Parkinson’s disease (PD) is a chronic progressive neurological disorder that involves both motor and nonmotor symptoms [
Falls in PD are largely multifactorial and intrinsic in nature [
When examining patients with PD, it is important to assess the mechanisms of postural instability and underlying balance impairments to identify fall risk and guide physical therapy interventions. The Parkinson Evidence Database to Guide Effectiveness and the Movement Disorder Society (MDS) developed recommendations for balance examination in the PD population [
The NeuroCom force platform system is a computerized system used to examine underlying balance deficits in patients with neurologic conditions and in the elderly at fall risk. Standardized force platform (FP) measures on this system include limits of stability (LOS) test, sensory organization test (SOT), and motor control test (MCT), which assess voluntary postural control, sensory integration, and reactive postural control, respectively. These FP measures provide quantitative data on postural sway, movement amplitude and velocity, sensory strategies, and latency of postural responses. These measures have been applied in elderly to identify balance impairments and fall risk. The SOT and LOS test validly identify postural control impairments in the elderly fallers, differentiate fallers from nonfallers, and predict fall risk [
Regarding the PD population, limited research supports that FP measures may identify balance impairments, even in early disease stages, and may detect balance decline with disease progression [
Further research on the clinical utility of the FP measures in the PD population is warranted due to the current gaps in the literature. Although there are valid and reliable clinical measures of balance at the functional level, there is a lack of rigorous research on balance measures at the impairment level. Further research is needed on computerized FP measures to assess if these measures identify balance deficits in PD across the disease stages and are able to sensitively assess changes in balance function with disease progression. In addition, further research is needed to examine if these measures are able to distinguish between balance impairments in persons with PD and age-related changes in balance in healthy older adults.
The purpose of this study is to compare balance performance on the LOS test, SOT, and MCT in persons with PD with age- and gender-matched healthy adults. Secondarily, this study will assist clinicians in determining if these standardized FP measures provide diagnostic and clinically meaningful information about the underlying balance impairments in the PD population. This research may help clinicians select examination measures to identify balance deficits across disease stages in persons with PD, which may direct balance interventions to reduce fall risk.
This study examines the differences in postural control between individuals with PD and an age- and gender-matched healthy cohort, as measured by standardized tests on the NeuroCom force platform system. The study analyzes between-group comparisons on the LOS test, MCT, and SOT, and examines if there were differences in test performance within the PD cohort based on disease stage, PD subtype, and age grouping.
This study’s recruitment and participant enrollment took place from June 2014 to March 2016. Recruitment methods included posting information in local Parkinson association newsletters, posting flyers in the community and local university, and conducting informational meetings at local PD support groups and community exercise classes. Recruitment of age- and gender-matched healthy adult controls was based on the following age groupings: 41–50, 51–60, 61–70, and 71–80 years old. Inclusion and exclusion criteria used for both cohorts are listed in Table
Inclusion and exclusion criteria for Parkinson’s disease cohort and age-matched healthy cohort.
Participant cohort | Inclusion criteria | Exclusion criteria |
---|---|---|
Parkinson’s disease cohort | (1) Idiopathic PD |
(1) Other neurological diagnosis |
Healthy age-matched cohort | (1) Between 20 and 80 years of age |
Researchers conducted phone interviews to determine if participants met the inclusion and exclusion criteria, followed by an in-person visit to assess walking and stair ability and to administer the Montreal Cognitive Assessment (MoCA) and the Semmes Weinstein monofilament (SWME). A MoCA cutoff score of 21/30 points for identifying dementia was used for both groups (sensitivity 81%, specificity 95% in PD) [
Medical history, activity self-report, and history of falls within the last six months were collected from all participants and/or their spouses. Falls were defined as any instance in which the individual lost their balance, causing them to fall to the ground or hit an object below them. Participants were classified as “fallers” if they reported 2 or more falls in the last 6 months. Information regarding disease characteristics and severity for the PD cohort was gathered through the freezing of gait questionnaire (FOGQ) and the Movement Disorder Society-sponsored revision of the unified Parkinson’s disease rating scale (MDS-UPDRS), which was administered by the principal investigator who had completed training and certification by the MDS. Participants were classified as “freezers” if they scored greater than one on the third item of the FOGQ. PD participants were classified into three PD subtypes based on their MDS-UPDRS score as described by Stebbins et al.: (1) tremor predominant (TD), (2) postural instability and gait difficulty (PIGD), and (3) indeterminate (I) [
As depicted in Figure
Participant flow diagram.
