Factors Affecting Reliability of the Biodex Balance System: A Summary of Four Studies

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1 Factors Affecting Reliability of the Biodex Balance System: A Summary of Four Studies Martha R. Hinman Purpose: To describe differences in test-retest reliability of balance measures produced by the Biodex Balance System. Methods: Subjects performed balance tests in 4 studies, each involving two 30-second tests under varying conditions. Test-retest reliability coefficients were calculated for the overall stability index (St) under each condition. Test-retest reliability was computed for subjects' limits of stability (LOS). Results: lntraclass correlation coefficients (ICC,,) ranged from.44 to.89 for static balance tests; variability between the 2 tests ranged from 21% to 37%. Tests at lower stability levels or without visual input produced the highest ICCs. ICCs for the LOS tests ranged from.64 to.89. These measures were less variable (6% to 8%) than the static measures of balance. Conclusions: Test-retest reliability of the SI produced by the Biodex Balance System is acceptable for clinical testing and is comparable to other balance measures currently in use. Key Words: balance testing, postural stability, limits of stability, rehabilitation Hinman MR. Factors affecting reliability of the Biodex Balance System: a summary of four studies. ) Sport Rehabil. 2000;9: Human Kinetics Publishers, Inc. Balance, or postural stability, is generally defined as the ability to maintain the body's center of mass (COM) within its base of support (BOS).l A variety of instruments and tests have been developed over the past few decades to assess balance performance. Tests of static balance require movement of a person's COM within a stationary BOS2; examples of static balance tests include the Romberg and Sharpened Romberg tests, l-legged stance time, the Functional Reach Test, the Postural Stress Test, and the balance portion of the Fugl-Meyer Sensorimotor Asse~sment.~,~ These tests, however, do not represent all the domains of balance that are needed to perform dynamic, functional activities. In tests of dynamic balance, both the COM and the BOS are mo~ing.~ Gait tests such as tandem walking, the Timed Up-and-Go Test, and obstacle courses have been used to assess dynamic balance, and tests such as The author is with the Department of Physical Therapy, School of Allied Health Sciences, at the University of Texas Medical Branch, Galveston, TX

2 Reliability of the Biodex Balance System 241 the Sensory Organization Test, Tmetti's Performance-Oriented MobilityAssessment, and the Berg Balance Scale assess both static and dynamic mane~vers.3,~ Most of these balance tests have traditionally been used in clinical settings because they require little specialized instrumentation or training. Numerous investigative studies have validated clinical tests of balance against biomechanical measures of balance (ie, forceplate measures of postural activities-of-daily-living s~ales,5.~,'~-'~ and fall occurrence.5~11~12~1421 In addition, reliability studies conducted on many of these clinical tests demonstrate interrater reliability coefficients ranging from.75 to.99 and test-retest reliability coefficients ranging from.57 to.98.7,22-28 Despite their value in assessing geriatric and patient populations, clinical tests of balance often have limited usefulness with younger or more physically active individuals who must function in more challenging domains of balance. Examples of these balance domains include both predictable and unpredictable functional tasks such as standing or walking on a moving or compliant surface, extreme leaning or reaching in different directions, sudden directional changes when walking or running, responding to postural perturbations caused by other people or objects, and moving in darkness or other sensory-altered condition^.^^ Technologic advances in recent years have led to the development of more sophisticated and sensitive instruments that can quantify balance performance in athletic individuals who function at these higher le~els.3~ Examples of these devices include the Balance Master and EquiTest Systems (NeuroCom International, Clackamas, Ore), the Kinesthetic Ability Training (KAT) Balance Platform (Breg Inc., Vista, Calif), and the Biodex Balance System (Biodex Medical Systems, Shirley, NY), which use computerized force platforms to analyze changes in the body's COM under stable and unstable conditions. NeuroCom has conducted extensive tests to establish normal values, validity, and reliability data on their balance systems under a variety of test conditions. In Nashner's31 summary report, he cites test-retest reliability coefficients ranging from.70 to.94 for balance tests conducted under different visual conditions with different types of surface support. Testretest reliability studies using NeuroComfs test protocols for limits of stability have produced reliability coefficients for movement time and path ranging from.57 to.91.31"3 Despite the wealth of data that NeuroCom provides for their devices, the size and cost of these balance systems prohibit their use in most clinical or sports rehabilitation settings. The KAT Balance Platform and the Biodex Balance System are less expensive, more portable devices that include moveable platforms that can be adjusted to provide varying degrees of stability. Although the design and features of the KAT and Biodex systems might be useful and appealing, little information has been published about the test reliability of either instrument. The purpose of this article is to describe the test-retest reliability of the balance measures produced by the Biodex Balance System under a variety of test conditions. This information is needed to help rehabilitation

