Authors: Hans-Georg Palm, MD, MBA Patricia Lang, MD Johannes Strobel, MD Hans-Joachim Riesner, MD Benedikt Friemert, MD
Affiliations:
Posture
ORIGINAL RESEARCH ARTICLE
From the Trauma Research Group, Department of Orthopedics and Trauma Surgery, German Armed Forces Hospital of Ulm, Ulm, Germany.
Correspondence: All correspondence and requests for reprints should be addressed to Hans-Georg Palm, MD, MBA, Trauma Research Group, Department of Orthopedics and Trauma Surgery, German Armed Forces Hospital of Ulm, Oberer Eselsberg 40, D-89081 Ulm, Germany.
Disclosures: Financial disclosure statements have been obtained, and no conflicts of interest have been reported by the authors or by any individuals in control of the content of this article.
0894-9115/14/9301-0049 American Journal of Physical Medicine & Rehabilitation Copyright * 2013 by Lippincott Williams & Wilkins DOI: 10.1097/PHM.0b013e3182a39019
Computerized Dynamic Posturography The Influence of Platform Stability on Postural Control ABSTRACT Palm H-G, Lang P, Strobel J, Riesner H-J, Friemert B: Computerized dynamic posturography: the influence of platform stability on postural control. Am J Phys Med Rehabil 2014;93:49Y55.
Objective: Postural stability can be quantified using posturography systems, which allow different foot platform stability settings to be selected. It is unclear, however, how platform stability and postural control are mathematically correlated. Design: Twenty subjects performed tests on the Biodex Stability System at all 13 stability levels. Overall stability index, medial-lateral stability index, and anteriorposterior stability index scores were calculated, and data were analyzed using analysis of variance and linear regression analysis.
Results: A decrease in platform stability from the static level to the second least stable level was associated with a linear decrease in postural control. The overall stability index scores were 1.5 T 0.8 degrees (static), 2.2 T 0.9 degrees (level 8), and 3.6 T 1.7 degrees (level 2). The slope of the regression lines was 0.17 for the men and 0.10 for the women.
Conclusions: A linear correlation was demonstrated between platform stability and postural control. The influence of stability levels seems to be almost twice as high in men as in women. Key Words:
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Postural Control, Posturography, Instability, Biodex Balance System
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P
ostural stability is required for the maintenance of an upright posture and gait in humans. Stability regulation is a dynamic process that involves the continuous central processing of afferent inputs from visual, vestibular, and proprioceptive receptors in a hierarchical structure. Motor responses are then generated to correct posture.1,2 The complex interactions between the sensory systems result in a multitude of conditions that can affect the ability to maintain a stable posture. This was shown in a number of studies involving patients with impaired vision,3,4 diseases of the vestibular system,5 or changes in proprioception and sensorimotor function.6,7 A decrease in postural control is usually quantified using posturography. This method involves the use of a static platform that assesses displacements of a subject’s center of gravity or a dynamic platform that records passive displacements (computerized dynamic posturography). Multiple levels of platform instability can be selected and allow examiners to tailor tests to the needs and the requirements of patients or athletes. Studies, however, have not yet investigated whether and, if so, to what extent a decrease in postural control is correlated with an increase in platform instability and especially whether the results are identical for men and women. Although a linear relationship seems to be most likely, it is also possible that healthy and physically active subjects, for example, are able to fully compensate for high levels of instability until a certain level is reached. After a plateau phase, they may then show a sudden loss of postural control. The relationship between postural control and level of platform instability is not only interesting from a purely scientific point of view but also of high practical relevance. Neither a literature review nor information obtained from the Biodex Balance System (BBS) manufacturer (Biodex, Shirley, NY) revealed a specific stability level that is used as a standard level in tests (Paterno et al.8: level 4, Rozzi et al.9: level 2, and Schmitz and Arnold10: dynamic levels 8Y1). As long as it remains unclear whether and, if so, how platform instability is associated with a decrease in postural control, the comparability of available studies and the comparability of this study’s own data and that of available studies are limited. In addition, this study on healthy subjects describes a pilot approach that provides a basis for further investigations involving patients with reduced postural control such as patients with anterior cruciate ligament rupture. Other studies may address the
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question as to whether patients with injuries that are known to reduce proprioception and thus postural control show different correlations or decompensation without mathematical relationships. For this reason, the objectives of this prospective randomized clinical and experimental study were to investigate the relationship between postural control as reflected by the overall stability index (OSI) and the level of platform instability and to assess whether there are differences between the male and female subjects. A further objective was to express any possible relationships for the first time in a mathematical formula. Owing to the explorative nature of this pilot study, however, this formula should be regarded as a potential approach to describing the relationship between the stability levels and postural control and should, in particular, illustrate a linear correlation.
SUBJECTS AND METHODS Subjects Twenty subjects took part in this study. Of these, ten were men (25.3 T 1.9 yrs, 180 T 11 cm, 77.3 T 5.3 kg) and ten were women (23.9 T 2.4 yrs, 172 T 9 cm, 66.4 T 6.4 kg). No subject had a history of diseases of the neurologic, the vestibular, or the visual system. Subjects with diseases or injuries of the lower extremities, the spine, or the pelvis and especially injuries of the cruciate, medial, or lateral ligaments were excluded. Standardized clinical examinations of the musculoskeletal system were carried out to rule out unknown diseases or previous injuries. If there was any doubt about the presence or the absence of a disease, the subject was not included in the study. This clinical and experimental study was approved by the ethics committee of the University of Ulm (No. 65/08) and was conducted as part of a special research project of the German Armed Forces (16K3-S-100712) in accordance with the Declaration of Helsinki of 1975 as revised in 1983. All subjects gave their informed consent.