Demographics for Parkinson’s disease cohort and age-matched healthy cohort.
PD cohort | Healthy cohort | |
---|---|---|
Number of participants |
|
|
Age | 66.21 ± 7.92 | 64.75 ± 8.50 |
Gender | ||
Male |
|
|
Female |
|
|
Disease duration (months) | 53.90 ± 37.86 | N/A |
Hoehn and Yahr stage | 2.33 ± 0.77 | |
Stage 1 |
|
N/A |
Stage 2 |
| |
Stage 3 |
| |
Stage 4 |
| |
MDS-UPDRS total score | 47.98 ± 23.70 | N/A |
MDS-UPDRS motor score | 25.95 |
N/A |
Number of medications | 4.30 ± 3.01 | 2.31 ± 1.69 |
Percentage of fallers | 24% | 0% |
Percentage of freezers | 24% | N/A |
PD subtype | ||
(1) Tremor dominant |
|
N/A |
(2) Posture instability and gait difficulty |
|
N/A |
(3) Indeterminate |
|
N/A |
This study was part of a larger study examining the reliability and concurrent validity of FP measures in individuals with PD, with detailed description of the full study’s methods published by Harro et al. [
Testing protocol utilized for both Parkinson’s disease and healthy cohorts.
The NeuroCom™ Smart Equitest Clinical Research System/Balance Master System 9.1 was used to administer the LOS test, MCT, and SOT (Natus Medical Inc., 9570 SE Lawnfield Rd, Clackamas, OR, 97015). Participants were barefoot for the FP testing. Foot placement on the platform was carefully controlled, and standardized instructions were given for each test. Per testing protocol guidelines, participants wore an overhead harness and were closely guarded during testing. A “fall” during any of the tests was defined as needing to take a step, using hands on the walls, or requiring assistance from the researcher to regain balance. Reliability and validity of these measures in persons with PD was discussed earlier in this paper [
This test assesses voluntary postural control as demonstrated by participants moving their center of gravity towards eight targets when cued by a visual signal on-screen. The test variables recorded for this study were endpoint excursion (EPE), average movement velocity (Avg MV), endpoint excursion to posterior targets 4, 5, and 6 (EPE 4–6), total number of falls, and number of falls during posterior targets 4, 5, and 6.
This test assesses reactive postural control in response to anterior and posterior perturbations induced by movements of the force platform surface. The test variables recorded for this study were the average response latency (Avg Lat), average amplitude of response to large perturbations (Avg Amp L), average posterior amplitude (Avg Posterior Amp), and average posterior latency (Avg Posterior Lat) in response to posterior perturbations.
This test measures postural sway in response to six different sensory conditions that challenge the integration of vision, somatosensory, and vestibular senses. The test variables recorded in this study were composite equilibrium score (Comp Eq), vestibular ratio (Vestib Ratio), visual preference ratio, total number of falls, and number of falls in conditions 5 and 6.