3 242 Hinman specialists determine the best testing protocol to use with this instrument and to compare its test reliability with other tests and instruments that measure balance in a similar manner. Instrumentation Methods The Biodex Balance System challenges an individual to maintain his or her balance while standing on a moveable platform that tilts a maximum of 20" (from the horizontal plane) in all directions. The amount of stiffness in the platform is controlled mechanically and ranges from stability level 8 (stiffest) to stability level 1 (loosest). An LCD screen provides subjects with visual feedback on where their COM is in relation to the periphery of the platform. Safety handrails are attached, and an optional overhead harness system can be added if more support is needed. When used in the testing mode, the Biodex system provides an overall stability index (SI) that represents the variance of the foot platform dislevel position, in all motions during a test; /(number of samples).34 Greater amounts of body movement associated with an unstable posture produce a high SI; a low SI indicates little body movement and is associated with a more stable posture. SIs are also calculated for the anterior-posterior (AP) and mediallateral (ML) directions, which represent the variance of foot-platform displacement for motions occurring in the sagittal and frontal planes, respectively. In addition, the Biodex system displays the platform as a series of concentric circles, or zones, that are divided into 4 quadrants. The innermost circle, zone A, represents 0-5" of foot-platform deflection from the level position. Zones B, C, and D represent 6-10", 11-15", and 16-20" of deflection, re~pectively.~~ The percentage of the test time that an individual spends in each zone and quadrant is reported along with the SI values. The test protocol can be set up for either bilateral or unilateral stance and includes a grid on the foot platform to record the foot position. Test time can be adjusted in 10-second increments from 10 seconds up to 10 min. Although the test protocol is designed to assess the subject's ability to maintain a static position, the Biodex can also be used to assess balance in a more dynamic manner by asking subjects to volitionally move their COM within their limits of stability (LOS). LOS refers to the outermost range of an area in space that a person can lean from the vertical position in any direction without changing his or her BOS.1,3 However, when using the Biodex system, the supporting surface (ie, BOS) moves along with the body's COM. According to E. Behan of Biodex Medical Systems (written cornmunication, February 1996), the percentage of normal LOS in the AP direction is calculated by dividing the subjects' AP stability index (maximum of 20") by a normal reference value of 12" and multiplying that number by 100. Similarly, subjects' ML stability index (maximum of 20") is divided by a

