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International Journal of Rehabilitation Research:
doi: 10.1097/MRR.0b013e3283588bac
Original articles

Organization of functional postural responses following perturbations in multiple directions in elderly fallers standing quietly

Matjačić, Zlatkoa; Sok, Davidb; Jakovljević, Miroljubc; Cikajlo, Imrea

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Author Information

aUniversity Rehabilitation Institute, Republic of Slovenia

bDepartment of Orthopaedic Surgery, University Medical Centre Ljubljana

cFaculty of Health Sciences, University of Ljubljana, Ljubljana, Slovenia

Correspondence to Zlatko Matjačić, PhD, University Rehabilitation Institute, Republic of Slovenia, Linhartova 51, SI-1000 Ljubljana, Slovenia Tel: +386 1 47 58 159; fax: +386 1 43 72 070; e-mail: zlatko.matjacic@ir-rs.si

Received June 7, 2012

Accepted July 21, 2012

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Abstract

The objective of the study was to assess functional postural responses by analyzing the center-of-pressure trajectories resulting from perturbations delivered in multiple directions to elderly fallers. Ten elderly individuals were standing quietly on two force platforms while an apparatus delivered controlled perturbations at the level of pelvis in eight directions: ‘forward (FW)’ and ‘backward (BW)’ [anterioposterior plane (AP)], ‘left (LT)’ and ‘right (RT)’ [mediolateral plane (ML)] and four combinations of these principal directions: forward-left (FL), forward-right (FR), backward-left (BL) and backward-right (BR). Perturbations were repeated randomly four times in each direction. Peak amplitude responses (PARAP and PARML) and times to reach peak amplitude responses (TPAR-AP and TPAR-ML) were extracted for each perturbation direction from acquired center-of-pressure data. One-way analysis of variance was used to test differences between directionally-similar perturbation directions. Balance abilities were assessed by means of Berg Balance Scale. Average Berg Balance Scale for the group resulted in 33.9±5.8 points, which means that the participants were at greater risk of fall. The comparison of the group averaged PARAP and TPAR-AP in all ‘forward’ directions (FW, FL, FR) as well in all ‘backward’ directions (BW, BL, BR) have not shown statistically significant differences. The comparison of the group averaged PARML and TPAR-ML in all ‘left’ directions (LT, FL, BL) as well in all ‘right’ directions (RT, FR, BR) have not shown statistically significant differences. These results suggest that a principle of decoupled control may to a large extent be preserved also in elderly with clearly diminished balancing abilities, which implies that the accuracy in the assessment of perturbation direction may be well preserved also in very old age.

Ziel der vorliegenden Studie war die Beurteilung der funktionalen posturalen Reaktionen mithilfe der Analyse der Bahnen des Druckmittelpunktes infolge von in verschiedenen Richtungen wirkenden Störungen bei älteren sturzgefährdeten Menschen. Zehn ältere Probanden standen ruhig auf zwei Kraftmessdruckplatten, während ein Apparat unterdessen in Hüfthöhe kontrollierte Störreize in acht verschiedenen Richtungen ausübte: “vorne” und “hinten” [Standfläche in Richtung anterio-posterior], “links” und “rechts” [Standfläche in Richtung medial-lateral] sowie vier Kombinationen dieser Hauptauslenkungen der Standfläche: vorne links, vorne rechts, hinten links und hinten rechts. Die Störreize erfolgten wahllos viermal in jede Richtung. Die höchsten Amplitudenreaktionen (PARAP und PARML) sowie die Dauer beim Erreichen der höchsten Amplitudenreaktionen (TPAR-AP und TPAR-ML) wurden aus erworbenen Druckmittelpunkt-Daten für jede Störungsrichtung ermittelt. Unter Zuhilfenahme einer einseitigen Varianzanalyse wurden die Unterschiede zwischen direktional ähnlichen Störungsrichtungen überprüft. Die Gleichgewichtsregulation wurde mithilfe der Berg-Balance-Skala beurteilt. Die mediane Berg-Balance-Skala für die Gruppe ergab 33.9±5.8 Punkte, was auf eine gröβere Sturzgefahr bei den Teilnehmern hinwies. Der Vergleich der Gruppe ergab im Mittel PARAP und TPAR-AP in allen “vorwärts”-Richtungen (vorne, vorne links, vorne rechts) sowie in allen “rückwärts”-Richtungen (hinten, hinten links, hinten rechts) und somit keine statistisch signifikanten Unterschiede. Der Vergleich der Gruppe ergab im Mittel PARML und TPAR-ML in allen “links”-Richtungen (links, vorne links, hinten links) sowie in allen “rechts”-Richtungen (rechts, vorne rechts, hinten rechts) und somit keine statistisch signifikanten Unterschiede. Diese Ergebnisse legen den Schluss nahe, dass ein Prinzip der entkoppelten Kontrolle in hohem Maβe auch bei älteren Menschen mit eindeutig eingeschränkter postularer Stabilität gewahrt werden kann, was andeutet, dass die Präzision bei der Beurteilung der Störungsrichtung möglicherweise bis ins hohe Alter hinein gewahrt werden kann.

