from various stance positions indicate that older people with seemingly intact balance may suffer from early-onset deterioration in their ability to control posture. In addition, the present finding that even women without fall history, but decreased balance performance, had increased risk for indoor falls, highlights early fall prevention. Our finding that force platform tests predict indoor but not outdoor or overall falls is in line with the results of Bergland and colleagues (10). Older people who fall indoors often have multimorbidity and poor mobility (10,18). In the Outdoor Falls present data, those who fell indoors were also heavier, used more medication, and more often reported mobility problems. Thus, our study accumulates evidence for the relationship between indoor falls and intrinsic risk factors and shows that the force platform method serves as a sufficiently sensitive balance measure to identify older persons who are susceptible to falls due to intrinsic risk factors. The majority of falls during the follow-up year of the present study occurred outdoors. The participants who only fell outdoors showed better values in the balance tests as well as in the variables related to physical health and functioning than did those who had either only indoor falls or both indoor and outdoor falls. It may be that people with delicate health or incipient balance impairment consciously or unconsciously restrict their outdoor activities, and their time at risk for outdoor and indoor falls differs from that of healthier and more mobile older people. Falling both indoors or outdoors may be involved in environmental or situational factors. However, our study suggests that, in older people, outdoor falls especially are related to the intensity or difficulty of the activity done or to a hazardous environment. Indoor falls, in contrast, seem to be associated with balance impairment precipitated by such intrinsic risk

7 FORCE PLATFORM BALANCE AND FALL RISK 177 factors as poor health, impaired mobility, multimorbidity, and medication use. It is important to prevent as many falls as possible, as falls tend to be recurrent and frailty predisposes older people to fall-induced injuries. Half of the present participants fell during the follow-up period, and almost every other faller fell at least twice. Force platform measures have previously been considered to best predict risk for recurrent falls (7,11,12). Most of the previous prospective fall studies have used dichotomized (falls/no falls) fall data. However, to capture the cumulative nature of falls, the usage of all falls occurring during the follow-up period as an outcome has been recommended (35). As this approach leads to a highly skewed distribution of the count data, some form of Poisson analysis is suggested to provide the most accurate risk estimates (35,36). Therefore, in the present analysis, we chose to apply the negative binomial regression modeling. This method also accommodates the dependence of the fall events (i.e., overdispersion) as well as each individual s follow-up time. Regardless of the representative sample of independently living older women and meticulous followup of falls, the present data are insufficient to assess the risk for more specific subgroups of falls, for example, falling indoors at night. Force platform based COP measures differentiate postural control between young and older people, capture the decrease in the ability to control posture along with age, and predict mortality (31,37,38). Existing studies suggesting that force platform based balance measures can be used in identifying older people with an elevated risk for future falls are limited (5). The present results add knowledge by showing that the force platform method provides valid data on the quality of upright posture control, especially among older women. These data are useful in the detection of early signs of deterioration in balance among older people without perceptible balance problems and in the identification of people at risk for falls. ACKNOWLEDGMENTS This work was supported by the Academy of Finland, the Ministry of Education of Finland, Juho Vainio Foundation grants, and a Finnish Cultural Foundation grant. We thank Michael Freeman and Tiina Aho for revising the language of the manuscript. The Finnish Twin Cohort study is part of the Academy of Finland Centre of Excellence in Complex Disease Genetics. CORRESPONDENCE Address correspondence to Satu Pajala, PhD, The Finnish Centre for Interdisciplinary Gerontology, Department of Health Sciences, University of Jyväskylä, P.O. Box 35 (Viv), FIN University of Jyväskylä. pajalass@mail.nih.gov REFERENCES 1. 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