Test–retest reliabty of center of pressure measures of postural stability during quiet standing in a group with musculoskeletal disorders consisting of low back pain, anterior cruciate ligament injury and functional ankle instability
Introduction
Musculoskeletal disorders (MSDs) significantly contribute to the causes of absence from work and disability throughout the world, having a substantial influence on health and quality of life, and inflicting a significant economic burden on health services [1]; therefore, knowledge of the associated impairments is critical for reducing the burden.
As balance is the foundation for all voluntary motor skills, considerable research has been conducted to evaluate the balance performance in patients with MSDs [2], [3], [4], [5], [6], [7]. Most of this work focused on evaluating balance control in low back pain (LBP) [2], [3], anterior cruciate ligament (ACL) injury [4], [5] and functional ankle instability (FAI) [6], [7]. Higher mean total velocity and root mean square of amplitude in mediolateral (ML) direction in LBP patients [2], [3], increased mean total velocity and area in ACL-injured group [4], [5] and higher mean velocity in anteroposterior (AP) and ML directions in patients with ankle instability [6], [7] compared with their control groups have been reported in the literature.
Balance performance as assessed by force platform studies is most often quantified by various parameters derived from the COP data [8]. COP parameters will provide different information on the mechanism or strategy used to control posture [8].
COP measures are subject to measurement errors with three potential sources: instrument, observer (i.e. variability in procedure adopted) and variability in biologic phenomena being measured [9]. An essential part of COP variability has been attributed to the intrinsic variability of the postural control system since the precision of instrument and procedure was satisfactory in numerous studies [9], [11]. Although high level of reliability cannot guarantee the validity of a measure [10], identifying the measurement error as a prerequisite for discriminative and evaluative purposes is a major concern for clinicians when they use COP parameters [11].
To date most of the studies conducted to estimate the various types of reliability of COP measures have targeted healthy and elderly individuals as the study population [9], [11], [12], [13]. According to Domholdt [10], the reliability is not a fixed property but is dependent on the studied population. Although some researchers have attempted to identify reliable COP measures to be used in the comparative studies between patients with LBP [2], [14], ACL injury [15] or FAI [16] and their control groups, the results of these studies can be generalized only to healthy subjects who were targeted but not the whole population of MSDs. The inappropriate use of healthy subjects to establish the reliability of clinical measures has the potential to inflate reliability estimates which may be overlooked given the fact that healthy participants may be easier to measure than patients [10]. Addressing this knowledge void encouraged us to conduct a purely methodological study to estimate the test–retest reliability of some COP measures in a group of patients with MSDs, divided in three levels of postural difficulty over two sessions.
According to the dynamic model of postural control, the central nervous system must tightly control not only the relative position between COP and base of support but also the relative velocity to maintain stability [17]. Based on this rationale, some commonly used parameters, each representing a unique dimension of position, velocity and both position and velocity of COP were included in our study.
Section snippets
Subjects
A sample of 33 subjects with MSDs was recruited to provide representative subgroups of LBP (nine males, two females), ACL (12 males) and FAI (nine males, one female). The inclusion criteria for each of the following subgroups were: (1) recurrent nonspecific LBP: LBP for 12 months or more, no history of serious spinal pathology, nerve root pain, previous history of spinal surgery, structural deformity of the spine and pain extending the gluteal fold; (2) ACL injury: unilateral, non-operated (to
Results
Table 3 shows the mean and S.D. of COP measures for test and retest sessions and Table 4 demonstrates ICC and its 95% CI, SEM, MMDC and CV.
There was no significant difference between test and retest mean scores for any COP measures in all condition, which indicates absence of any systematic bias (p > 0.05).
In rigid-open condition, high reliability was found for mean total velocity, phase plane (ML), S.D. of velocity (ML) and phase plane (AP–ML) with ICC levels of 0.84, 0.79, 0.77 and 0.71,
Discussion
The results obtained indicate high to very high relative reliability of mean total velocity, consistently in all conditions of postural difficulty. This indicates that the measurement error was small in comparison to the variability between subjects and therefore the possibility of type II error would be limited [26], [27]. To detect differences of balance performance between groups of LBP and healthy subjects, Luoto et al. [2] reported that among different COP parameters, mean total velocity
Conclusion
In agreement with previous studies, our results showed high to very high reliability of mean total velocity in all conditions of postural difficulty among some routinely used COP parameters. This suggests that mean total velocity can be used for both discriminative and evaluative purposes in the population of MSDs. The next best parameter was phase plane portrait in AP–ML and ML direction. Further research is needed to determine the discriminative value of the above parameters by including
Conflict of interest
None of the authors have any financial or other interests relating to the manuscript to be submitted for publication in Gait and Posture.
Acknowledgments
The study was supported by University of Social Welfare and Rehabilitation and Tehran University of Medical Sciences, Tehran, Iran. Special thanks to Dr. Anushirvan Kazemnejad and Dr. Mohammad Reza Ebrahimian for their contribution in statistical analysis and patient selection. The experiment was conducted in Biomechanics Lab., Rehabilitation Research Center, Iran University of Medical Sciences and Biomechanics Lab., School of Rehabilitation sciences, Tehran University of Medical Sciences.
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