GRAPHICAL ABSTRACT

INTRODUCTION

People with Parkinson’s disease (PwPD) have impaired bed mobility, including daytime movements such as turning in bed, transitioning from supine-to-sitting, and transitioning from sitting-to-supine, from the early stage of the disease [1-3], and the need for assistance increases as the disease progresses [4]. These impairments can negatively impact nocturnal sleep [5-8] and quality of life [9,10]. Daytime bed mobility is essential for performing daily activities such as eating, toileting, and transferring. Independence in daytime bed mobility is directly related to functional independence and is highly relevant for patients with moderate to advanced Parkinson’s disease (PD). The progressive loss of bed mobility independence [1] can lead to complications such as bedsores [11,12], positional asphyxia [13], and increased caregiver burden [14,15].

Pharmacotherapy [10,16,17] and surgical treatments such as deep brain stimulation [18] play key roles in mitigating disabilities in PwPD. However, studies on unmet medical needs indicate that pharmacological treatments for PwPD are often insufficient for improving bed mobility [19,20]. It is widely acknowledged that incorporating rehabilitation with these treatments is essential for addressing bed mobility impairments [21]. Several research groups have examined the effectiveness of rehabilitation for bed mobility [22-24], yet its efficacy remains unclear. Because multidisciplinary interventions improve activities of daily living [25], identifying the precise factors associated with bed mobility impairments through comprehensive assessments of motor and cognitive symptoms, as well as physical functions such as muscle strength, is essential for developing targeted interventions.

Impaired turning in bed, defined as a score ≥1 point on the Movement Disorders Society-Unified Parkinson’s Disease Rating Scale (MDS-UPDRS) Part 2 Item 9, has been linked to axial symptoms in at-risk individuals and bradykinesia in de novo PD patients [3]. Wearable device assessments of nocturnal turning movements in PwPD have revealed associations between movement frequency, velocity, and duration and axial symptoms, rigidity, bradykinesia, and cognitive dysfunction [2]. Additionally, experimental evaluations of supine-to-sitting movements in PwPD have demonstrated associations between movement duration and upper-limb rigidity and bradykinesia [26]. Rigidity, bradykinesia, and axial symptoms may be associated with independence in each bed mobility task, although the specific contributing factors and their impact may vary by mobility task. Bed mobility requires coordinated muscle activity, particularly of the neck, trunk, and hips, to generate sufficient strength for initiating and controlling movement. Furthermore, as these movements are sequential and goal-directed, the involvement of cognitive functions, especially attention and executive functions, is considered important in planning and executing these movements. Therefore, the evaluation of muscle strength, as well as cognitive function, via assessments such as the Mini-Mental State Examination (MMSE) and Trail Making Test (TMT) may provide clinically useful insights into the factors contributing to bed mobility impairments in PwPD. However, to our knowledge, no previous studies have investigated factors contributing to daytime bed mobility independence, and no research has examined factors associated with impaired sitting-to-supine movements.

The primary aim of this cross-sectional study was to identify factors associated with nonindependence in turning in bed, supine-to-sitting, and sitting-to-supine movements among PwPD classified as Hoehn and Yahr (HY) stage 2–4. Additionally, we examined PwPD at HY stage 4, a stage believed to mark the onset of bed mobility decline, and compared independent and nonindependent groups within this population to account for variations in disease severity.