El objetivo del estudio fue evaluar las respuestas posturales funcionales mediante el análisis de las trayectorias del centro de presión resultantes de las perturbaciones en múltiples direcciones que tienen lugar en las caídas en ancianos. Diez individuos de edad avanzada permanecieron de pie sobre dos plataformas de fuerza mientras un aparato provocaba una serie de perturbaciones a la altura de la pelvis en las siguientes ocho direcciones: “hacia adelante (FW)” y “hacia atrás (BW)” [plano anteroposterior (AP)], “izquierda (LT)” y “derecha (RT)” [plano mediolateral (ML)] y cuatro combinaciones de dichas direcciones principales: hacia adelante-izquierda (FL), hacia adelante-derecha (FR), hacia atrás-izquierda (BL) y hacia atrás-derecha (BR). Las perturbaciones se repitieron de forma aleatoria cuatro veces en cada dirección. Las respuestas de la amplitud pico (PARAP y PARML) y los tiempos necesarios para alcanzar las respuestas de la amplitud pico (TPAR-AP y TPAR-ML) se obtuvieron para cada una de las direcciones a partir de los datos del centro de presión obtenidos. Se utilizó un análisis de varianza en un solo sentido con el fin de determinar las diferencias entre las direcciones de las perturbaciones más similares. La capacidad de equilibrio fue evaluada mediante la Escala de Equilibrio de Berg. El resultado medio del grupo obtenido mediante la Escala de Equilibrio de Berg fue de 33.9±5.8 puntos, lo que implica que los participantes se encontraban bajo un riesgo mayor de caída. La comparación del grupo demostró que PARAP y TPAR-AP en todas las direcciones “hacia adelante”(FW, FL, FR) y en todas las direcciones “hacia atrás”(BW, BL, BR) no presentaban diferencias estadísticamente significativas. La comparación del grupo demostró que PARML y TPAR-ML en todas las direcciones “hacia la izquierda” (LT, FL, BL) y en todas las direcciones “hacia la derecha” (RT, FR, BR) no presentaban diferencias estadísticamente significativas. Estos resultados sugieren que el control separado puede utilizarse también en el caso de ancianos con una menor capacidad de equilibrio, lo cual significa que la evaluación de las direcciones de la perturbación puede ser precisa también en casos de edades avanzadas.

Cette étude avait pour objet d'évaluer les réponses fonctionnelles posturales en analysant les trajectoires du centre de pression résultant de perturbations imposées dans de multiples directions à des personnes âgées lors d’une chute. Dix personnes âgées se tenaient debout sur deux plates-formes de force tandis qu'un appareil leur imposait des perturbations contrôlées dans huit directions au niveau du bassin: «vers l’avant (AV)» et «vers l'arrière (AR)» [plan antério-posterieur (AP)], «à gauche (G)» et «à droite (D)» [plan médio-latéral (ML)] et quatre combinaisons de ces directions principales: avant-gauche (AVG), avant-droite (AVD), arrière-gauche (ARG) et arrière-droite (ARD). Les perturbations ont été répétées quatre fois au hasard dans chaque direction. Les réponses d'amplitude de crête (RACAP et RACML) et les délais pour parvenir à des réponses d'amplitude de crête (DRACAP et DRACML) ont été extraits pour chaque direction de perturbation depuis les données de centre de pression obtenues. Une analyse de variance unilatérale a été utilisée pour tester les différences entre les directions de perturbation similaires. Les capacités d'équilibre ont été évaluées au moyen de l'échelle d'équilibre de Berg. L’échelle d'équilibre de Berg moyenne pour le groupe a donné 33.9±5.8 points, ce qui signifie que les participants présentaient un plus grand risque de chute. La comparaison des données moyennes de groupe RACAP et DRACAP dans toutes les directions «avant» (AV, AVG, AVD) ainsi que dans toutes les directions «arrière» (AR, ARG, ARD) n'a pas fait apparaître de différences statistiquement significatives. La comparaison des données moyennes de groupe RACML et DRACML dans toutes les directions «gauche» (G, AVG, ARG) ainsi que dans toutes les directions «droite» (D, AVD, ARD) n’a pas fait apparaître de différences statistiquement significatives. Ces résultats suggèrent que le principe du contrôle découplé peut dans une large mesure être également préservé chez les personnes âgées avec des capacités d'équilibre nettement diminuées, ce qui implique que la précision dans l'évaluation de la perturbation de la direction peut aussi être bien conservée à un âge très avancé.