Instrumentation The BBS was used, which has a circular platform that is free to move in the medial-lateral and anterior-posterior axes. It permits up to 20 degrees of tilt in any plane and has a measuring accuracy of 0.1 degrees. The system records the degree of displacement from the initial level under dynamic conditions and calculates a score for the mediallateral stability index, the anterior-posterior stability
Am. J. Phys. Med. Rehabil. & Vol. 93, No. 1, January 2014
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index, and the OSI. The focus of this study was on the OSI because it is the most commonly used and the most relevant parameter. The BBS allows testing at 1 static and 12 dynamic levels. At any level, a shift in a subject’s center of gravity leads to a passive displacement of the foot platform. A setting of 1 is the least stable platform setting. A setting of 12 is the
most stable released setting. According to the manufacturer, the position of the platform is secured by locking screws that can be electronically unlocked and thus allows higher levels of platform instability to be selected gradually and evenly. When level 1 is selected after level 2, the tilting platform is fully unlocked in a discontinuous manner.
TABLE 1 Stability index scores Part A OSI
Mean
SD
SE
95% CI (Lower Bound)
95% CI (Upper Bound)
Min
Max
4.7 3.6 2.9 2.5 2.4 2.6 2.3 2.2 2.2 1.9 1.9 1.7 1.5
2.1 1.7 1.7 0.9 1.0 1.1 1.0 0.9 1.1 0.9 0.8 0.8 0.8
0.5 0.4 0.4 0.2 02 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
3.7 2.8 2.1 2.1 2.0 2.1 1.9 1.7 1.7 1.5 1.5 1.3 1.1
5.7 4.4 3.7 2.9 2.9 3.1 2.8 2.6 2.7 2.3 2.2 2.1 1.8
1.7 1.3 1.3 1.0 1.2 1.1 0.6 0.7 0.7 0.8 0.9 0.9 0.6
8.3 7.0 7.5 3.6 4.6 4.5 4.5 4.6 4.0 4.3 3.2 4.2 3.5
Mean
SD
SE
95% CI (Lower Bound)
95% CI (Upper Bound)
Min
Max
3.4 2.5 1.9 1.7 1.7 1.9 1.7 1.5 1.6 1.3 1.4 1.2 1.1
1.5 1.3 1.1 0.7 0.9 0.8 0.8 0.6 0.9 0.6 0.6 0.7 0.7
0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.1 0.2 0.1 0.1 0.2 0.1
2.7 1.9 1.4 1.4 1.3 1.5 1.3 1.2 1.2 1.0 1.1 0.8 0.7
4.0 3.1 2.5 2.0 2.1 2.2 2.0 1.7 2.0 1.6 1.7 1.5 1.4
1.4 0.7 0.7 0.5 0.7 0.7 0.3 0.4 0.6 0.5 0.6 0.6 0.3
5.8 4.8 4.2 2.9 3.7 3.1 3.6 2.7 3.1 2.7 2.5 3.6 2.8
MLSI
Mean
SD
SE
95% CI (Lower Bound)
95% CI (Upper Bound)
Min
Max
Level 1 Level 2 Level 3 Level 4 Level 5 Level 6 Level 7 Level 8 Level 9 Level 10 Level 11 Level 12 Static
2.6 2.0 1.7 1.5 1.3 1.4 1.3 1.3 1.1 1.1 1.0 0.9 0.8
1.3 1.0 1.1 0.6 0.5 0.7 0.6 0.6 0.6 0.6 0.5 0.4 0.5
0.3 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
2.0 1.5 1.2 1.2 1.1 1.1 1.0 1.0 0.8 0.8 0.8 0.7 0.5
3.2 2.4 2.2 1.7 1.6 1.8 1.6 1.6 1.4 1.4 1.2 1.2 1.0
0.6 0.7 0.7 0.7 0.6 0.6 0.4 0.5 0.2 0.5 0.5 0.4 0.2
4.7 4.0 5.4 2.4 2.3 2.7 2.5 3.2 2.5 2.8 2.2 2.2 2.2
Level 1 Level 2 Level 3 Level 4 Level 5 Level 6 Level 7 Level 8 Level 9 Level 10 Level 11 Level 12 Static Part B APSI Level 1 Level 2 Level 3 Level 4 Level 5 Level 6 Level 7 Level 8 Level 9 Level 10 Level 11 Level 12 Static Part C
All values are in degrees. APSI indicates anterior-posterior stability index; CI, confidence interval; max, maximum; min, minimum; MLSI, medial-lateral stability index; OSI, overall stability index; SD, standard deviation; CI, confidence interval.
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To test postural control under realistic conditions, the authors covered the computer screen and instructed the subjects to gaze straight ahead with their eyes open. The authors were thus able to exclude the presence of biofeedback that can increase stability. On the basis of their previous studies, auditory influences were not eliminated during the investigation.4
PROCEDURES During the tests, which were performed in a double-leg stance, all subjects wore their own athletic shoes. They were permitted to move their arms freely and to use the support handles as needed. No subject, however, had to prevent himself/herself from falling. The tests were performed at all stability levels (levels 1Y12 and the static level). Each test consisted of three 20-sec trials. Means (stability index scores) were calculated. Care was taken to ensure that the feet were placed in the same position on the platform during all tests. The sequence of the stability levels was strictly randomized for each subject using sealed envelopes. This minimized possible, although unlikely, learning effects during the tests. Learning effects, however, were not expected on the basis of the authors’ own research findings and the results reported by Pincivero et al.11
Statistical Analysis In addition to descriptive statistics, stability index scores (in degrees) were calculated for the subjects on the basis of the mean tilt of the platform from the horizontal. All subjects underwent measurements at each of the 13 levels. These measurements were repeated three times to increase the validity of the results and to reduce the number of outliers. Despite an apparently small sample size (20 subjects), a total of 780 measurements were performed and used for further calculations and analyses. Linear regression analysis was used to investigate a possible relationship between stability levels and index scores (P G 0.05). The authors conducted the sample size calculation (on the basis of previous data) and the statistical analyses with the assistance of the Institute of Biometrics at the University of Ulm, which was able to establish a mathematical formula to describe the relationship between postural control and platform stability. The main purposes of this formula were to provide the basis for a graphical representation of the linear correlation and to illustrate sex-specific differences in the sample investigated here.