MATERIALS & METHODS

Study design and subjects

This cross-sectional observational study was conducted from April 1, 2021, to May 31, 2023. PwPD admitted to Hokkaido Neurological Hospital (Hokkaido, Japan) for rehabilitation and prescribed physical therapy were recruited. PwPD were eligible if they met the following criteria: 1) a confirmed PD diagnosis on the basis of the International Parkinson and Movement Disorder Society clinical diagnostic criteria [27] and 2) a MMSE score of at least 21 points [28,29], ensuring that they understood the study’s objectives and could follow verbal instructions. The exclusion criteria included 1) the concurrent presence of another neurological disorder, 2) an orthopedic condition that could affect bed mobility, 3) dyskinesia, 4) orthostatic hypotension, and 5) deep brain stimulation. The study procedures were approved by the ethics committees at Hokkaido Neurological Hospital (approval no. 2021_No.2) and conducted in strict accordance with the principles of the Declaration of Helsinki. Prior to participation, all enrolled patients provided written informed consent. Assessments were conducted in a medically stable state at least 2 weeks after admission. All evaluations took place during the “ON” phase, 1–2 hours after taking antiparkinsonian medication. Before each assessment, the participants were interviewed about their motor status. If any clinical signs or subjective reports suggested an “OFF” state, the assessment was postponed and rescheduled to ensure that it was performed during the “ON” state. This procedure was applied consistently across all participants to minimize the potential impact of motor fluctuations.

Bed mobility tasks

We assessed the patients’ ability to perform three bed mobility tasks (turning in bed, supine-to-sitting, and sitting-to-supine), as these movements are essential for daily activities. All patients were instructed to perform these movements on both their left and right sides at a preferred speed without using bed rails [30]. Assessments were conducted on the same hospital bed model (Espacia, Paramount Bed Corp.; 209.6 cm long×93.9 cm wide× 43.0 cm high) with the same mattress model (Everfit C3, Paramount Bed; 191 cm long×91 cm wide×10 cm high) as in the hospital ward, providing patients an experience typical of hospitalization. Each task was performed three times in a randomized order.

Specifically, the “turning in bed” task involved transitioning from a supine position to a side-lying position; “supine-to-sitting” required moving from a supine position to a sitting position; and “sitting-to-supine” entailed transitioning from a sitting position to a supine position. Independence was defined as the ability to complete each movement without using bed rails or requiring physical assistance (Figure 1).

Patients’ performances were videotaped, and independence for each bed-mobility task was evaluated separately. For each of the three movements, patients who were able to perform the task independently on both sides were classified into the independent group for that task. Patients who were unable to complete the movement on either side without assistance or use of bed rails were classified into the nonindependent group for that task. Two experienced physical therapists (M.N. and K.S.) assessed each patient’s independence or nonindependence, and their assessments were in complete agreement for all patients.

Motor symptoms

Motor symptoms were assessed using the MDS-UPDRS Part 3 during the “ON” state. Following previous studies [31,32], four motor symptom components were analyzed via the MDS-UPDRS Part 3 items: 1) rigidity (mean of subitem 3.3), 2) bradykinesia (mean of subitems 3.2, 3.4–3.8, and 3.14), 3) tremor (mean of subitems 3.15–3.18), and 4) axial symptoms (mean of subitems 3.9–3.13). Bradykinesia, tremor, and rigidity were separately assessed for the upper and lower limbs, with rigidity also evaluated for the neck. A trained evaluator (M.N.), certified through MDS-UPDRS online training, conducted all the assessments.

Muscle strength and cognitive function

Muscle strength during neck flexion, trunk flexion, trunk extension, trunk rotation, and hip flexion was assessed in a seated position during isometric contraction using a hand-held dynamometer (Power Gauge, Namba Works). Each test was performed twice, and the average, adjusted for body weight, was recorded. Trunk rotation and hip flexion strength were averaged across both sides. A trained physical therapist (M.N.) conducted all muscle strength assessments. The intrarater reliability of the muscle strength measurements was also evaluated. We also assessed the intrarater reliability of the muscle strength values. Global cognitive function was evaluated using the MMSE, and executive function was assessed with the TMT-A and -B.

Statistical analyses

The Shapiro–Wilk test was used to assess data normality. Intraclass coefficients (ICC) were calculated for each muscle strength test to assess intrarater reliability. PwPD were classified into independent and nonindependent bed mobility groups on the basis of their performance in turning in bed, supine-to-sitting, and sitting-to-supine tasks. Student’s t test, the Mann–Whitney U test, and the χ² test were used to compare group differences for each variable.

