### Abstract

This study compared the effects of visual feedback and stability level on standing balance performance using the Biodex Balance System. The analysis was performed on a 2 x 2 factorial design for the purpose of testing the main effects of the type of feedback (augmented visual feedback or none) and balance condition (less stable – Biodex level 2 or more stable – Biodex level 7). Four randomly assigned groups performed nine 20-second dynamic balance trials at stability level 2 or at level 7, depending on group assignment. The dependent variable was the mean stability index calculated as an average of the nine 20-seconds trials. A significant feedback by stability level interaction was found (P = .04). At stability level 7, augmented visual feedback mean stability index scores were better when compared to no augmented visual feedback (P < .001). No significant differences were found at stabilty level 2. Our data indicate that when balancing on a Biodex Balance System, as the degree of difficulty increases the effect of concurrent augmented visual-feedback is reduced.

**Key words:** balance, visual feedback, posture, augmented

### Introduction

Dynamic balance is critical for the acquisition and execution of motor skills. Balance training is used for injury rehabilitation, fall reduction, and sport and motor skill development. One commercial device used to quantify the degree of dynamic balance is the Biodex Balance System (4). The Biodex Balance System is an instrumented device that allows the tilting of a circular flat platform. The degrees of tilt from horizontal are measured and used to calculate an overall stability index (1). This index is a quantitative estimate used for the evaluation of an individual’s neuromuscular control as it pertains to the ability to maintain postural stability on an unstable surface (4).

One feature unique to the Biodex Balance System is that the stability of the balance platform can be increased or decreased, thus enabling control of the level of difficulty of the standing balance task. Biodex platform stability levels range from 1 to 8, with 8 being the most stable or least difficult to perform. Another feature of the Biodex Balance System is an attached LCD monitor that provides augmented visual feedback. The monitor provides information, via a screen tracing, concerning the subject’s ability to balance on the platform as the subject tries to maintain the cursor in the middle of the screen’s grid (4).

#### Theoretical Rationale

We were unable to find studies that compared the efficacy of augmented visual feedback at different levels of balance difficulty. As the stabilometer platform becomes less stable and thus more difficult, the ability to effectively process both intrinsic and augmented visual feedback may become increasingly difficult. This would be caused by a decrease in the amount of time available to process feedback information while balancing (11). The increase in time constraints as balance difficulty increases may also bring about a change in the type of motor control strategy used, i.e., open versus closed loop. During open looped motor control, the movement is executed entirely by the motor program without the use of sensory feedback (5,6). During closed looped motor control, an initial command is sent to the muscles which start the movement. The actual execution of closed loop movements, then, depends on sensory feedback which is used to monitor the movement (6). As the balance task becomes increasingly difficult, information processing demands may be increased because of the greater number and rate of balance adjustments that must be monitored. The less stable platform also brings about the rapid initiation of postural responses which limits the effectiveness of feedback mechanisms because of the inherent time delays (11). Horack and Nashner (1986) suggests that rapid postural actions are organized by a limited repertoire of open looped motor programs which do not require feedback for execution. Open looped strategies simplify the process of complex movement by incorporating knowledge of past experiences into motor programs enabling anticipation of events and reducing reliance on the slower feedback mediated responses associated with closed looped monitoring (6).
The purpose of this study was to determine the effects of concurrent augmented visual feedback and balance condition on standing balance performance using the Biodex Balance System. We postulate that concurrent augmented visual feedback will not be as effective at the less stable condition (Biodex stability level 2) when compared to the more stable condition (Biodex stability level 7). This hypothesized difference in the effects of visual feedback at the different levels of stability will be demonstrated in the form of a feedback by stability level statistical interaction.

### Methods

#### Participants

Forty healthy, male university students (age = 21.4 ± 3.6 years, mass = 70.3 ± 14.3 kg, height = 170 ± 3.1 cm) volunteered to participate in this study. No participants reported any sensory impairment or physical injury that hindered performance of the balance task, nor did any of the participants have previous experience with balance training on the Biodex Balance System. The study was approved by the university’s institutional review board, and informed consent was obtained from each individual before testing.