4 Reliability of the Biodex Balance System 243 normal reference value of 16" and multiplied by 100 to determine the percentage of normal LOS in the lateral direction. Subjects and Data Collection Procedures Four studies were conducted using different test conditions and subjects to estimate the test-retest reliability of the SI measures produced by the Biodex Balance System. In each of these studies, healthy subjects were provided with 1-2 rnin of practice before testing in order to become familiar with the equipment and minimize training effects. Intraclass correlation coefficients (ICCs) were calculated for each study based on 2 trials performed on the same day, with 30- to 60-second rest periods between trials to minimize fatigue. The order in which subjects performed each test condition (independent variable) was randomized in each study. Subjects were asked to stand on the platform with their feet apart at a comfortable stance width. The positions of their feet were recorded using the coordinates on the platform's grid to ensure a similar stance for all subsequent tests. The Institutional Review Board at The University of Texas Medical Branch approved each study, and written informed consent was obtained from each subject. Subjects were excluded if they had a neurological or orthopedic condition that interfered with their ability to stand or ambulate independently, recent surgery that interfered with their ability to maintain an erect posture, or dementia that interfered with their ability to follow directions. Specific subject samples and test procedures are described below. Study 1. This study included 50 subjects (31 women and 19 men) between the ages of 18 and 65 (mean = years). Measures of body weight and height were not recorded in this study. Subjects were tested for 30 seconds at 2 different stability levels (levels 3 and 6) with their eyes open, looking straight ahead. Study 2. Study 2 included 50 subjects (37 women and 13 men) between the ages of 21 and 52 (mean = 28.1 f 8.1 years). The body mass index (BMI) of these subjects ranged from 17.7 to 31.4 (mean = ). Subjects' overall SIs were compared under 3 different visual conditions: eyes open, looking straight ahead; eyes open, receiving visual feedback; and eyes closed. Each test was conducted for 30 seconds at stability level 6. Study 3. Study 3 enrolled 79 older adults (47 women and 32 men) be- tween the ages of 65 and 92 (mean = years). Subjects' body weights ranged from 39.9 to kg, with a mean of 74.4 f 15.3 kg; heights were not recorded. Subjects were tested both while wearing hard-soled shoes and while wearing soft-soled shoes. All tests were conducted for 30 seconds at stability level 7 with subjects' eyes open, receiving visual feedback. Study 4. This study evaluated the reliability of 2 LOS test protocols under 2 different visual conditions. The sample included 44 subjects (37 women and 7 men) between the ages of 21 and 50 (mean = years). The BMIs for these subjects ranged from 18.2 to 28.1 (mean = 21.7f 2.3). As opposed to static tests, in which subjects are asked to maintain a stable

5 244 Hinman position, these LOS tests required subjects to move the platform in various directions by leaning as far as possible in different directions without changing their base of support or grabbing the safety rails. The linear protocol required subjeds to move the platform in an AP direction for 30 seconds, followed by 30 seconds in a side-to-side, or ML, direction. The circular protocol required subjeds to move the platform in a clockwise direction for 30 seconds, followed by 30 seconds in a counterclockwise direction. Both testing protocols were repeated 4 times, twice with visual feedback and twice with vision impaired by semitransparent goggles. All tests were conducted at stability level 6. Data Analysis Procedures Using an analysis of variance (ANOVA) model, an ICC,, was calculated for each test protocol included in each study. This ICC model uses a repeated-measures ANOVA design in which each subject is assessed multiple times by a single rater; the reliability coefficient is calculated using the following ANOVA terms:35 between-subjects MS - error MS ICC,,, = between-subjects MS + (number of trials - l)(error MS) When using the ICC model, it should be noted that reliability is based on the proportion of the total observed variance that is attributed to error; thus, ICCs tend to increase when the total amount of variance among subjects gets larger and the error component of that variance gets ~maller.~~,~~ ICCs and their confidence intervals were calculated for the overall SI measures in the first 3 studies. In the LOS study, ICCs were calculated separately for SI measures recorded in the AP and ML directions to correspond with the planes in which normative LOS data have been established. In addition, the amount of measurement error associated with the test-retest measures was analyzed by calculating the method error (ME); the ME is a measure of discrepancy between 2 sets of repeated scores that is based on the standard deviation of the difference scores (SD,; ME = SD,/~). The coefficient of variation for the method error (CVm) was then calculated to estimate the percentage of variation between the 2 SI measures (CV,, = 2ME/(T1 + Y2) X 100). This statistic is not affected by the lack of variability among subjects as is the ICC.36 All data were analyzed using SPSS statistical software (SPSS Inc., Chicago, IL). The ICCs for studies involving static tests ranged from.44 to 29, depending on the test condition; CV,, for these tests ranged from 21% to 37% (Table 1). Overall, the highest ICCs had the narrowest confidence intervals and were obtained under the most challenging test conditions (ie, low