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RESULTS In this study, postural control decreased when platform stability decreased from the static level to level 1. This finding was reflected in increasing stability index scores. The authors obtained an OSI score of 1.5 T 0.8 degrees in the static mode, 2.2 T 0.9 degrees at level 8 (more stable), and 4.7 T 2.1 degrees at level 1 (least stable). The difference between the two extreme levels is 3.1 degrees and corresponds to an increase in instability by 206.7% or, in other words, to an ability to compensate for platform displacements that is reduced by a factor of 3. Similar results were obtained for the anterior-posterior stability index and the medial-lateral stability index. All scores are shown in Table 1 (parts a to c). The authors were able to establish a linear formula on the basis of the results obtained. This formula should be regarded as a possible approach to describing and illustrating the results of this study. Separate formulas were developed for the men and the women as a result of anthropometric differences. The two formulas (Table 2) describe a significant linear correlation between the two quantities (P G 0.05). The slope of the line is 0.17 for the male subjects and is thus nearly twice as steep as that for the female subjects (0.10; Fig. 1) A graphical representation where the platform stability levels are indicated on the x axis and where the OSI scores (in degrees) are indicated on the y axis shows an apparently linear relationship between platform TABLE 2 Linear relationship between stability levels Formula Male subjects Female subjects Level 1 2 3 4 5 6 7 8 9 10 11 12 Static level
OSI = 1.86 + 0.17 LF OSI = 1.15 + 0.10 LF Level Factor 13 12 11 10 9 8 7 6 5 4 3 2 1
The two formulas describe the significant linear relationship (P G 0.05) that exists between stability levels (from level 2 to the static level) and OSI scores. As a result of anthropometric differences between the men and the women, the slope of the regression line for the male subjects is steeper than that for the female subjects.
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FIGURE 1 Approximate straight lines indicating the relationship between stability levels and the OSI scores that were obtained for the men (blue bars) and the women (red bars) during double-leg stance. From the static level to level 2, the solid red line (women) and the solid blue line (men) show a linear relationship between the two parameters.
instability and increasing stability index scores (Fig. 2). A linear regression analysis confirmed a significant linear correlation. As explained in the BMETHODS[ section, level 1 was excluded from this study’s data analysis because there is a disproportionate difference between level 1 and level 2 for technical reasons (see BMETHODS[).
DISCUSSION The purposes of this study were to demonstrate a relationship between the level of platform stability and stability index scores in posturography, to express this relationship (if any) mathematically, and to determine (if possible) differences according to sex. The authors were able to demonstrate that stability index scores increased in a linear fashion with increasing levels of difficulty and that the influence of stability levels on the men is almost twice as high as that on the women.
When it comes to the use of posturography, however, it is still unclear whether an increase in platform instability is correlated with a decrease in postural control. Although practical experience suggests a relationship, available data do not allow the authors to conclude how strong this correlation is or whether the relationship is linear. Because a standard experimental setup has not been defined and posturography tests are performed at different levels, the comparability of available studies is limited. In addition, information on expected stability index scores can be obtained only from tables of data, based on experience. A simple formula that would allow the authors to make rough estimates is not available. Moreover, sex-based differences have been largely ignored.
Stronger Effect of Stability Levels on Men Regression lines were calculated from the OSI scores, which have the greatest clinical relevance.
FIGURE 2 A linear relationship seemed to exist between stability levels (with the exception of level 1) and OSI scores and was confirmed by a linear regression analysis. Because the platform is fully unlocked at level 1, there is a disproportionately large difference between level 2 and level 1, which explains the outlier in the figure. www.ajpmr.com
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Because the authors’ previous studies gave them reason to suspect considerable differences between male and female subjects, OSI scores were differentiated according to sex. In the literature, the authors found only a few publications supporting their assumption that there might be sex-based differences. Their literature review revealed a paucity of studies that investigated sex-specific aspects. None of these studies, however, examined the relationship between stability levels and stability index scores. In addition, no significant differences were reported.12,13 The equations of the calculated regression lines are OSI = 1.15 + 0.10 LF for women (P G 0.05) and OSI = 1.86 + 0.17 LF (P G 0.05) for men, where LF is a level-related factor that must be selected from a table in which a level factor is assigned to each stability level (Table 2). For example, a factor of 1 is assigned to the static level; 2, to level 12; and 3, to level 11. The slope factors (0.10 for women and 0.17 for men) indicate the steepness of the line. Because the line for the male subjects is almost twice as steep as that for the female subjects, it is likely that postural control in men depends more on the (in)stability of the platform or surface than in women. Because the medical histories and clinical examinations had revealed that the members of both groups were physically healthy and differed only in height and weight, anthropometric differences between men and women are a possible explanation. The authors did not find any studies investigating such differences using posturography. Nevertheless, anthropometric data must necessarily be taken into consideration in studies involving subjects of both sexes and different levels of difficulty. Although this study was designed and conducted with the assistance of the Institute of Biometrics, it could be improved by recruiting more subjects and also including elderly participants. This study’s formulas, however, seem to be reliable for young and healthy subjects and provide an approach to investigating the correlation between stability levels and postural control in other groups involving healthy or injured subjects who may be investigated in further studies.