#### Instrumentation

The Biodex Stability System 945-300 (Biodex Medical Systems; Shirely, New York) was used to quantify bilateral standing balance (4). The system consists of a multi-axial tilting platform interfaced with a computer which records and calculates stability indices of standing balance. The platform stability can be varied by adjusting the resistance applied to the platform via one of 8 stability settings controlled by the system’s microprocessor-based actuator (14). Setting 1 represents the least stable platform and setting 8 the greatest platform stability. An 11.5 x 8.5 cm LCD display screen, located at eye level, provides visual feedback via a circular grid that visually shows a cursor tracing of the subject’s stability performance. The goal of dynamic balance testing on the Biodex Balance System during the augmented visual feedback condition is for the subject to maintain the cursor on the center of the circular grid for as long as possible during the test trial (8). During the no augmented visual feedback condition the goal was to keep the balance platform in a horizontal position while focusing straight ahead on a covered LCD screen. The Biodex Balance System has been shown to have high reliability.(8,14).

#### Procedures

Four randomly assigned groups with ten subjects in each group performed nine 20-second dynamic balance trials. The platform balance task required the subject to stand barefooted in a comfortable upright position with feet shoulder width apart with arms at sides. Groups 1 and 3 received augmented visual feedback during the balance task, while groups 2 and 4 received no augmented visual feedback. During the augmented visual feedback trials, the subject was instructed to keep the cursor directly in the middle of the screen while balancing on the platform. Group 1 performed the balance task at platform stability level 2 with augmented visual feedback. Group 2 performed at stability level 2 with no augmented visual feedback, which involved the subject performing the balance task while focusing straight ahead on a covered screen. Group 3 performed the balance task at stability level 7 with identical augmented visual feedback as used with group 1. Group 4 performed at stability level 7 with no augmented visual feedback which involved performing the balance task while focusing on a covered screen.

A familiarization session was conducted in which the participants were introduced to the testing protocol. Four 20-second practice trials were performed either at stability level 2 or at level 7, depending on group assignment. A 20-second rest period was allowed between trials. Participants assigned to the augmented vision condition practiced the balance task while being allowed to watch the balance tracing on the screen. Participants assigned to the no visual feedback condition practiced the balance task while viewing a covered screen.

Prior to the data acquisition trials, all subjects achieved a stable upright stance by positioning their feet shoulder width apart on the center of platform while looking straight ahead. The screen was either left uncovered or covered which was dependent on the assigned treatment group. The platform was then unlocked, requiring balance at the given stability level. Nine 20-second dynamic balance trials were performed. The same examiner (S.F.P.) administered the balance task in a non-distracting environment. If a participant lost control of balance that required grabbing the handrail, the trial was repeated. Three participants repeated one trial each. Two participants lost balance control more than once and were not included in the data analysis.

Platform stability levels 2 and 7 were chosen based on testing recommendations found in the literature and from pilot data (12). In addition, enough disparity between groups in terms of balance difficulty was necessary in order to ensure that statistical differences between feedback groups, if it in fact existed, could be found. A previous study reported that approximately nine 20-second trials could be safely performed in one practice session before participants reported fatigue (12). No participant was told their stability index scores or given any other information concerning their performance other than that given in the visual feedback conditions.

#### Statistical Analysis

A 2 x 2 factorial design was used to examine the effects of feedback and balance condition on dynamic balance performance using the Biodex Balance System. The first independent variable was type of feedback with two levels (augmented visual and no augmented visual). The second independent variable was balance condition with two levels (stability level 2 or stability level 7). The dependent variable was the mean stability index calculated as an average of the nine 20-seconds trials. The stability index is determined from the amount of platform tilt in degrees from a zero-centered balance-point (level). The index was calculated as the standard deviation of the platform displacement from horizontal obtained from each 20-second trial (4). A low stability index score indicates good dynamic stability or balance, whereas a high stability index scores indicates poor balance control.

A two-way univariate analysis of variance was conducted to examine the effects of the type of feedback and balance condition for the stability index score data. The α level was set a priori at .05. We used SPSS (version 18; SPSS Inc, Chicago, IL) to analyze the data.

### Results

Means and standard deviations for stability index scores are presented in Table 1. A significant main effect was found for balance condition (F1,36 = 105.134, P = .001), which means participants assigned to the easier balance condition had better balance scores than those assigned to the more difficult balance task. No significant differences were found for the type of feedback groups (F1,36 = 2.145, P = .152). More importantly, a significant feedback by balance condition statistical interaction was found (F1,36 = 4.107, P = .04). At stability level 7, augmented visual feedback stability index scores were better when compared to no augmented visual feedback stability index scores (P < .001). However, for stability level 2, no difference was found between the feedback and no feedback conditions (P = .778).

