ISway: a sensitive, valid and reliable measure of postural control

Abstract

BackgroundClinicians need a practical, objective test of postural control that is sensitive to mild neurological disease, shows experimental and clinical validity, and has good test-retest reliability. We developed an instrumented test of postural sway (ISway) using a body-worn accelerometer to offer an objective and practical measure of postural control.MethodsWe conducted two separate studies with two groups of subjects. Study I: sensitivity and experimental concurrent validity. Thirteen subjects with early, untreated Parkinson’s disease (PD) and 12 age-matched control subjects (CTR) were tested in the laboratory, to compare sway from force-plate COP and inertial sensors. Study II: test-retest reliability and clinical concurrent validity. A different set of 17 early-to-moderate, treated PD (tested ON medication), and 17 age-matched CTR subjects were tested in the clinic to compare clinical balance tests with sway from inertial sensors. For reliability, the sensor was removed, subjects rested for 30 min, and the protocol was repeated. Thirteen sway measures (7 time-domain, 5 frequency-domain measures, and JERK) were computed from the 2D time series acceleration (ACC) data to determine the best metrics for a clinical balance test.ResultsBoth center of pressure (COP) and ACC measures differentiated sway between CTR and untreated PD. JERK and time-domain measures showed the best test-retest reliability (JERK ICC was 0.86 in PD and 0.87 in CTR; time-domain measures ICC ranged from 0.55 to 0.84 in PD and from 0.60 to 0.89 in CTR). JERK, all but one time-domain measure, and one frequency measure were significantly correlated with the clinical postural stability score (r ranged from 0.50 to 0.63, 0.01 < p < 0.05).ConclusionsBased on these results, we recommend a subset of the most sensitive, reliable, and valid ISway measures to characterize posture control in PD: 1) JERK, 2) RMS amplitude and mean velocity from the time-domain measures, and 3) centroidal frequency as the best frequency measure, as valid and reliable measures of balance control from ISway.

The within-day and between-day reliability of using sacral accelerations to quantify balance performance

“…Whitney et al (2011) demonstrated that the path length (PL), a measure of the length of the mediolateral acceleration data plotted against the anteroposterior acceleration data, was the most reliable measure across a range of balance tasks (ICC range 0.63-0.80). Mancini et al, (2012) also reported that PL was the most reliable balance performance metric however also suggested Jerk, the time derivative of acceleration, was reliable, a finding supported by Marchetti et al, (2013). In addition to PL and jerk previous authors have also reported the root mean square value (RMS) as a method of quantifying postural sway.…”

“…Accelerometers, on the other hand, measure acceleration of the COM and therefore describe the body's attempt to control movement of the COM (Adlerton & Moe-Nilssen 2003). As the two devices measure different metrics they are not necessarily interchangeable, however the correlation of the attempts to maintain balance are good (Mayagoitia et al, 2002;Whitney et al, 2011;Mancini et al, 2012). Therefore it may be possible to obtain sway signatures using the accelerometer method which has distinct advantages over the force plate method for the clinician being smaller, cheaper and not constrained to a specific environment.…”

“…This commonly requires the use of expensive laboratory mounted force plates (Mancini et al, 2012). Clinicians often measure balance using crude measures such as time spent on one leg or star excursion balance test (O'Sullivan et al, 2009;Coughlan et al, 2013).…”

“…However, there is still a requirement to explore the reliability of these devices. Test-retest reliability for various stance tasks, including M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT double leg stance (ICC 95% confidence interval (CI) = 0.35-0.89; Mancini et al, 2012), single leg stance (ICC 95%CI = 0.62-1; Moe-Nilssen 1998b) and tandem stance tasks (ICC 95% CI = 0.75-0.89) have been reported. These values represent the combination of biological (human) and equipment (sensor) variability and the spread of the confidence intervals demonstrate the inherently variability in human movement.…”

Robust evaluation of time since awakening using force platform posturography

“…When a human subject is asked to stand as still as possible, he actually exhibits small, involuntary, movements called postural sway (Winter, 1995;Duarte and Zatsiorsky, 2000). This sway can be quantifi ed from the trajectories of the Center of Pressure (COP), which is typically recorded by force platforms (Winter, 1995), or alternatively quantifi ed from body acceleration (Mancini et al, 2012). Other possible methods for detecting fatigue related to sleepiness include using electroencephalography and electrooculography (Hirvonen et al, 2010;Liu et al, 2013;Hallvig et al, 2014).…”