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Introduction

Diminishing balancing abilities in the elderly frequently cause falls that may lead to injuries and reduction in the quality of life (Bloem et al., 2003). Underlying reasons for deteriorated balance and postural control are related to physiological changes that occur in aging (decrease in nerve conduction velocity, deterioration of visual, vestibular and somatosensory function, and reduction in muscle strength) leading to reduction in functional balancing abilities (Laughton et al., 2003). Spontaneous-sway and induced-sway balance tests are frequently used to examine the balancing abilities during standing. In particular, induced-sway balance tests utilizing a moving platform to some extent simulate potential falling circumstances and may therefore provide valuable insight into the postural control scheme of an individual. A substantial number of studies using the above referenced techniques examined changes in the postural control due to aging, but the majority of these studies were confined to a single plane of movement (Maki et al., 1990).

Several studies investigated the organization of postural responses during standing following multidirectional perturbations in anterioposterior (AP) and mediolateral (ML) directions (Matjačić et al., 2001; Jones et al., 2008). These studies showed that the organization of functional postural responses in humans, as reflected in support surface reaction forces and net joint torques, is such that follows a principle of functionally decoupled control in the AP and ML planes. This means that when a perturbation occurs in the AP and ML directions simultaneously, the corresponding postural responses in both directions are exactly the same as in case when perturbation occurs only in either of both directions. To be able to execute postural responses organized in the described way accurate information on perturbation is essential. As in aging both sensory and motor systems deteriorate, there is a question to what extent these changes have influence on the ‘directional’ appropriateness of postural responses induced by perturbations generated in multiple directions.

The aim of this study was to determine the organization of functional postural responses during quiet standing following perturbations in multiple directions of transverse plane in a group of elderly with reduced balancing abilities and confirmed history of at least one falling incident. We specifically examined whether the principle of functionally decoupled postural control also applies in the elderly with diminished balancing abilities.

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Materials and methods

Participants

Ten elderly women (age 84.6±4.2 years; height 163.5±6.3 cm; weight 75.7±12.8 kg) volunteered from the local nursing home. All the participants were able to maintain quiet stance for at least 1 min, they had no diagnosis of neurological diseases and no cognitive impairment, as was shown by all successfully passing the Slovenian version of the Mini Mental State Examination short test on cognitive abilities (Granda et al., 2003). In addition, based on the data log on falls maintained by the physiotherapy service at the local elderly home, all participants experienced at least one reported and recorded fall. The study was carried out in accordance with ethical standards and was approved by the national ethics committee and all participants gave written informed consent.

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BBS

Postural control and balancing abilities were assessed by a Berg Balance Scale (BBS) that has proven to be reliable and valid for an elderly population (Berg et al., 1992). The BBS consists of 14 items that include simple mobility tasks (e.g. transfers, standing unsupported and sit-to-stand) and more difficult tasks (e.g. tandem standing, turning 360°, single-leg stance). Each item is scored on a scale of 0–4. A score of 0 is given if the participant is unable to perform the task, and a score of 4 is given if the participant is able to complete the task based on the criterion that has been assigned to it. The maximum total score on the test is 56. A score of less than 45 indicates that individuals may be at a greater risk of falling (Perell et al., 2001; Scott et al., 2007). The test was subjectively scored by a well-trained physiotherapy student.