6 Reliability of the Biodex Balance System 245 Table 1 Reliability Coefficients, 95% Cls, and CV,$ for the Stability Index During Static Tests Under Different Test Conditions* Biodex Stability Level Test Condition Level 3 Level 6 Level 7 Eyes open, looking straight ahead Eyes open, receiving visual feedback Eyes closed Soft-soled shoes Hard-soled shoes.89 (n = 50),.87 (n = 50), CI = , CI = , CV,, = 23% CV,, = 24%.44 (n = 50), CI =.I 8-.64, cv,, = 37%.49 (n = 50), CI = , cv,, = 3 5%.83 (n = 50), CI = , cv,, = 21%.79 (n = 79), CI = , CV,, = 26%.82 (n = 79), CI = , CV,, = 27% *CI indicates confidence interval, and CV, coefficient of variation for the method error. stability level or eyes closed), in which the SI values had the greatest amount of variability among the subjects. ICCs for the LOS study ranged from.77 to.89, and the CV,, ranged from 6% to 8% (Table 2). Because these tests were inherently more difficult, ICCs for the LOS tests were similar to those for static tests performed under the most challenging test conditions (ie, lower stability levels or eyes closed). Higher ICCs were found when using the circular test protocol versus the linear one. The reliability of AP tests increased slightly during both test protocols when vision was impaired; however, visual conditions did not affect the reliability of the ML tests. Variability in SI scores for the LOS tests (CV,, = 64%) was considerably less than that found with the static tests. Reliability of Static Measures Discussion In summarizing the effects of various test conditions on the reliability of the Biodex SI measures, one must consider the factors that influence variance

7 246 Hinman Table 2 Reliability Coefficients, 95% Cls, and CV,s for Biodex LOS Tests Under Different Test Conditions (N = 44)* Plane of Movement Linear, eyes open.77, CI = , CV, = 8%.79, CI = , CV, = 7% Linear, vision.84, CI = , CV, = 8%.64, CI = , CV, = 7% impaired Circular, eyes open.87, Cl = , CV, = 6%.86, CI = , CV, = 7% Circular, vision.89, CI = , CV, = 6%.87, CI = , CV, = 6% impaired *CI indicates confidence interval; CV, coefficient of variation for the method error; LOS, limits of stability; AP, anterior-posterior; ML, medial-lateral. both among and within the subjects in the sample. The greatest amounts of variance in overall SI measures were found in study 1 when subjects were tested on a looser platform (ie, stability level 3; variance = and 8.374) and in study 2 when subjects were tested with their eyes closed (variance = and ). Table 3 compares sources of variance based on the ANOVA tests. These data generally indicate that the more difficult tests produced proportionally greater variances among subjects, with the exception of the eyes-closed test, which demonstrated similar amounts of variance between subjects and between tests (subjects tended to perform significantly better on the second test). When tested under easier conditions, subjects' SI measures were fairly homogeneous, and the ICCs were lower. Similar findings have been reported in studies conducted on NeuroComfs Balance Master in which the easiest test conditions produced the poorest test-retest reliability.32,38 The relatively easy test protocol used in Study 3 with older subjects produced ICCs that were comparable to those found under more challenging test conditions in the first two studies using younger subjects. Although one explanation for this finding might be greater heterogeneity in the balance performance of the older subjects, variances in their SI measures were not substantially greater than those found for younger subjects (when tested under similar conditions). However, the older adults' SI measures appear to be more consistent, as evidenced by the lower CV,, and relatively small amounts of error variance. Perhaps the motor strategies used by the older adults were less divergent than those used by younger adults, who might have had a greater repertoire of motor strategies. The type of footwear worn by subjects in study 3 did not affect the reliability of their SI measures, but subjects tended to slide on the Biodex platform more when wearing leatherhoes than when wearing rubber-soled shoes, so a piece of nons material had to be added to the platform to ensure adequate coupling.