Data collected were used to examine if there were differences in performance on the FP tests between the PD cohort and the age-matched healthy cohort. Using multiple linear regression, it was determined that 46 participants were needed in the PD cohort to maintain a statistical power of 0.80 and achieve an effect size of 0.35 and an alpha level of 0.05 [
Discriminative validity was analyzed between the PD and healthy cohorts by using unpooled independent
The secondary research purpose was addressed using subgroup analysis within the PD cohort to examine if FP performance differed based on disease severity, age grouping, and PD subtype (TD and PIGD). Comparison of H&Y stage 4 was excluded as there was only one participant in this group. One-way ANOVA was utilized to assess if there were differences in mean performance on the LOS Avg EPE and Avg MV and MCT Avg Lat among the disease stages. Post hoc analysis was completed with adjusted Bonferroni corrections for multiple comparisons. Kruskal–Wallis analysis was used to assess if there were differences in median performance on the SOT Comp Eq among disease stages, as equality of variance could not be assumed. Post hoc analysis was completed using a Wilcoxon rank-sum test with adjusted Bonferroni correction for multiple comparisons (
Subgroup analyses were performed for the PD cohort based on the PD subtype (PIGD and TD). Subgroup analyses for the primary FP variables (SOT Comp Eq, LOS Avg EPE, and MCT Avg Lat) are reported by Harro et al. in a previous paper [
A one-way ANOVA was used to assess if there were differences in mean performance on primary FP variables based on the age group within the PD cohort. Age groups were established as group 1 (41–60 years), group 2 (61–70 years), and group 3 (71–80 years) [
Descriptive statistics for FP balance measures for both the PD and healthy cohorts are summarized in Table
Descriptive statistics of force platform measures.
Cohort | FP measures | Mean | SD | Minimum | Q1 | Median | Q3 | Maximum |
---|---|---|---|---|---|---|---|---|
Parkinson’s disease cohort ( |
SOT composite equilibrium (1–100) | 68.52 | 12.93 | 24.00 | 64.00 | 72.50 | 77.00 | 83.00 |
SOT vestibular ratio (0-1) | 0.54 | 0.24 | 0.00 | 0.47 | 0.61 | 0.72 | 0.85 | |
SOT visual preference | 0.98 | 0.13 | 0.69 | 0.91 | 0.98 | 1.04 | 1.32 | |
SOT fall number | 1.21 | 2.07 | 0.00 | 0.00 | 0.00 | 1.00 | 8.00 | |
SOT fall number-conditions 5 and 6 | 1.12 | 1.82 | 0.00 | 0.00 | 0.00 | 1.00 | 6.00 | |
LOS average endpoint excursion (%) | 70.58 | 13.71 | 36.63 | 61.25 | 74.50 | 80.00 | 104.60 | |
LOS movement velocity (m/s) | 3.35 | 1.02 | 1.35 | 2.64 | 3.31 | 4.01 | 7.35 | |
LOS number of falls | 0.43 | 0.77 | 0.00 | 0.00 | 0.00 | 1.00 | 3.00 | |
LOS number of falls on targets 4, 5, and 6 | 0.33 | 0.61 | 0.00 | 0.00 | 0.00 | 1.00 | 2.00 | |
LOS average endpoint excursion on targets 4, 5, and 6 (%) | 61.99 | 16.45 | 26.33 | 49.67 | 61.84 | 72.00 | 103.33 | |
MCT average latency (ms) | 144.12 | 9.21 | 118.00 | 139.00 | 145.50 | 150.00 | 161.00 | |