No Defined Standard Setup The first step in planning posturography studies is to determine a setup for the measurements. Should the subjects perform balance tests during single-leg or double-leg stance?8,14,15 Should a static platform or dynamic posturography be used?16,17 Should a low level of platform stability be selected, which has the disadvantage that subjects with a propensity to fall may have to discontinue
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the tests? Or should a stable level be selected, which has the disadvantage that physically active athletes may feel underchallenged? These considerations and the fact that different types of posturography systems are available explain why it is impossible to define a standard level of posturographic measurements that can be universally used in all studies. For example, a multitude of studies are available in which the BBS was used (as in this study). In these studies, tests were performed at widely varying levels of stability, whereas, for example, Paterno et al.8 investigated 41 high school athletes at level 4 and Rozzi et al.9 examined athletes who played soccer and/or basketball at level 2. By contrast, Schmitz and Arnold10 and Pereira et al.18 took measurements during 30-sec balance tests in which platform stability gradually decreased from level 8 to level 1. It is, of course, both appropriate and important to select stability levels in accordance with the subjects’ abilities. The use of different stability levels, however, limits the comparability of study results.
Linear Decrease in Postural Control with Increasing Platform Instability The results of this study show that there is a linear relationship between the different levels of difficulty and stability index scores. This finding provides a basis for comparing studies that use different levels of stability. Apart from a lack of comparability of studies, there was an absence of data on whether and, if so, to what extent the level of platform stability is correlated with postural stability indices. The authors were able to demonstrate a significant decrease in balance ability with decreasing platform stability. A comparison of OSI scores showed, for example, that the result for the fixed platform condition (1.5 T 0.8 degrees) was 3.1 times higher than the result for level 1 (4.7 T 2.1 degrees), which is the least stable setting (Table 1, parts a to c). The authors, however, found clinically relevant differences not only when they examined the results for extremely different stability levels but also when they compared the results for level 6 (2.6 T 1.1 degrees), level 8 (2.2 T 0.9 degrees), and level 10 (1.9 T 0.9 degrees), which are used in many studies and are similarly stable. A graphical representation of the results suggested a linear relationship between platform stability level and postural control (Fig. 2). A subsequent linear regression analysis confirmed this observation (P G 0.05). The authors can therefore conclude that a linear correlation exists between stability level and
Am. J. Phys. Med. Rehabil. & Vol. 93, No. 1, January 2014
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the postural control of the subjects. There are, however, a few limitations of this study that must be noted. First of all, these findings apply only to healthy subjects, and data for patients with a disease or injury are not yet available. As a result of this homogeneous sample of subjects, additional factors such as age, height, and weight were not included in this first explorative study. In addition, a linear relationship was found only from level 2 to the static level because the platform is fully unlocked at level 1 and the difference between level 1 and level 2 is disproportionately large. Finally, the findings of this study apply only to the BBS, which was used in this study, although it is assumed that similar results will likely be obtained with other systems. Owing to the aforementioned limitations, the authors regard the study presented here as a pilot that provides interesting findings and the first evidence for a linear correlation between the level of platform stability and stability index scores, which warrants further investigation in larger samples of subjects.
CONCLUSIONS The authors were able to demonstrate for the first time a linear correlation between stability index scores and platform stability levels. Balance ability therefore increases with increasing stability of the computerized dynamic posturography platform. The authors were able to express these findings in two sex-specific linear equations that allowed them to compare the results of available studies and of this study, irrespective of the stability level used. Because the slope of the regression line for the male subjects is almost twice as steep as that for the female subjects, the authors assume that the influence of platform stability on postural control is almost twice as high in men as in women as a result of anthropometric differences. Further studies should address aspects of particular clinical relevance and compare the correlation lines of healthy subjects and those of patients with injuries that are known to adversely affect postural control (e.g., injuries involving the lower extremities). ACKNOWLEDGMENTS
The authors thank Birgit Hay (Institute of Biometrics, University of Ulm, Germany) for her skillful help with the statistical analysis and Barbara Isenberg (Federal Office of Languages, Huerth, Germany) for her valuable assistance in translating this article. REFERENCES 1. Biedert RM, Zwick EB: Ligament-muscle reflex arc after anterior cruciate ligament reconstruction: www.ajpmr.com
Electromyographic evaluation. Arch Orthop Trauma Surg 1998;118:81Y4 2. Maurer C, Mergner T, Peterka RJ: Multisensory control of human upright stance. Exp Brain Res 2006; 171:231Y50 3. Friedrich M, Grein HJ, Wicher C, et al: Influence of pathologic and simulated visual dysfunctions on the postural system. Exp Brain Res 2008;186:305Y14 4. Palm HG, Strobel J, Achatz G, et al: The role and interaction of visual and auditory afferents in postural stability. Gait Posture 2009;30:328Y33 5. Morrison G, Hawken M, Kennard C, et al: Dynamic platform sway measurement in Meniere’s disease. J Vestib Res 1994;4:409Y19 6. Cuccia A, Caradonna C: The relationship between the stomatognathic system and body posture. Clinics (Sao Paulo) 2009;64:61Y6 7. Rein S, Fabian T, Krishnan K, et al: Evaluation of the proprioceptive influence of the cutaneous afferents to the ankle in patients after sural nerve harvesting. Neurosurgery 2009;64:519Y26 8. Paterno MV, Myer GD, Ford KR, et al: Neuromuscular training improves single-limb stability in young female athletes. J Orthop Sports Phys Ther 2004; 34:305Y16 9. Rozzi SL, Lephart SM, Gear WS, et al: Knee joint laxity and neuromuscular characteristics of male and female soccer and basketball players. Am J Sports Med 1999;27:312Y9 10. Schmitz R, Arnold B: Intertester and intratester reliability of a dynamic balance protocol using the Biodex Stability System. J Sport Rehabil 1998;7:95Y101 11. Pincivero D, Lephart