To examine the relationships between independence in each movement and factors such as motor symptoms, muscle strength, and cognitive function, univariate logistic regression analyses were performed. In each analysis, independence or nonindependence in turning in bed, supine-to-sitting, and sitting-to-supine movements were set as the dependent variables, whereas motor symptoms, muscle strength, and cognitive function were the independent variables. Confounding factors were identified as those showing significant differences (p<0.05) between independent and nonindependent groups in bivariate analyses. Variance inflation factors (VIFs) were assessed, and variables with VIFs greater than 10 were excluded from the model to prevent multicollinearity [33]. The final logistic regression model was constructed using forward stepwise selection, incorporating all variables significantly associated with independence in bivariate analyses (p<0.05).

Receiver operating characteristic (ROC) curves were generated to predict nonindependence, with a focus on covariates significantly associated with each bed mobility task. The area under the curve (AUC) was calculated for each model, along with the sensitivity and specificity. The optimal cutoff probability value for each task was determined using the Youden index.

Additionally, subgroup analyses were conducted for PwPD at HY stage 4, to compare independent and nonindependent groups for each bed mobility task, considering the influence of disease stage. Statistical significance was set at p<0.05, and all analyses were performed using SPSS for Windows, ver. 29 (IBM Corp.).

RESULTS

Patient characteristics and bed mobility independence

A total of 109 individuals with PD participated in the study. Table 1 summarizes their characteristics. The HY stages ranged from 2 to 4, with a median disease duration of 8.0 years. The median MMSE score was 27.0, with the lowest score being 21.0. Among the 109 PwPD, 102 (93.6%) had no changes in their medication during hospitalization. The remaining seven patients (6.4%) had minor adjustments to their antiparkinsonian medication after admission, with minimal impact. Furthermore, Table 2 summarizes the characteristics of each HY stage. Muscle strength assessments demonstrated excellent interrater reliability (ICC: 0.97–0.99).

Among the 109 PwPD, 89 (81.7%) were classified as independent in the turning in bed task, whereas 20 (18.3%) were classified as nonindependent. In the supine-to-sitting task, 87 patients (79.8%) were in the independent group, whereas 22 (20.2%) were in the nonindependent group. For the sitting-to-supine task, 92 patients (84.4%) were in the independent group, whereas 17 (15.6%) were in the nonindependent group.

For the turning in bed task, the independent group had a median time of 3.98 sec (interquartile range [IQR]: 2.97–5.29) on the right and 3.78 sec (IQR: 3.12–4.99) on the left, with a maximum of 15.7 sec on the right and 11.0 sec on the left. The independent group required a median of 6.06 sec (IQR: 4.76–8.30) to complete the supine-to-sitting task on the right and 6.16 sec (IQR: 4.74–8.17) on the left, with a maximum of 21.0 sec on the right and 19.0 sec on the left. For the sitting-to-supine task, the median time was 5.62 sec (IQR: 4.16–7.26) on the right and 5.79 sec (IQR: 4.25–7.71) on the left, with a maximum of 15.9 sec on the right and 21.2 sec on the left. No patient exhibited significantly prolonged bed immobility.

Analysis of nonindependent movement directions based on the disease-dominant side revealed the following: in the turning in bed task, 13 out of the 20 nonindependent patients (65.0%) had difficulty bilaterally, four of the 20 (20.0%) had difficulty only on the more-affected side, and three of the 20 (15.0%) had difficulty only on the less-affected side. For the supine-to-sitting task, 20 of the 22 nonindependent patients (90.9%) had difficulty bilaterally, 0 (0.0%) had difficulty only on the more-affected side, and two of the 22 (9.1%) had difficulty on the less-affected side. For the sitting-to-supine task, all 17 nonindependent patients (100%) had difficulty bilaterally. The nonindependent patients in each bed mobility task were unable to complete any of the three trials for their affected direction.

