Design and Participants
This was an ancillary study to a cluster randomized clinical trial (RCT) that investigated the effect of two different group exercise programs conducted at their residence facility on walking ability, disability and self-reported function . This study took place from April 2014 to May 2016. A subsample of 131 participants from the RCT were invited during their baseline assessment to take part in measurement validation. For test–retest reliability, a subsample of 38 participants returned one week later to take part in a retest session, also at their residence facility. This study has been approved by the Institutional Review Board of the University of Pittsburgh. Inclusion and exclusion criteria followed that of the parent study [13,14].
The accelerometer was developed as a part of the National Institutes of Health (NIH) Toolbox project as a balance measurement . The dual axis accelerometer (ADXL213AE, with range of ±1.2 g and resolution of 1mg; Analog Devices, Inc., Norwood, MA) is oriented to record acceleration of the body in both AP and ML axis. The acceleration is transmitted through Bluetooth transmitter to a laptop computer at 50 Hz. A custom written Labview program was used to acquire the data. The accelerometer was attached to the participant’s back at the level of the iliac crest using Velcro and gait belt.
For assessment of test-retest reliability, participants repeated the following procedures on two separate occasions with one week apart. One week between testing sessions was chosen based on previous reliability studies,[16–19] and to avoid the expected effect of an improvement in balance over the course of the intervention. Balance measurements included the following six standing conditions in order: (1) feet together on a firm surface with eyes open; (2) feet together on a firm surface with eyes closed; (3) feet together on a foam surface with eyes open; (4) feet together on a foam surface with eyes closed; (5) semi-tandem stance (one foot halfway in front of the other) on a firm surface with eyes open; and (6) tandem stance on a firm surface with eyes open. All conditions were performed with the participant’s own pair of comfortable shoes. The foam surface that was used in the balance protocol is an AIREX® Balance Pad (Airex AG, Switzerland). For the semi-tandem and tandem stance conditions, the participants placed their feet according to their preference. Each condition was performed for a maximum of 30 s, and a rest of 30 s was provided between each trial.
Outcome Measures for the Convergent Validity
In order to examine convergent validity, the balance measurements at baseline were compared with mobility measures that were collected by in the parent study. These measures included the Six-Minute Walk Test (6MWT) , gait speed , Figure-of-8 Walk Test (F8WT) , Short Physical Performance Battery (SPPB) , and Gait Efficacy Scale (GES) .
Six-Minute Walk Test (6MWT)
The Six-Minute Walk Test (6MWT) is a well-validated measure of walking capacity. The test was included to measure walking endurance by calculating the maximum distance walked in six minutes, that includes rest time if needed . Better performance is indicated by a greater distance covered during six minutes.
Participants were asked to walk at their usual speed on an instrumented walkway . Participants performed six passes and the average of the six passes was used in the analysis. Two practice trials were done before the real testing.
Figure-of-8 Walk Test (F8WT)
The Figure-of-8 Walk Test (F8WT) measures motor skill in walking . Participants walked a figure-of-8 pattern that was made by two cones with 1.5 meters apart. Number of steps and time to finish the test were measured.
Gait Efficacy Scale (GES)
The Gait Efficacy Scale (GES) is a 10 item scale used to address elderly’s perception of confidence during a challenging walking tasks such as walking over different surfaces, curbs, or stairs .
Short Physical Performance Battery (SPPB)
The SPPB was originally developed as a measure of physical performance for a longitudinal study of aging conducted by the National Institutes on Aging . The SPPB measures three aspects of functional mobility: the time to perform five consecutive transfers from sitting to standing (chair stands), time to ambulate on level surfaces for 4 meters, and the ability to stand with decreasing medial-lateral base of support. Scores from 0 to 4 are assigned to each of the tasks based on quartile scores of the timed chair stands and ambulation, and degree of difficulty of the standing balance test. A summary performance score is equal to the sum of the three sub-scores.
The first and last five seconds of the recording were excluded from the data analysis in order to eliminate transient effects . Using a custom written Matlab code, the acceleration data were lowpass filtered using a 4th order Butterworth filter with a cutoff frequency of 2 Hz. The Root Mean Square (RMS) and the Normalized Path Length (NPL) were calculated for both the antero-posterior (AP) and medio-lateral (ML) axis; a higher value indicates more sway. The RMS and NPL were computed as follows:
See formulas 1 and 2 in the supplementary files.
where t is the time duration, N is the number of time samples, and pj is the acceleration data at time sample j. mG stands for milli-Gravitational acceleration, where 1 mG = 0.0098 m/s2.
Data were analyzed using SAS software version 9.4 (SAS Institute, Inc., Cary, NC). Descriptive statistics of participant demographic characteristics were reported. The level of statistical significance was set at α ≤ 0.05 for all analyses.
Test–retest reliability one week apart was estimated using intraclass correlation coefficients (ICC, model 3.1, two-way mixed-effects model) and 95% confidence intervals (95% CI). Absolute reliability of the balance accelerometry measurements was examined using the standard error of the measurement (SEM). The SEM is an estimate of the within-subject variability after repeated measures. The SEM was calculated using the sample standard deviation (SD) and the ICC as follows: SEM = SD √ (1 – ICC). In addition, the minimal detectable change (MDC) at the 95% level of confidence will be calculated for the outcome measures using the SEM values, as follows: MDC95 =SEM×1.96×√2. Bland–Altman plots were used to assess the agreement between test-retest measurements.
Face validity was examined by examining how body sway changed as the balance conditions became more difficult. These balance conditions were chosen to alter sensory feedback and reduce the base of support. A Friedman test was used to examine if there was a significant difference between the balance tests. Post hoc pairwise comparisons were performed with Wilcoxon signed ranks tests. The convergent validity was examined by calculating the correlation of balance measurements with the mobility measurements at the initial baseline assessment, using Spearman’s rank correlation coefficients.