S, Henry T: Learning effects and reliability of the Biodex Stability System. J Athl Train 1995;30:35 12. Bryant EC, Trew ME, Bruce AM, et al: Gender differences in balance performance at the time of retirement. Clin Biomech (Bristol, Avon) 2005;20:330Y5 13. Lee AJ, Lin WH: The influence of gender and somatotype on single-leg upright standing postural stability in children. J Appl Biomech 2007;23:173Y9 14. Gstottner M, Neher A, Scholtz A, et al: Balance ability and muscle response of the preferred and nonpreferred leg in soccer players. Motor Control 2009; 13:218Y31 15. Hinman M: Factors affecting reliabilty of the Biodex Balance System, a summary of four studies. J Sport Rehabil 2000;9:240Y50 16. Arnold BL, Schmitz RJ: Examination of balance measures produced by the Biodex Stability System. J Athl Train 1998;33:323Y7 17. Chaudhry H, Findley T, Quigley K, et al: Measures of postural stability. J Rehabil Res Dev 2004;41:713Y20 18. Pereira HM, de Campos TF, Santos MB, et al: Influence of knee position on the postural stability index registered by the Biodex Stability System. Gait Posture 2008;28:668Y72
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55
Affiliations:
Posture
ORIGINAL RESEARCH ARTICLE
From the Trauma Research Group, Department of Orthopedics and Trauma Surgery, German Armed Forces Hospital of Ulm, Ulm, Germany.
Correspondence: All correspondence and requests for reprints should be addressed to Hans-Georg Palm, MD, MBA, Trauma Research Group, Department of Orthopedics and Trauma Surgery, German Armed Forces Hospital of Ulm, Oberer Eselsberg 40, D-89081 Ulm, Germany.
Disclosures: Financial disclosure statements have been obtained, and no conflicts of interest have been reported by the authors or by any individuals in control of the content of this article.
0894-9115/14/9301-0049 American Journal of Physical Medicine & Rehabilitation Copyright * 2013 by Lippincott Williams & Wilkins DOI: 10.1097/PHM.0b013e3182a39019
Computerized Dynamic Posturography The Influence of Platform Stability on Postural Control ABSTRACT Palm H-G, Lang P, Strobel J, Riesner H-J, Friemert B: Computerized dynamic posturography: the influence of platform stability on postural control. Am J Phys Med Rehabil 2014;93:49Y55.
Objective: Postural stability can be quantified using posturography systems, which allow different foot platform stability settings to be selected. It is unclear, however, how platform stability and postural control are mathematically correlated. Design: Twenty subjects performed tests on the Biodex Stability System at all 13 stability levels. Overall stability index, medial-lateral stability index, and anteriorposterior stability index scores were calculated, and data were analyzed using analysis of variance and linear regression analysis.
Results: A decrease in platform stability from the static level to the second least stable level was associated with a linear decrease in postural control. The overall stability index scores were 1.5 T 0.8 degrees (static), 2.2 T 0.9 degrees (level 8), and 3.6 T 1.7 degrees (level 2). The slope of the regression lines was 0.17 for the men and 0.10 for the women.
Conclusions: A linear correlation was demonstrated between platform stability and postural control. The influence of stability levels seems to be almost twice as high in men as in women. Key Words:
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Postural Control, Posturography, Instability, Biodex Balance System
Computerized Dynamic Posturography Copyright © 2013 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
49
P
ostural stability is required for the maintenance of an upright posture and gait in humans. Stability regulation is a dynamic process that involves the continuous central processing of afferent inputs from visual, vestibular, and proprioceptive receptors in a hierarchical structure. Motor responses are then generated to correct posture.1,2 The complex interactions between the sensory systems result in a multitude of conditions that can affect the ability to maintain a stable posture. This was shown in a number of studies involving patients with impaired vision,3,4 diseases of the vestibular system,5 or changes in proprioception and sensorimotor function.6,7 A decrease in postural control is usually quantified using posturography. This method involves the use of a static platform that assesses displacements of a subject’s center of gravity or a dynamic platform that records passive displacements (computerized dynamic posturography). Multiple levels of platform instability can be selected and allow examiners to tailor tests to the needs and the requirements of patients or athletes. Studies, however, have not yet investigated whether and, if so, to what extent a decrease in postural control is correlated with an increase in platform instability and especially whether the results are identical for men and women. Although a linear relationship seems to be most likely, it is also possible that healthy and physically active subjects, for example, are able to fully compensate for high levels of instability until a certain level is reached. After a plateau phase, they may then show a sudden loss of postural control. The relationship between postural control and level of platform instability is not only interesting from a purely scientific point of view but also of high practical relevance. Neither a literature review nor information obtained from the Biodex Balance System (BBS) manufacturer (Biodex, Shirley, NY) revealed a specific stability level that is used as a standard level in tests (Paterno et al.8: level 4, Rozzi et al.9: level 2, and Schmitz and Arnold10: dynamic levels 8Y1). As long as it remains unclear whether and, if so, how platform instability is associated with a decrease in postural control, the comparability of available studies and the comparability of this study’s own data and that of available studies are limited. In addition, this study on healthy subjects describes a pilot approach that provides a basis for further investigations involving patients with reduced postural control such as patients with anterior cruciate ligament rupture. Other studies may address the
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question as to whether patients with injuries that are known to reduce proprioception and thus postural control show different correlations or decompensation without mathematical relationships. For this reason, the objectives of this prospective randomized clinical and experimental study were to investigate the relationship between postural control as reflected by the overall stability index (OSI) and the level of platform instability and to assess whether there are differences between the male and female subjects. A further objective was to express any possible relationships for the first time in a mathematical formula. Owing to the explorative nature of this pilot study, however, this formula should be regarded as a potential approach to describing the relationship between the stability levels and postural control and should, in particular, illustrate a linear correlation.
