Validity and reliability of the Swaymeter device for measuring postural sway (2024)

Abstract

Background

The control of standing balance is a task of maintaining the body's centre of mass (COM) within the limits of the base of support, achieved by producing forces on the support surface/s (predominantly under the feet while standing). Excursions of the centre of pressure (COP), the point of application of the ground reaction force, measured by a forceplate has been widely used to represent postural sway, as an index of balance control. However, these measures involve technical devices that can be costly and require processing protocols that can make them unfeasible for many clinics and research facilities.

The need for a simple measure of postural sway exists due to the issue of balance problems and the risk of falls in older people. Increasing age is associated with increases in the magnitude and velocity of postural sway during standing [1,2]. Indeed, postural sway has been shown to be a risk factor for falls in numerous older populations (see [3] for review). One study has shown multiple fallers to have 33-46% greater sway than those who did not experience a fall in the previous 12 months [4]. In a prospective study of 100 adults aged 62-96, the root mean square of the mediolateral COP displacement while standing blindfolded was found to predict those people who fell in a follow-up period of 12 months with a predictive accuracy of 67% [5]. These studies suggest that measuring postural sway, particularly while sensory information is reduced, can provide an indication of an individual's risk of a future fall.

A low tech Swaymeter was designed to address the needs of clinicians and researchers with limited resources (e.g. no access to forceplates or motion laboratories). It is a useful field test, as it is compact, lightweight, has short administration and data processing time. Unlike other lightweight and easily applied systems, such as accelerometers and gyroscopes, the Swaymeter involves no electronics or computer processing. Thus, assessment can be conducted in a variety of community settings and health care facilities. Several research groups have found the Swaymeter to be feasible for use in different populations of young and older people [6-8].

The Swaymeter has been used in numerous studies of balance [6-8] as well as retrospective [9,10] and prospective [11,12] investigations of falls risk in older people. Sway path length or sway displacement measures have been found to discriminate between fallers and non-fallers in each of these studies. For example, a prospective study of 341 community-living older women, postural sway assessed with participants standing on a foam mat with eyes open was a significant and independent risk factor for multiple falls [12]. A companion study [20] examined 136 younger women (aged 20-64 years) and reported significant correlations between Swaymeter-recorded postural sway and age, but did not reveal whether postural sway data discriminated between the young and older subgroups. Furthermore, the immediate (intrasession) reliability of the Swaymeter device has not been reported, nor has it been validated against accepted measures of postural sway.

The purpose of this study was to examine: 1) the concurrent validity of the Swaymeter in discriminating between young and older adults; 2) the convergent validity of the Swaymeter against a forceplate system; and 3) the immediate test-retest repeatability of postural sway measures obtained from the Swaymeter. Older adults were hypothesised to have increased postural sway obtained from the Swaymeter, compared to young adults. It was hypothesised that the Swaymeter would show good-to-excellent agreement with forceplate (COP) data, in addition to acceptable reliability across 3 repeated trials.

Methods

Results

Data are missing for 2 older adults in the foam eyes closed condition, as they were concerned about their stability and refused to complete the trials. Otherwise, all subjects successfully completed all trials without incident. Table ​Table11 presents participant characteristics and indicates that the study sample comprised relatively healthy groups of young and older people without cognitive impairment.

Descriptive statistics for sway displacement and path length from Swaymeter recordings in the young and older participants are presented in Table ​Table2.2. Significant group differences existed in Swaymeter variables, with older adults having greater AP displacement, (F_1,473 = 23.84, p < 0.0001), ML displacement (F_1,473 = 50.94, p < 0.0001) and path length (F_1,473 = 54.29, p < 0.0001) than young. Furthermore, there was a significant main effect for condition (F_3,473 = 53.47, p < 0.0001), with Bonferroni post-hoc tests showing significant differences between all conditions (p < 0.024). The group*condition main effect was not significant (F_3,473 = 0.13, p = 0.943).

Discussion

Control of the whole body COM is the primary goal of the balance system. The Swaymeter provides an indirect measure of COM movement as it is fixed at approximately the level of the COM (pelvis) and records motion of the body in 2D while standing. In order to stabilise the COM, an individual produces forces on the support surface/s (predominantly under the feet while standing). Due to the relative ease of forceplate recordings, the COP has commonly been used in previous studies of standing balance control as an indicator of balance stability. However, while COM and COP measures are related, they are not synonymous [17]. The COP-COM position separation varies across individuals and conditions. Despite this, the Swaymeter showed good agreement with forceplate COP measures for AP and ML displacement (average offset = 6 mm) and moderate-excellent correlations for AP (r > 0.743) and ML displacement (r > 0.692).