MCT average amplitude to large perturbations (degrees/second) | 10.23 | 3.52 | 3.00 | 7.75 | 9.63 | 12.50 | 19.75 | |
MCT average posterior amplitude (degrees/second) | 7.68 | 2.72 | 2.17 | 5.33 | 7.17 | 10.00 | 13.67 | |
MCT average posterior latency (ms) | 151.98 | 10.84 | 125.00 | 145.00 | 151.67 | 161.67 | 170.00 | |
|
||||||||
Healthy cohort ( |
SOT composite equilibrium (1–100) | 74.35 | 8.05 | 48.00 | 70.00 | 75.00 | 80.00 | 86.00 |
SOT vestibular ratio (0-1) | 0.64 | 0.16 | 0.00 | 0.57 | 0.67 | 0.75 | 0.92 | |
SOT visual preference | 0.99 | 0.11 | 0.72 | 0.93 | 1.00 | 1.05 | 1.42 | |
SOT fall number | 0.36 | 0.80 | 0.00 | 0.00 | 0.00 | 0.00 | 4.00 | |
SOT fall number-conditions 5 and 6 | 0.35 | 0.80 | 0.00 | 0.00 | 0.00 | 0.00 | 4.00 | |
LOS average endpoint excursion (%) | 72.97 | 14.19 | 36.00 | 64.13 | 75.88 | 83.50 | 97.75 | |
LOS movement velocity (m/s) | 4.45 | 1.45 | 2.29 | 3.26 | 4.18 | 5.79 | 8.06 | |
LOS number of falls | 0.40 | 0.89 | 0.00 | 0.00 | 0.00 | 0.00 | 4.00 | |
LOS number of falls on targets 4, 5, and 6 | 0.16 | 0.57 | 0.00 | 0.00 | 0.00 | 0.00 | 3.00 | |
LOS average endpoint excursion on targets 4, 5, and 6 (%) | 62.42 | 16.57 | 14.67 | 50.00 | 62.33 | 74.67 | 97.67 | |
MCT average latency (ms) | 142.62 | 8.77 | 126.00 | 136.00 | 141.00 | 148.00 | 165.00 | |
MCT average amplitude to large perturbations (degrees/second) | 10.19 | 2.82 | 5.50 | 7.75 | 10.00 | 11.75 | 16.50 | |
MCT average posterior amplitude (degrees/second) | 7.18 | 2.22 | 3.50 | 5.50 | 7.00 | 8.17 | 12.50 | |
MCT average posterior latency (ms) | 149.91 | 11.15 | 131.67 | 141.67 | 146.67 | 158.33 | 185.00 |
SOT, sensory organization test; LOS, limits of stability test; MCT, motor control test; SD, standard deviation; Q1, lower quartile; Q3, upper quartile.
Discriminative validity of force platform measures between PD and healthy cohorts.
Force platform variable |
|
|
95% confidence interval |
---|---|---|---|
SOT composite equilibrium (1–100) | 2.56 (64.563) | 0.013 |
1.286, 10.357 |
SOT vestibular ratio (0-1) | 2.26 (68.555) | 0.027 |
0.012, 0.185 |
SOT visual preference | 0.37 (80.477) | 0.710 | −0.039, 0.057 |
SOT fall number | −2.53 (50.466) | 0.015 |
−1.527, −0.175 |
LOS average endpoint excursion (%) | 0.81 (89.681) | 0.422 | −3.367, 7.964 |
LOS movement velocity (m/s) | 4.36 (94.456) | <0.001 |
0.597, 1.595 |
LOS number of falls | −0.17 (93.594) | 0.866 | −0.365, 0.308 |
LOS number of falls on targets 4, 5, and 6 | −1.39 (85.006) | 0.167 | −0.412, 0.072 |
LOS average endpoint excursion on targets 4, 5, and 6 (%) | 0.13 (88.294) | 0.899 | −6.288, 7.512 |
MCT average latency (ms) | −0.81 (86.129) | 0.419 | −5.175, 2.174 |
MCT average amplitude to large perturbations (degrees/second) | −0.06 (76.892) | 0.949 | −1.364, 1.279 |
Independent
Comparative boxplot of sensory organization test composite equilibrium score for Parkinson’s disease and age-matched healthy cohorts.
Comparative boxplot of limits of stability average movement velocity performance for Parkinson’s disease and age-matched healthy cohorts. LOS, limits of stability.