SUBJECTS AND METHODS Subjects Twenty subjects took part in this study. Of these, ten were men (25.3 T 1.9 yrs, 180 T 11 cm, 77.3 T 5.3 kg) and ten were women (23.9 T 2.4 yrs, 172 T 9 cm, 66.4 T 6.4 kg). No subject had a history of diseases of the neurologic, the vestibular, or the visual system. Subjects with diseases or injuries of the lower extremities, the spine, or the pelvis and especially injuries of the cruciate, medial, or lateral ligaments were excluded. Standardized clinical examinations of the musculoskeletal system were carried out to rule out unknown diseases or previous injuries. If there was any doubt about the presence or the absence of a disease, the subject was not included in the study. This clinical and experimental study was approved by the ethics committee of the University of Ulm (No. 65/08) and was conducted as part of a special research project of the German Armed Forces (16K3-S-100712) in accordance with the Declaration of Helsinki of 1975 as revised in 1983. All subjects gave their informed consent.
Instrumentation The BBS was used, which has a circular platform that is free to move in the medial-lateral and anterior-posterior axes. It permits up to 20 degrees of tilt in any plane and has a measuring accuracy of 0.1 degrees. The system records the degree of displacement from the initial level under dynamic conditions and calculates a score for the mediallateral stability index, the anterior-posterior stability
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index, and the OSI. The focus of this study was on the OSI because it is the most commonly used and the most relevant parameter. The BBS allows testing at 1 static and 12 dynamic levels. At any level, a shift in a subject’s center of gravity leads to a passive displacement of the foot platform. A setting of 1 is the least stable platform setting. A setting of 12 is the
most stable released setting. According to the manufacturer, the position of the platform is secured by locking screws that can be electronically unlocked and thus allows higher levels of platform instability to be selected gradually and evenly. When level 1 is selected after level 2, the tilting platform is fully unlocked in a discontinuous manner.
TABLE 1 Stability index scores Part A OSI
Mean
SD
SE
95% CI (Lower Bound)
95% CI (Upper Bound)
Min
Max
4.7 3.6 2.9 2.5 2.4 2.6 2.3 2.2 2.2 1.9 1.9 1.7 1.5
2.1 1.7 1.7 0.9 1.0 1.1 1.0 0.9 1.1 0.9 0.8 0.8 0.8
0.5 0.4 0.4 0.2 02 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
3.7 2.8 2.1 2.1 2.0 2.1 1.9 1.7 1.7 1.5 1.5 1.3 1.1
5.7 4.4 3.7 2.9 2.9 3.1 2.8 2.6 2.7 2.3 2.2 2.1 1.8
1.7 1.3 1.3 1.0 1.2 1.1 0.6 0.7 0.7 0.8 0.9 0.9 0.6
8.3 7.0 7.5 3.6 4.6 4.5 4.5 4.6 4.0 4.3 3.2 4.2 3.5
Mean
SD
SE
95% CI (Lower Bound)
95% CI (Upper Bound)
Min
Max
3.4 2.5 1.9 1.7 1.7 1.9 1.7 1.5 1.6 1.3 1.4 1.2 1.1
1.5 1.3 1.1 0.7 0.9 0.8 0.8 0.6 0.9 0.6 0.6 0.7 0.7
0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.1 0.2 0.1 0.1 0.2 0.1
2.7 1.9 1.4 1.4 1.3 1.5 1.3 1.2 1.2 1.0 1.1 0.8 0.7
4.0 3.1 2.5 2.0 2.1 2.2 2.0 1.7 2.0 1.6 1.7 1.5 1.4
1.4 0.7 0.7 0.5 0.7 0.7 0.3 0.4 0.6 0.5 0.6 0.6 0.3
5.8 4.8 4.2 2.9 3.7 3.1 3.6 2.7 3.1 2.7 2.5 3.6 2.8
MLSI
Mean
SD
SE
95% CI (Lower Bound)
95% CI (Upper Bound)
Min
Max
Level 1 Level 2 Level 3 Level 4 Level 5 Level 6 Level 7 Level 8 Level 9 Level 10 Level 11 Level 12 Static
2.6 2.0 1.7 1.5 1.3 1.4 1.3 1.3 1.1 1.1 1.0 0.9 0.8
1.3 1.0 1.1 0.6 0.5 0.7 0.6 0.6 0.6 0.6 0.5 0.4 0.5
0.3 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
2.0 1.5 1.2 1.2 1.1 1.1 1.0 1.0 0.8 0.8 0.8 0.7 0.5
3.2 2.4 2.2 1.7 1.6 1.8 1.6 1.6 1.4 1.4 1.2 1.2 1.0
0.6 0.7 0.7 0.7 0.6 0.6 0.4 0.5 0.2 0.5 0.5 0.4 0.2
4.7 4.0 5.4 2.4 2.3 2.7 2.5 3.2 2.5 2.8 2.2 2.2 2.2
Level 1 Level 2 Level 3 Level 4 Level 5 Level 6 Level 7 Level 8 Level 9 Level 10 Level 11 Level 12 Static Part B APSI Level 1 Level 2 Level 3 Level 4 Level 5 Level 6 Level 7 Level 8 Level 9 Level 10 Level 11 Level 12 Static Part C
All values are in degrees. APSI indicates anterior-posterior stability index; CI, confidence interval; max, maximum; min, minimum; MLSI, medial-lateral stability index; OSI, overall stability index; SD, standard deviation; CI, confidence interval.