8 Reliability of the Biodex Balance System 247 The results of these studies indicate that the test-retest reliability of the Biodex SI measures is similar to that reported by Schmitz and Arnold39 for the same instrument. They found intratester and intertester reliabilities of.82 and.70, respectively, for overall SI scores obtained when subjects performed a series of five 30-second tests using a unilateral stance at progressively decreasing stability levels. The difficulty level of these test conditions would be somewhat comparable to the tests performed with eyes closed or those performed at very low stability levels. These reliability coefficients are also comparable to those produced by static forceplate systems. In their study of postural sway measures, Le Clair and Riach40 tested subjects in 2 stances (feet together and in tandem position), under 2 visual conditions (eyes open and closed), for 5 test durations (10,20,30,45, and 60 seconds). They reported test-retest coefficients ranging from.81 to.86 for these postural sway measures and noted that tests of 20 or 30 seconds in duration were most reliable. Another static forceplate system, the Balance Performance Monitor (SMS Healthcare, Harlow, Essex, UK), produces a sway coefficient that represents the average amount of weight shift during unsupported stance on a stable base. In a study of testretest reliability using the Balance Performance Monitor, ICCs of.62 and.80 were found for tests conducted with eyes open and eyes closed, respectively41 Similarly, static tests performed by subjects with hemiparesis on the Balance Master demonstrated higher reliability when tests were conducted with subjects' eyes closed (ICC =.63) than with their eyes open (ICC =.56) or when receiving visual feedback (ICC =.50).32 Ford-Smith et all4 reported test-retest reliability coefficients for older adults who performed the Sensory Organization Test using NeuroComfs EquiTest System. The Sensory Organization Test protocol includes 6 test conditions; half are conducted on a stable surface and half on a moving surface. Likewise, visual conditions are altered in some test conditions. The conditions that are most comparable to the Biodex test conditions are those conducted with a moving surface, a fixed screen, and either vision or no vision. The ICCs reported for these 2 test conditions ranged from.34 to.47 with eyes open and.68 to.70 with eyes closed; lower ICCs were reported for tests conducted on a stable surface. Therefore, it would seem that higher test reliability is obtained when more challenging test parameters are used, regardless of which forceplate system the balance test is performed on. Reliability of LOS Measures Reliability data from the LOS tests conducted on the Biodex system can be compared to coefficients reported for the Functional Reach (FR) test and LOS testing using the NeuroCom systems. FR is a simple test that requires subjects to lean as far forward, backward, and laterally as they can while keeping their feet in a stable position. The distances that subjects can reach in each direction are recorded using a wall-mounted yardstick or tape

9 248 Hinman measure. Duncan et a17 compared center of pressure (COP) excursion measures recorded by a static forceplate with yardstick measures during the FR test in a group of older adults. ICCs for repeated measures were.81 when using forceplate COP measures and.92 when using the yardstick method. Thapa et a1,9 however, reported a much lower test-retest reliability (ICC =.57) when using FR to test balance in a group of nursing home residents. NeuroCom's protocol for testing LOS involves linear movements from the center of the platform to 8 targets that are spaced at 45O intervals in an oval pattern that represents the theoretical limits of stability (100%) from the center target. Measures from this test that are most comparable to the Biodex measures are the endpoint and maximum excursion scores. Based on studies conducted by Rose and M~Killop,~~ NeuroCom reports that the reliability coefficients for these measures range from.73 to.76. In a generalizability study by Clark et al,33 estimates for endpoint and maximum excursions ranged from.80 to.91 for LOS tests conducted on 3 separate days. When comparing reliability data from the Biodex LOS tests with the other LOS tests cited previously, one must keep in mind that other LOStesting methods use a stable platform rather than a moving one. Thus, the difficulty of performing an LOS test using the Biodex system is much greater and would be expected to cause more variability both within and among subjects. Table 3 illustrates this increased subject and test variability along with a relatively small error variance. Although the LOS tests in the present study (study 4) might be useful for assessing athletic performance, they are probably too difficult for individuals who have sigruficant balance impairments resulting from biomechanical or neurological disorders. In fact, 6 of the original subjects in study 4 had to be eliminated from the sample because they could not complete one or more tests without holding on to the handrails. Another factor that might affect the reliability of the Biodex LOS test is the design of the equipment. Most subjects quickly learned that the excursion of the Biodex platform was limited to 20 in every direction. When using the linear test protocol, most subjects were able to forcefully move the platform back and forth within its maximal limits without losing their balance, especially in the lateral direction. The greater consistency of this particular movement (linear pattern, h4l direction) can be seen in the relatively small variance between measures in Table 3. Subjects were unable to take advantage of these platform limits when moving in a circular pattern; thus, the circular protocol produced greater variances between the 2 measures. Because subject variance increased while error variance remained small, the circular protocols produced higher ICCs than the linear protocols, even though the variance between tests was greater. Nonetheless, the reliability of the linear protocol is probably superior to that of the circular protocol because it maximizes subject variance while minimizing both error variance and variance between test