ANCOVA analysis examined whether FP performance varied between the PD and healthy cohorts when controlling for age. The overall models for all three primary variables were significant (SOT Comp Eq
Within the PD cohort, significant differences were found in MCT Avg Lat among H&Y stages (
Comparative boxplot of sensory organization test composite equilibrium performance of Parkinson’s disease cohort based on the Hoehn and Yahr stage. Hoehn and Yahr stage 4 was not included in this analysis as only one participant was represented in this stage. SOT, sensory organization test.
Previous analysis of the primary FP measures in this study, Harro et al. [
Comparative boxplot of motor control test posterior latency performance of Parkinson’s disease cohort based on the PD subtype. Subtype 3 was not included in this analysis as it is an intermediate group with symptoms specific to both subtypes 1 and 2. MCT, motor control test. 1: tremor dominant. 2: postural instability and gait dysfunction.
Significant differences were found between age groups within the PD cohort in LOS Avg EPE (
This study compared FP measures of balance impairment in persons with PD and an age-matched healthy cohort and demonstrated that sensory organization test (SOT) measures and limits of stability (LOS) movement velocity were significantly different between the two groups. The PD cohort had slower movement velocity during voluntary postural control in the LOS test as compared to healthy controls. Individuals with PD had reduced postural stability on the SOT (Comp Eq score), lower equilibrium scores in conditions requiring effective use of vestibular cues (Vestib ratio), and an increased number of falls during the SOT when compared to healthy controls, reflective of sensory integration deficits affecting balance control. The SOT differentiated between PD and healthy controls, as the Comp Eq score was estimated to be 5.88 higher in healthy individuals than those with PD when controlling for age.
Our findings regarding identification of postural instability based on SOT results in persons with PD compared to healthy individuals are consistent with previous research [
Individuals with PD displayed reduced movement velocity on the LOS test, which may reflect underlying bradykinesia affecting voluntary movements and impaired feed-forward balance strategies and is consistent with two other studies [
Currently, there is limited published research on MCT FP performance, a measure of reactive postural control, in persons with early-to-middle stage PD. Dimitrova et al. and Horak et al. [
When examining performance differences within the PD cohort among H&Y stages, the SOT distinguished between those individuals in stages 1 and 3 and those in stages 2 and 3. Additionally, the SOT showed a moderate relationship to disease severity (MDS-UPDRS) [
Our research findings support that individuals with the posture instability gait Difficulty (PIGD) subtype of PD have greater underlying balance impairments than those with the tremor dominant (TD) subtype. Based on analysis of this PD cohort’s performance on primary FP measures, Harro et al. [
Comparing FP balance performance between age groups within the PD cohort, the LOS Avg EPE was the only variable that demonstrated significant differences. Endpoint excursion to targets was significantly reduced in the 71–80 year-old group when compared to both younger age groups. Since LOS Avg EPE showed no differences between H&Y stages, these results may indicate that age is a greater contributing factor to reduced endpoint excursion than disease stage. Older age (>70 years) combined with advancing PD stage may be a cause for concern of a decline in feedforward, voluntary postural control needed for daily functional tasks.
Force platform measures sensitively detect balance impairments in persons with PD and may be especially valuable for individuals with suspected balance underlying impairments based on clinical functional balance measures. The SOT provides clinically valuable information regarding sensory organization strategies for postural control and identifies a decline in these strategies with increasing disease severity in PD. In contrast to the modified clinical test of sensory interaction and balance (Mod-CTSIB), which has fair reliability and validity at best for measuring sensory organization deficits, the SOT has excellent reliability (ICC = 0.90) and good validity [
The LOS test assesses voluntary postural control and provides quantitative information about limits of stability and speed of center of mass (COM) movement. This study’s findings regarding reduced movement velocity on the LOS test in the PD cohort may indicate a need for increased attention in dynamic balance training to address full body bradykinesia. Interventions to improve balance should emphasize full body, self-generated, dynamic movements targeting feed-forward anticipatory strategies for control of balance. Based on our findings, the clinical use of the LOS test may be especially beneficial for those with PD who are categorized in the PIGD subtype and for those in middle to later disease stages to detect balance deficits in feedforward balance mechanisms [
Results from this study indicated that there were not deficits in reactive postural control strategies based on MCT results in those in early stages (H&Y 1 and 2). Our findings did support that the MCT appears to be an appropriate diagnostic test to identify reactive balance control deficits for patients in H&Y stages 3 and 4 and in those individuals with PIGD subtype who are at greater risk for early balance decline. If clinical balance assessments indicate suspected deficits in reactive balance strategies, then the MCT may be a useful next step to provide objective, quantitative assessment of this impairment and to assess the efficacy of reactive balance training interventions.