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To test postural control under realistic conditions, the authors covered the computer screen and instructed the subjects to gaze straight ahead with their eyes open. The authors were thus able to exclude the presence of biofeedback that can increase stability. On the basis of their previous studies, auditory influences were not eliminated during the investigation.4
PROCEDURES During the tests, which were performed in a double-leg stance, all subjects wore their own athletic shoes. They were permitted to move their arms freely and to use the support handles as needed. No subject, however, had to prevent himself/herself from falling. The tests were performed at all stability levels (levels 1Y12 and the static level). Each test consisted of three 20-sec trials. Means (stability index scores) were calculated. Care was taken to ensure that the feet were placed in the same position on the platform during all tests. The sequence of the stability levels was strictly randomized for each subject using sealed envelopes. This minimized possible, although unlikely, learning effects during the tests. Learning effects, however, were not expected on the basis of the authors’ own research findings and the results reported by Pincivero et al.11
Statistical Analysis In addition to descriptive statistics, stability index scores (in degrees) were calculated for the subjects on the basis of the mean tilt of the platform from the horizontal. All subjects underwent measurements at each of the 13 levels. These measurements were repeated three times to increase the validity of the results and to reduce the number of outliers. Despite an apparently small sample size (20 subjects), a total of 780 measurements were performed and used for further calculations and analyses. Linear regression analysis was used to investigate a possible relationship between stability levels and index scores (P G 0.05). The authors conducted the sample size calculation (on the basis of previous data) and the statistical analyses with the assistance of the Institute of Biometrics at the University of Ulm, which was able to establish a mathematical formula to describe the relationship between postural control and platform stability. The main purposes of this formula were to provide the basis for a graphical representation of the linear correlation and to illustrate sex-specific differences in the sample investigated here.
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RESULTS In this study, postural control decreased when platform stability decreased from the static level to level 1. This finding was reflected in increasing stability index scores. The authors obtained an OSI score of 1.5 T 0.8 degrees in the static mode, 2.2 T 0.9 degrees at level 8 (more stable), and 4.7 T 2.1 degrees at level 1 (least stable). The difference between the two extreme levels is 3.1 degrees and corresponds to an increase in instability by 206.7% or, in other words, to an ability to compensate for platform displacements that is reduced by a factor of 3. Similar results were obtained for the anterior-posterior stability index and the medial-lateral stability index. All scores are shown in Table 1 (parts a to c). The authors were able to establish a linear formula on the basis of the results obtained. This formula should be regarded as a possible approach to describing and illustrating the results of this study. Separate formulas were developed for the men and the women as a result of anthropometric differences. The two formulas (Table 2) describe a significant linear correlation between the two quantities (P G 0.05). The slope of the line is 0.17 for the male subjects and is thus nearly twice as steep as that for the female subjects (0.10; Fig. 1) A graphical representation where the platform stability levels are indicated on the x axis and where the OSI scores (in degrees) are indicated on the y axis shows an apparently linear relationship between platform TABLE 2 Linear relationship between stability levels Formula Male subjects Female subjects Level 1 2 3 4 5 6 7 8 9 10 11 12 Static level
OSI = 1.86 + 0.17 LF OSI = 1.15 + 0.10 LF Level Factor 13 12 11 10 9 8 7 6 5 4 3 2 1
The two formulas describe the significant linear relationship (P G 0.05) that exists between stability levels (from level 2 to the static level) and OSI scores. As a result of anthropometric differences between the men and the women, the slope of the regression line for the male subjects is steeper than that for the female subjects.
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FIGURE 1 Approximate straight lines indicating the relationship between stability levels and the OSI scores that were obtained for the men (blue bars) and the women (red bars) during double-leg stance. From the static level to level 2, the solid red line (women) and the solid blue line (men) show a linear relationship between the two parameters.
instability and increasing stability index scores (Fig. 2). A linear regression analysis confirmed a significant linear correlation. As explained in the BMETHODS[ section, level 1 was excluded from this study’s data analysis because there is a disproportionate difference between level 1 and level 2 for technical reasons (see BMETHODS[).
DISCUSSION The purposes of this study were to demonstrate a relationship between the level of platform stability and stability index scores in posturography, to express this relationship (if any) mathematically, and to determine (if possible) differences according to sex. The authors were able to demonstrate that stability index scores increased in a linear fashion with increasing levels of difficulty and that the influence of stability levels on the men is almost twice as high as that on the women.
When it comes to the use of posturography, however, it is still unclear whether an increase in platform instability is correlated with a decrease in postural control. Although practical experience suggests a relationship, available data do not allow the authors to conclude how strong this correlation is or whether the relationship is linear. Because a standard experimental setup has not been defined and posturography tests are performed at different levels, the comparability of available studies is limited. In addition, information on expected stability index scores can be obtained only from tables of data, based on experience. A simple formula that would allow the authors to make rough estimates is not available. Moreover, sex-based differences have been largely ignored.
Stronger Effect of Stability Levels on Men Regression lines were calculated from the OSI scores, which have the greatest clinical relevance.
