The objectives of this study were to examine the test-retest reliability of postural control measurements of a portable force plate commonly used by clinicians and to explore the effect of using the mean value from multiple repetitions on reliability. Our results demonstrated that for measures obtained by the MPSA, single trials do not provide reliable estimates of postural control and that averaging multiple measures is necessary to achieve acceptable levels of measurement error. The number of repetitions necessary to achieve reliable results varied depending of the outcome variable and ranged from two to five. Clinicians should take this into account when measuring postural control on their patients.
ICC values for measures of relative weight bearing appeared lower than ICC values of COP mean velocity and average location of COP. However, inspection of the descriptive statistics (mean and SD) in Table 1 for each of these variables indicates less inter-subject variability in relative weight bearing as compared to the other dependent variables. Low levels of inter-subject variability are known to artificially lower ICC estimates, as this increases the relative magnitude of the error term in the ICC equation . Thus, it can be difficult to interpret ICC values derived from homogenous measures such as relative weight distribution.
We also assessed COP mean velocity, average location of COP, and sway area. Consistent with our results, COP mean velocity has been reported by others to be the most reliable estimate of COP [23–25]. In contrast, others have examined samples of healthy participants and reported low reliability for measures of COP mean velocity . However, it should be noted that in that study, the ICC statistics were calculated by averaging data from 3 10-second repetitions. The longer duration of recording used in our protocol may explain our higher reliability estimates. While longer duration trials of up to 120 seconds are recommended to reduce measurement error , sampling duration should be matched to the abilities of participants. For instance, children with cerebral palsy or the elderly may not tolerate standing for an ideal duration of time.
For measures of sway area, it was necessary to average values from five repetitions to achieve an acceptable level of measurement error. Two studies reported similarly low ICC values for sway area [27, 28]. Alternatively, others have reported acceptable levels of ICC for this variable; however, these latter studies were conducted under eyes closed.
A potential issue with relying on measures obtained from a suboptimal number of repetitions can arise in clinical practice. For instance, when examining for differences in postural stability over using single measures, a practitioner needs to observe an improvement of at least 12.1 mm/sec in COP mean velocity to be 95% confident that a true change has occurred. However, when using a mean of 3 repetitions, a practitioner can be just as confident that true change has occurred with a change of 2 mm/sec.
The results of this study are limited by several factors. This study was conducted on healthy individuals and the results may or may not generalize to clinical populations. Moreover, the MPSA as an instrument of measurement has several limitations. The MPSA is limited to a fixed duration of data acquisition of between 5 and 60 seconds. It is not possible to set up the recording time to durations longer than 60 seconds, which may be desirable is some circumstances. This technology also has a fixed sampling rate and cut-off frequency and altering these frequencies is not possible. Finally, the MPSA software does not report some variables such as sway area, which we calculated from the raw data.
Future research should examine the validity of the force plates commonly used by clinicians by comparing their measures to those obtained using force plates with known validity. Additionally, it would be useful to assess the reliability of similar force plates in a clinical population, such as those individuals with neurological impairments.