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Equipment

Induced-sway postural responses were generated by means of computer-controlled mechanical apparatus as shown in Fig. 1. The apparatus is described in detail in Cikajlo and Matjačić (2007); here, we provide only a brief description. The apparatus is a standing frame made of aluminum and fixed to the base through compliant springs enabling movement in two degrees of freedom: AP and ML sway. On the top of the standing frame, a wooden table with a safety belt enables firm and comfortable contact of the standing individual at the level of pelvis. In this way, the individual’s pelvis follows the inclinations of the standing frame in the transverse plane. Four battery powered electric motors (Iskra Avtoelektrika d.d., Šempeter, Slovenia), connected by ropes to the frame, were used to generate postural perturbations in eight directions [(forward (FW), forward-right (FR), right (RT), backward-right (BR), backward (BW), backward-left (BL), left (LT) and forward-left (FL)]. Each electrical motor delivered constant torque during selected duration of perturbation. The realization of perturbation and moving the standing frame in combined principal direction was managed by simultaneous action of two electro-motors each for corresponding principal direction (FW, BW, RT, LT) on the command from personal computer. The command was realized as pulses generated by a multipurpose PCI board (NI 6259; National Instruments, Austin, Texas, USA) and dedicated computer software. To perturb the frame in a certain direction the appropriate electric motor winded up the rope and pulled the frame away from its upright position. This led to a corresponding perturbation being applied to the frame and consequently to the individual standing in the frame. Participating individuals stood with each foot on separate force plates (AMTI OR6-5; AMTI Inc., Watertown, Massachusetts, USA) assessing 6-DOF data (three forces, three moments, filtered within AMTI amplifier). Data assessment (sampled F’s=100 Hz) were managed by custom-developed Matlab (The MathWorks Inc., Natick, Massachusetts, USA) software based graphical user interface controlling the multipurpose PCI board.

Fig. 1
Fig. 1
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Protocol

Participants were instructed to maintain the upright posture with eyes open and gaze directed forward while standing barefooted in the perturbation apparatus. Their standing area formed by feet was in the AP direction, determined by the length of the feet, and in the ML direction, determined by the distance between the outer edges of the feet, which was approximately the width of pelvis. The participants were instructed to recover the upright position after each perturbation. The participants were allowed to hold the table, which moved together with the frame. The height of the table was adjusted for each individual. The perturbation direction and the perturbation commencement were defined randomly, but within 1 s after the operator pressed the button. The imposed perturbation provoked a pull on the participating individual that lasted 1 s. The perturbation intensity was set to 3 Nm constant torque of a motor. The total assessment time was set to 5 s. Each participant underwent 32 trials, four trials for each perturbation direction. For each perturbation trial, a set of 6-DOF data (forces and moments in the AP and ML directions and vertical axis) for each foot were recorded using two force platforms.

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Data analysis

Data assessed from both force platforms were filtered (4th order Butterworth filter, 15 Hz cut-off frequency) and used to calculate common center-of-pressure (COP) (Cikajlo and Matjačić, 2007). For each perturbation direction mean responses were calculated from four trials. An example of COP trajectories for perturbation direction FL is shown in Fig. 2. As we were interested only in the automated part of the postural response, the amplitude of the first peak and the time needed to reach the first peak were determined in COP trajectories for both AP and ML directions (Fig. 2). The mean values and SDs of BBS were calculated for the group of participants. Also the mean values and SDs of peak amplitude response in AP and ML directions (PARAP and PARML) and time to peak amplitude response (TPAR-AP and TPAR-ML) were calculated for each perturbation direction separately for the group of tested individuals.

Fig. 2
Fig. 2
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Statistical analysis

One-way analysis of variance was used to compare the PARAP and TPAR-AP in all three ‘forward’ directions (FW, FL, and FR) and in all three ‘backward’ directions (BW, BL, and BR) as well as PARML and TPAR-ML in all three ‘left’ directions (LT, FL, and BL) and in all three ‘right’ directions (RT, FR, and BR). The level of statistical significance was set to P less than 0.05.

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Results

The BBS score for the group was 33.9±5.8 (range 28–42), confirming that the participants were at a greater risk for falling.