10 Reliability of the Biodex Balance System 249 Table 3 Sources of Variance in SI Measures (from ANOVA tables)* Between Subjects Between Measures Error Comparison of stability levels Level 3 (eyes open) Level 6 (eyes open) Comparison of visual conditions Eyes open, no feedback Visual feedback Eyes closed Comparison of footwear Soft-soled shoes Hard-soled shoes Comparison of LOS protocols Linear, eyes open (AP) Linear, eyes open (ML) Linear, vision impaired (AP) Linear, vision impaired (ML) Circular, eyes open (AP) Circular, eyes open (ML) Circular, vision impaired (AP) Circular, vision impaired (ML) *SI indicates stability index; ANOVA, analysis of variance; AP, anterior-posterior; ML, rnediallateral. Conclusions The balance measures (SI) provided by the Biodex Balance System appear to be reliable measures of postural stability, particularly when tests are conducted without visual input and/or using a less stable base. Because these studies were conducted on a normal population, little variability was found under less challenging test conditions, which might have contributed to the lower ICC values associated with tests conducted with subjects' eyes open and tests conducted at more stable platform levels. LOS tests conducted on the Biodex system demonstrate less variability than static tests do, although the design of the equipment and the test protocol used might distort the reliability coefficients to some extent. Although the test-retest reliability of the SI measures produced by the Biodex system is acceptable and comparable to that of other balance tests, further studies are needed to determine whether the reliability of these

11 250 Hinman balance measures might differ among a patient population and whether they are sensitive to changes that occur through participation in a training or rehabilitation program. Acknowledgments Funding for these studies was provided from The University of Texas Medical Branch School of Allied Health Sciences, the Sealy Center on Aging, and the Texas Physical Therapy Education and Research Foundation. The author wishes to thank the following physical therapists, who participated in the data collection for these studies: Deborah Heyl, Pam Ingram, Jennifer Metevier, Nori Morales, Angelica Castillo, Cathy Ibe, Julie Stevens, Theresa Lundy, and Kirsten Koechley. No financial support was received from Biodex Medical Systems to fund any of the studies reported in this article, nor do the findings of these studies necessarily represent an endorsement of this company's products. References 1. Shurnway-Cook A, Woollacott MH. Motor Control: Theory and Practical Applications. Baltimore, Md: Williams & Wilkins; Woollacott MH, Tang P. Balance control during walking in the older adult: research and its implications. Phys Ther. 1997;77: Ragnarsdottir M. The concept of balance. Physiotherapy. 1996;82: Russo SG. Clinical balance measures: literature resources. Neurol Rep. 1997;21: Berg KO, Wood-Dauphinee SL, Williams JI, Maki B. Measuring balance in the elderly: validation of an instrument. Can J Public Health. 1992;83:S7-S Berg KO, Maki BE, Williams JI, Holliday PJ, Wood-Dauphinee SL. Clinical and laboratory measures of postural balance in an elderly population. Arch Phys Med Rekabil. 1992;73: Duncan PW, Weiner DK, Chandler J, et al. Functional reach: a new clinical measure of balance. J Gerontol. 1990;45:M192-M Mathias S, Nayak L, Issacs B. Balance in elderly patients: the "get-up and go" test. Arch Phys Med Rekabil. 1986;67: Thapa PB, Gideon P, Fought RL, Kormicki MI Ray WA. Comparison of clinical and biomechanical measures of balance and mobility in elderly nursing home residents. J Am Geriatr Soc. 1994;42: DiFabio RP, Badke MB. Relationship of sensory organization to balance function in patients with hemiplegia. Phys Ther. 1990;70: Means KM, Rodell DE, O'Sullivan PS. Obstacle course performance and risk of falling in community-dwelling elderly persons. Arch Phys Med Rehabil. 1998;79: Means KM, Rodell DE, O'Sullivan PS. Use of an obstacle course to assess balance and mobility in the elderly: a validation study. Am J Phys Med Rehabil. 1996;75:88-95.

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