Maintaining balance is a complex function involving visual, vestibular, and somatosensory input, as well as appropriate muscle activity to maintain the body's centerof-gravity over it's base of support during static and dynamic tasks (Emery, 2003). A variety of clinical (e.g. Star Excursion Balance Test) and laboratory devices (e.g. force platforms) have been developed to evaluate static and dynamic balance (Clifford and Holder-Powell, 2010). One of the most frequently used devices for dynamic balance is the Biodex Stability System (BSS) which provides quick and quantitative measures of postural control, center of pressure location, and (indirectly) the overall function of the sensorimotor system. The stability of the BBS platform can be varied by adjusting the level of spring resistance (i.e. support) from 1 (least stable) to 12 (most stable) (Hinman, 2000; Schmitz and Arnold, 1998). Numerous papers report high test-retest reliability for the BSS when using high resistance levels (Cachupe, et al., 2001; Parraca et al., 2011, Sherafat et al. 2013). However, reliability data for lower stability levels, which are more challenging and dynamic in nature, is almost non-existent within the literature (Cachupe et al, 2001). More challenging tasks, such as balancing on a highly unstable surface, have also been associated with more pronounced learning curves (Nordahl et al., 2000; Valovich et al, 2003). Thus, it is likely that BSS scores obtained from lower stability levels are inherently less stable and require greater familiarization (i.e. practice) to achieve a stable score both within and between test sessions. Unfortunately, no empirical data is available to determine an appropriate number of familiarization trials or the reliability of the low BSS resistance levels despite the continued use of lower stability levels in the evaluation of postural control during and after therapeutic interventions. Therefore, the primary purpose of this investigation was to determine if a commonly reported 6 trial sequence (3 practices, 3 test trials) used with the BSS is adequate to achieve a stable level 1 score within a single test session. The secondary purpose was to evaluate the test-retest reliability of the BSS at resistance level 1 over a 10-week period (ie. Does a 6 trial sequence achieve a stable score between test sessions?). Based on the existing literature we hypothesized that the standard 6 trial sequences would be adequate to achieve a stable BSS score within a single test session on resistance level 1 and that such a sequence would produce at least fair reliability scores.
Twenty sedentary university students (11 male; age 22 [+ or -] 2.24 years height: 1.78 [+ or -] 0.08 meters, weight: 73.91 [+ or -] 9.49 kilograms, Body Mass Index: 23.04 [+ or -] 2.06 kg-m-2; 9 female; age: 20.88 [+ or -] 1.16 years, height: 1.63 [+ or -] .5 meters, weight: 57.07 [+ or -] 7.76 kilograms, Body Mass Index: 21.34 [+ or -] 2.3 kg-m-2; voluntarily participated. Participants were required to have a sedentary life style (i.e.
A BSS SD (Biodex[R], Inc., Shirley, NY, USA) was used for assessments of dynamic balance. The BSS is comprised of a movable balance platform that provides up to 20[degrees] of surface tilt in a 360[degrees] arc of motion. The version of the BSS used in the study had 12 dynamic stability levels with level 12 being the most rigid (easiest) and level 1 the most unstable (difficult). Main outcome measures included the overall stability index (OSI), the anterior/posterior stability index (APSI), and the medial/lateral stability index (MLSI). The OSI represents the total variance of platform displacement (all directions), measured in degrees, with higher scores indicating worse postural control while the APSI and MLSI represent platform displacement in the sagittal and frontal planes respectively (Arnold and Schmitz, 1998, Riemer and Wikstrom, 2010, Sulewski et al., 2012). The following formulas (OSI=[[([sigma](0-Y).sup.2] + [sigma][[(0-X).sup.2] / [number of samples).sup.[conjunction]05], APSI = [([([sigma]0-Y).sup.2]/[ number of samples)].sup.[conjunction]0,5], MLSI = [([[sigma](0-X).sup.2]/ [number of samples)].sup.[conjunction]0,5]), where Y and X represent the degree of platform tilt in the sagittal and frontal plane respectively, were used to calculated the outcomes of interest.
For all trials, participants were tested barefoot with their eyes open and were allowed to visualize the real time feedback provided by the BSS computer interface. First, 6, 20-second trials of dual limb stance were completed. This stance required participants to stand with slight knee flexion (~15[degrees]), while looking straight ahead with their arms across their chest. Next, 6, 20-seconds trials of single leg stance were completed. These trials required an identical test position but were completed while standing on the dominant limb only. Limb dominance was defined as the limb a participant would use to kick a soccer ball. A 60-second rest was given between all trials. Participants returned to the laboratory 10 weeks later to repeat the above described testing protocol. This timeframe was chosen to establish reliability of level 1 BSS scores over an extended period of time because longer periods between assessments have been shown to neutralize initial motor learning in young adults (Wrisley et al., 2007).
To determine if a 6 trial sequence was adequate to achieve a stable within session score, the OSI, APSI, and MLSI for each stance (dual limb, single limb) were submitted to a 1-way repeated measures ANOVA [6 levels] (Robinson and Gribble, 2008). This analysis was also performed on data from the second test session. Because we were more concerned with making a Type II error, no adjustment was made on the a priori alpha level which was set at 0.05 despite multiple ANOVAs being run. If a statistically significant difference was noted...