Figure 3 shows a typical set of postural responses as assessed in one individual. For the principal perturbation directions (FW, BW, LT, RT) only postural response relevant for a given direction (COPAP for FW and BW and COPML for LT and RT) is displayed, whereas for the diagonal-combined perturbations (FL, FR, BL, BR) both postural responses (COPAP and COPML) are shown. One can notice considerable similarity between the three ‘forward’ (FW, FL, and FR) and three ‘backward’ (BW, BL, and BR) postural responses. Also considerable similarity may be observed by comparing three ‘left’ (LT, FL, and BL) and three ‘right’ (RT, FR, and BR) postural responses.

Fig. 3
Fig. 3
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Figure 4 shows mean values and SDs of PARAP and PARML for all eight perturbation directions for the group of tested individuals. One can observe considerable similarity between the three ‘forward’ (FW, FL, and FR) and three ‘backward’ (BW, BL, and BR) PARAP postural responses. Also considerable similarity may be observed by comparing three ‘left’ (LT, FL, BL) and three ‘right’ (RT, FR, BR) PARML postural responses. One-way analysis of variance did not detect statistically significant differences among the compared responses.

Fig. 4
Fig. 4
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Table 1 presents TPAR-AP and TPAR-ML for all perturbation directions for the group of tested individuals. Also here no statistically significant differences were detected among the compared responses.

Table 1
Table 1
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Discussion

In the present study, we examined postural responses as reflected in COP trajectories following perturbations delivered to the waist in multiple directions in a fall-safe environment to the group of elderly fallers. The results of our study show that the organization of postural responses resulting from perturbations in multiple directions to large extent follow a principle of decoupled control, which is similar as seen in population without postural control deficits (Matjačić et al., 2001; Jones et al., 2008). To our best knowledge, this is the first study that examined the directionality of postural responses following application of multiple-direction perturbations in a group of elderly fallers.

The main observation of the present study relates to the organization of postural responses following perturbations in multiple directions in the elderly. In the general population, decoupled control strategy for postural control following perturbed stance in the AP and ML planes has been demonstrated (Matjačić et al., 2001). That means that regardless whether a perturbation occurs, for example, only in the AP direction or it is accompanied by a perturbation also in the ML direction the magnitude of responses in AP does not change, which is from a biomechanical point of view understandable. Interestingly, our results also show that in the tested group of elderly fallers this organization of postural responses remains similar suggesting that despite deteriorated sensory and motor capacities the estimation of the direction of perturbation may not be impaired because of the age of the tested individuals.

However, the interpretation of the limited set of results data needs to be taken with caution due to the methodological limitations of this study. The nature of perturbation generation is such that it inevitably ‘breaks’ a tested subject into double inverted pendulum, thereby causing a postural response to be a combination of ‘ankle’ and ‘hip’ postural strategies (Horak and Nashner, 1986; Winter, 1995). The level of perturbing forces was selected such that it resulted in predominantly ‘ankle’ postural responses, which is largely determined by COP trajectories. However, there was also a ‘hip’ part of the postural responses influencing shear forces, which could not be evaluated because of the fact that the individuals were allowed to hold on the table. This was necessary to be able to assure fall-free experimental conditions, which was a prerequisite to be able to carry out this study in the tested elderly individuals of considerable age and with histories of falling. Described methodological shortcomings (which due to safety of participants could not be avoided) certainly added to variability of the presented results that relate to COP trajectories. However, the results related to timing of postural responses are not affected due to used perturbation methodology. The timing of postural responses displayed rather low variability and significant similarity when comparing appropriate perturbation directions in the tested group suggesting that the ‘accuracy’ of identifying a direction of perturbation may not be impaired also in the elderly individuals with obvious balancing deficiencies.

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Conclusion

Although several well-documented age-related changes in balance and postural control (delayed responses and diminished strength being most notable) have a definitive impact on the balancing and falls in elderly, it seems that inappropriate assessment of perturbation direction does not add to the list of potential causes of falling.

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Acknowledgements

Financial support by the Agency for Research, Republic of Slovenia is gratefully acknowledged (Grant No. P2-0228).

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Conflicts of interest

There are no conflicts of interest.

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Keywords:

balance; elderly; perturbed stance

© 2013 Lippincott Williams & Wilkins, Inc.

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