### Discussion

We propounded the question of whether or not concurrent augmented visual feedback influences balance on the Biodex Balance System at different stability levels. The results supported our postulation that concurrent augmented visual feedback did not influence balance at the more unstable level (Biodex stability level 2). The importance of vision on postural control has long been known (2), however, the effect of concurrent augmented visual feedback on postural control while balancing on an unstable surface is equivocal. Most of the reported clinical studies that examined the effects of augmented visual feedback on postural control have involved stroke patients (7,11). Barclay-Goddard et al (2009) conducted a meta-analysis of the efficacy of concurrent augmented feedback using force platform standing balance in stroke patients. Their results showed no clear evidence that the use of force platform visual feedback improved standing balance. O’Connor et al (2008) compared the effects of different visual cues on postural sway in healthy older and younger adults. The older adults were able to habituate to repeated visual perturbations, however, it took more exposures compared to the younger adults. This finding suggests that aging impacts the ability to quickly modify augmented visual feedback for postural control. Hlavackova et al (2009) studied the effects of concurrent mirror feedback on upright stance control in elderly transfemoral amputees. Their results showed mirror feedback improved upright stance control.

Normal postural sway and equilibrium produced while standing on a flat stable surface may be controlled by lower level closed-looped feedback corrections. Standing balance on a stable surface primarily involves activating automatic postural reactions that are based on reflex actions rather than conscious control (12). The Biodex Balance System is unique in that it uses a moveable platform to create different levels of stability. Our rationale was that at the more difficult stability level 2 the influence of augmented visual feedback would be reduced as a result of change in motor control strategies. As platform stability decreased, open-looped strategies may have been used in an effort to maintain the platform in a horizontal position. Gutierrez et al (2009) in their clinical review state that during dynamic balance, open-looped mechanisms operate faster than closed looped mechanisms when perturbations to balance are imposed. This contention is supported by the study of Horak and Nasher (1986) who investigated the extent to which standing automatic postural reactions are controlled by motor programs. They adduce the theory that postural actions are organized by a limited repertoire of central programs selected in advance of movement. Organization of movements into motor programs simplifies the process of modifying movement by reducing reliance on concurrent sensory feedback. Our data suggest that the motor control strategies used when balancing on the Biodex Balance System may not be universal at all levels of difficulty.

### Conclusion

The learning/relearning of balance is a primary goal in many types of sport and wellness rehabilitation. Because of the importance of balance, there is a constant need for the identification of efficient and successful methods of balance testing and training as well as the delineation of variables that influence balance. We conclude that when balancing on the Biodex Stabilometer, the way feedback is administered is important because it significantly affects balance performance. Our study implies that, when balancing on a Biodex Balance System, as the degree of difficulty increases the influence of concurrent augmented visual-feedback is mitigated.

### Application in Sport

During the early stages of balance training, where the stabilometer tasks are performed at the more stable (less difficult) levels, augmented visual feedback may improve the performance of the balance task. However, as task difficulty increases the ability to use augmented visual feedback to guide postural reactions may decrease. These results infer that during Biodex stability training both open and closed looped motor control strategies are being used depending on the stability level being practiced. Under these conditions, previous research (12) has shown that variable practice, where several difficulty levels are practiced in a random order during any given training session, is a more efficient means of balance training when compared to constant practice where only one stability level is practiced during a training session. Variable practice has been shown to be more efficient in the development of open loop motor programs where rapid movements are required (6). Therefore, when doing Biodex balance training for sport a protocol that involves practicing several different levels of difficulty during one training session would be recommended. Future studies need to examine additional variables such as disability, injury and age in order to determine the most appropriate rehabilitation protocols.

### Tables

#### Table 1
Mean (± SD) Stability Index Scores Averaged Across Nine 20-Second Trials

Type of Feedback Level 7 Level 2
Augmented Visual 1.62 ± .41 11.58 ± 4.54
No Augmented Visual 4.45 ± .83a 11.12 ± 2.20

a. Difference between type of feedback at level 7 (P < .001).

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### Corresponding Author

Dr. Steven Pugh, PhD.
University of South Alabama
HPE Building, RM 1016
171 Jaguar Drive
Mobile, Alabama 36688

(251) 461-8231

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