While moderate correlations (r > 0.560) were found for Swaymeter versus forceplate sway path length, Bland-Altman plots revealed measures were consistently longer for the forceplate compared to the Swaymeter (average offset = 376 mm). Since correlations tend to increase when the data are more widely spread, the Bland-Altman statistics provide more meaningful detail regarding measurement agreement. The Bland-Altman results show that the magnitude of difference between Swaymeter and forceplate measures of sway path increased with the magnitude of sway. This result may be due to the reduced precision of the Swaymeter (1 mm), the dampening of body motion through the Swaymeter and pen-paper interface friction, and the higher frequency motions recorded by the forceplate system. Increased filtering of COP data might improve the agreement between these measures. However, these results suggest that postural sway path length cannot be directly compared between Swaymeter and forceplate recordings. Subsequent analyses on log-transformed sway path datasets showed good agreement across the range, indicating that data from these sources may be compared following log-transformation. However, it should be noted that log transformations of Swaymeter data are not required in clinical settings, as raw scores can be contrasted to population norms to provide an indication of an individual's postural stability [16].

Immediate test-retest repeatability was found to be good for all Swaymeter measures across the 4 conditions. The correlation coefficients across trials were excellent for sway path length (ICC > 0.834) and ML displacement (ICC > 0.763), and good for AP displacement (ICC > 0.654). Data from all participants were included in these analyses to cover a fuller range (spread) of data points, as the issue of interest was the device, not the participant group. However, similar associations were evident when the analyses were restricted to the older group alone (data not shown). The superior Swaymeter ICCs in the ML plane, relative to AP, are in agreement with those previously reported for within-day COP velocity ICCs [18,19]. Within session reliability is largely related to the random variability of the measurement, as opposed to intersession reliability which may incorporate changes in postural stability over a longer period of time and errors associated with reapplication of setup protocols. Lin and colleagues [18] also found within-day reliability to be better than between-day reliability for both young and older adult groups. Similarly, we found higher reliability coefficients for immediate measures compared with those previously measured for repeats over a longer period [4], which suggests that an individual's postural sway may change over days or weeks.

Sway variables measured with the Swaymeter significantly differed between young and older participants, in line with previous findings [20]. These differences provide evidence of Swaymeter concurrent validity for examining age-related differences in postural sway. One quarter of our older adult group reported a fall in the previous year, suggesting that we recruited a representative sample of healthy community-dwelling older adults living in the Australian state of New South Wales [21]. The between-group differences reported here are, therefore, indicative of normal age-related declines in postural sway in healthy community-dwelling older people.

Maki and colleagues [5] have suggested that a quick, simple, and safe measure of postural sway provides a better prediction of future falling risk in older people compared with measures derived using more complicated and expensive moving platform protocols that induce balance responses. Piirtola and Era [3] reviewed nine prospective studies of forceplate sway measures and found only five to be significantly associated with falls outcomes in older people. In contrast, Swaymeter measures have consistently shown significant associations with future falls across multiple populations of older people (ie people living in both the community and residential care) [11,12,22]. The consistent findings most likely relate to portability and feasibility factors, which have allowed assessment of large sample sizes (with resultant increased statistical power) and assessment of older people across a broad stability spectrum (with resultant high inter-participant sway variability). It is also possible that apparent limitations of the Swaymeter, including the cross-talk between ML and yaw motion (twisting) resulting in amplified Swaymeter-recorded ML displacement and sway path length and/or the dampening of pelvis motion through the device and pen-paper interface friction, provide measures that are more predictive of future falls than forceplate COP measures. For example, future fallers may have more yaw motion while standing, which is detected by larger ML displacement from Swaymeter recordings but not seen with COP measures. It should be noted that the Swaymeter is subject to operator error, as sway variables are counted and computed manually.

Conclusions

The Swaymeter enables measurement of postural sway that is simple and less expensive than methods employing forceplate or motion capture systems. This study has found the Swaymeter to be a reliable measure of postural sway that discriminates between young and older people across multiple sensory conditions. Sway displacement measures from the Swaymeter agreed well with forceplate measures, while sway path measures showed an offset that increased with the magnitude of sway, which can be overcome by correcting the skewed data via log-transformation.

List of abbreviations

AP: Anteroposterior; CI: Confidence intervals; COM: Centre of mass; COP: Centre of pressure; ICC: Intraclass correlation coefficients; ML: Mediolateral; SEM: Standard error of the mean; SD: Standard deviation.

Competing interests

The Swaymeter is commercially available from Neuroscience Research Australia. The authors declare that they have no other competing interests

Authors' contributions

DS conceived the study, participated in its design and coordination, collected and processed the data, helped with statistical analyses and drafted the manuscript. RA reviewed literature, helped with statistical analyses and drafted the manuscript. SL participated in the study design and coordination, helped with statistical analyses and drafted the manuscript. All authors have read and approve the final manuscript.

Pre-publication history

The pre-publication history for this paper can be accessed here:

http://www.biomedcentral.com/1471-2318/11/63/prepub

Acknowledgements

Dr Sturnieks was supported by a NHMRC Population Health Capacity Building Grant in Injury Prevention, Acute Care and Rehabilitation. Prof Lord was supported by a NHMRC Fellowship.

References