FIGURE 2 A linear relationship seemed to exist between stability levels (with the exception of level 1) and OSI scores and was confirmed by a linear regression analysis. Because the platform is fully unlocked at level 1, there is a disproportionately large difference between level 2 and level 1, which explains the outlier in the figure. www.ajpmr.com
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Because the authors’ previous studies gave them reason to suspect considerable differences between male and female subjects, OSI scores were differentiated according to sex. In the literature, the authors found only a few publications supporting their assumption that there might be sex-based differences. Their literature review revealed a paucity of studies that investigated sex-specific aspects. None of these studies, however, examined the relationship between stability levels and stability index scores. In addition, no significant differences were reported.12,13 The equations of the calculated regression lines are OSI = 1.15 + 0.10 LF for women (P G 0.05) and OSI = 1.86 + 0.17 LF (P G 0.05) for men, where LF is a level-related factor that must be selected from a table in which a level factor is assigned to each stability level (Table 2). For example, a factor of 1 is assigned to the static level; 2, to level 12; and 3, to level 11. The slope factors (0.10 for women and 0.17 for men) indicate the steepness of the line. Because the line for the male subjects is almost twice as steep as that for the female subjects, it is likely that postural control in men depends more on the (in)stability of the platform or surface than in women. Because the medical histories and clinical examinations had revealed that the members of both groups were physically healthy and differed only in height and weight, anthropometric differences between men and women are a possible explanation. The authors did not find any studies investigating such differences using posturography. Nevertheless, anthropometric data must necessarily be taken into consideration in studies involving subjects of both sexes and different levels of difficulty. Although this study was designed and conducted with the assistance of the Institute of Biometrics, it could be improved by recruiting more subjects and also including elderly participants. This study’s formulas, however, seem to be reliable for young and healthy subjects and provide an approach to investigating the correlation between stability levels and postural control in other groups involving healthy or injured subjects who may be investigated in further studies.
No Defined Standard Setup The first step in planning posturography studies is to determine a setup for the measurements. Should the subjects perform balance tests during single-leg or double-leg stance?8,14,15 Should a static platform or dynamic posturography be used?16,17 Should a low level of platform stability be selected, which has the disadvantage that subjects with a propensity to fall may have to discontinue
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the tests? Or should a stable level be selected, which has the disadvantage that physically active athletes may feel underchallenged? These considerations and the fact that different types of posturography systems are available explain why it is impossible to define a standard level of posturographic measurements that can be universally used in all studies. For example, a multitude of studies are available in which the BBS was used (as in this study). In these studies, tests were performed at widely varying levels of stability, whereas, for example, Paterno et al.8 investigated 41 high school athletes at level 4 and Rozzi et al.9 examined athletes who played soccer and/or basketball at level 2. By contrast, Schmitz and Arnold10 and Pereira et al.18 took measurements during 30-sec balance tests in which platform stability gradually decreased from level 8 to level 1. It is, of course, both appropriate and important to select stability levels in accordance with the subjects’ abilities. The use of different stability levels, however, limits the comparability of study results.
Linear Decrease in Postural Control with Increasing Platform Instability The results of this study show that there is a linear relationship between the different levels of difficulty and stability index scores. This finding provides a basis for comparing studies that use different levels of stability. Apart from a lack of comparability of studies, there was an absence of data on whether and, if so, to what extent the level of platform stability is correlated with postural stability indices. The authors were able to demonstrate a significant decrease in balance ability with decreasing platform stability. A comparison of OSI scores showed, for example, that the result for the fixed platform condition (1.5 T 0.8 degrees) was 3.1 times higher than the result for level 1 (4.7 T 2.1 degrees), which is the least stable setting (Table 1, parts a to c). The authors, however, found clinically relevant differences not only when they examined the results for extremely different stability levels but also when they compared the results for level 6 (2.6 T 1.1 degrees), level 8 (2.2 T 0.9 degrees), and level 10 (1.9 T 0.9 degrees), which are used in many studies and are similarly stable. A graphical representation of the results suggested a linear relationship between platform stability level and postural control (Fig. 2). A subsequent linear regression analysis confirmed this observation (P G 0.05). The authors can therefore conclude that a linear correlation exists between stability level and
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the postural control of the subjects. There are, however, a few limitations of this study that must be noted. First of all, these findings apply only to healthy subjects, and data for patients with a disease or injury are not yet available. As a result of this homogeneous sample of subjects, additional factors such as age, height, and weight were not included in this first explorative study. In addition, a linear relationship was found only from level 2 to the static level because the platform is fully unlocked at level 1 and the difference between level 1 and level 2 is disproportionately large. Finally, the findings of this study apply only to the BBS, which was used in this study, although it is assumed that similar results will likely be obtained with other systems. Owing to the aforementioned limitations, the authors regard the study presented here as a pilot that provides interesting findings and the first evidence for a linear correlation between the level of platform stability and stability index scores, which warrants further investigation in larger samples of subjects.
CONCLUSIONS The authors were able to demonstrate for the first time a linear correlation between stability index scores and platform stability levels. Balance ability therefore increases with increasing stability of the computerized dynamic posturography platform. The authors were able to express these findings in two sex-specific linear equations that allowed them to compare the results of available studies and of this study, irrespective of the stability level used. Because the slope of the regression line for the male subjects is almost twice as steep as that for the female subjects, the authors assume that the influence of platform stability on postural control is almost twice as high in men as in women as a result of anthropometric differences. Further studies should address aspects of particular clinical relevance and compare the correlation lines of healthy subjects and those of patients with injuries that are known to adversely affect postural control (e.g., injuries involving the lower extremities). ACKNOWLEDGMENTS
The authors thank Birgit Hay (Institute of Biometrics, University of Ulm, Germany) for her skillful help with the statistical analysis and Barbara Isenberg (Federal Office of Languages, Huerth, Germany) for her valuable assistance in translating this article. REFERENCES 1. Biedert RM, Zwick EB: Ligament-muscle reflex arc after anterior cruciate ligament reconstruction: www.ajpmr.com
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