Limitations in this study related to sample characteristics include a lack of representation of H&Y stage 4 (
Future research regarding the use of FP measures to evaluate balance impairments and detect balance decline in individuals with PD is warranted. This study was representative of individuals in H&Y stages 1–3; therefore, more research is needed to assess the diagnostic value of these FP measures in later disease stages when balance declines more progressively, specifically stage 4. The PD cohort in our study was not a representative sample of those with freezing of gait characteristics and those with positive fall history (24% of our sample had FOG; 24% of sample were fallers). Further research examining the sensitivity of FP measures to identify balance impairments and assess fall risk in individuals with FOG deficits and positive fall history is warranted. It was not the purpose of this study to examine if FP measures were a valid assessment for fall risk and could predict future falls. Research specifically targeting these important clinical questions is needed. In addition, further research examining the utilization of the FP measures to track changes in postural control longitudinally in persons with PD is recommended to shed light on the temporal course of postural control decline in PD. Lastly, computerized posturography assesses postural control mechanisms in the standing position on a force plate system. Research is needed to evaluate the control of dynamic balance during walking tasks in PD, such as varied speeds, during turns, and walking over obstacles, using a motion analysis system. This research would provide valuable insight into postural control deficits that contribute to fall risk during mobility and walking activities in those with PD.
This study provides evidence for the discriminative validity of force platform measures in assessing postural control impairments in individuals with PD compared to a healthy, age-matched cohort. The sensory organization test measures and the limits of stability test movement velocity measure were able to differentiate between the two cohorts, supporting the premise that these measures detect PD-specific balance impairments related to sensory integration and voluntary postural control, respectively. The PD cohort demonstrated significantly greater postural instability on sensory organization test measures and slower movement velocity on the limits of stability test than the healthy cohort. Additionally, this study’s results support the diagnostic value of FP measures to examine underlying balance impairments related to advancing disease severity, PD subtype (PIGD), and older age in individuals with PD. The sensory organization test differentiated between Hoehn and Yahr stages, sensitively detecting postural instability and sensory integration deficits with disease progression. The motor control test also differentiated between disease stages and PD subtypes, with poorer performance found in later disease stages and in those with the postural instability and gait difficulty subtype. The limits of stability test differentiated between PD subtypes and age groups, with poorer balance performance found in those greater than 70 years old and those with PIGD subtype. These findings support that force platform measures may provide clinically meaningful, quantitative information in the examination of balance impairments in individuals with PD. Given the high fall rate and devastating sequelae of falls in individuals with PD, force platform measures may inform clinicians regarding an individual’s underlying balance deficits and direct targeted balance interventions to remediate postural control impairments and reduce fall risk.
The quantitative data on the force platform variables in the PD and healthy cohorts and statistical data used to support the findings of this study are included within the article.
This research abstract was presented as a Research Poster at the American Physical Therapy Association-Combined Sections Meeting, New Orleans, LA, Feb-2018.
This work was completed as professional scholarship related to primary author’s employment at Grand Valley State University, Grand Rapids, MI. The authors declare that there are no conflicts of interest regarding the publication of this article.
We would like to thank Dr. Sango Otieno, Department of Statistics, Grand Valley State University, for his expertise and collaboration in the statistical analyses for this study.