Turning in bed

In the turning in bed task, the nonindependent group had significantly greater values for several clinical variables, including MDS-UPDRS Part 3 upper-limb rigidity, lower-limb rigidity, neck rigidity, upper-limb bradykinesia, axial symptoms, and the TMT-A time. Trunk flexion and extension muscle strength were significantly lower in the nonindependent group than in the independent group. No other variables were significantly different between the groups (Table 1).

Univariate logistic analyses revealed significant associations between nonindependence in turning in bed and the following variables: upper-limb rigidity, lower-limb rigidity, neck rigidity, upper-limb bradykinesia, axial symptoms, trunk flexion muscle strength, and trunk extension muscle strength (Table 3). Stepwise multiple logistic regression analysis revealed significant relationships with upper-limb rigidity, axial symptoms, and trunk extension muscle strength. The VIFs ranged from 1.25 to 3.69 (all <10), confirming that there was no multicollinearity (Table 4). ROC analysis for the turning in bed task demonstrated good discriminative ability, with an AUC of 0.84. The optimal cutoff probability value, determined using the Youden index, was 0.127, yielding a sensitivity of 80.0% and a specificity of 67.4% (Figure 2A). Logistic regression analysis indicated that higher scores for upper-limb rigidity and axial symptoms were significantly associated with nonindependence, whereas greater trunk extension strength was significantly associated with independence.

Supine to sitting movements

In the supine-to-sitting task, the nonindependent group had significantly greater values for several clinical variables, including MDS-UPDRS Part 3 upper-limb rigidity, lower-limb rigidity, neck rigidity, upper-limb bradykinesia, lower-limb bradykinesia, axial symptoms, and the TMT-A score. Trunk flexion, trunk extension, and hip flexion muscle strength, as well as MMSE scores, were significantly lower in the nonindependent group than in the independent group. No other variables were significantly different between the groups (Table 1).

Univariate logistic analyses revealed significant associations between nonindependence in the supine-to-sitting task and the following variables: upper-limb rigidity, lower-limb rigidity, neck rigidity, upper-limb bradykinesia, lower-limb bradykinesia, axial symptoms, trunk flexion muscle strength, trunk extension muscle strength, trunk rotation muscle strength, hip flexion muscle strength, the MMSE score, and the TMT-A time (Table 3). Stepwise multiple logistic regression analysis revealed significant relationships with upper-limb rigidity and axial symptoms. ROC analysis demonstrated good discriminative performance of the logistic regression model for predicting nonindependence in the supine-to-sitting task, with an AUC of 0.78. The optimal cutoff probability value, determined using the Youden index, was 0.201, yielding a sensitivity of 72.7% and a specificity of 70.1% (Figure 2B). This model was developed using upper limb rigidity and axial symptoms as explanatory variables, indicating that increased severity of these motor symptoms is associated with a greater likelihood of nonindependence in this task. The regression equation and corresponding coefficients are presented in Figure 2B.

Sitting-to-supine movements

In the sitting-to-supine task, the nonindependent group had significantly greater values for several clinical variables, including MDS-UPDRS Part 3, upper limb rigidity, lower limb rigidity, neck rigidity, upper limb bradykinesia, lower limb bradykinesia, axial symptoms, and the TMT-A time. Trunk flexion, trunk extension, trunk rotation, and hip flexion muscle strength, as well as the MMSE score, were significantly lower in the nonindependent group than in the independent group. No other variables were significantly different between the groups (Table 1).

Univariate logistic analyses revealed significant associations between nonindependence in sitting-to-supine movements and the following variables: upper limb rigidity, lower limb rigidity, neck rigidity, upper limb bradykinesia, lower limb bradykinesia, axial symptoms, trunk flexion muscle strength, trunk extension muscle strength, trunk rotation muscle strength, hip flexion muscle strength, the MMSE score, the TMT-A time, and the TMT-B time (Table 3). Stepwise multiple logistic regression analysis revealed significant relationships with upper-limb rigidity and axial symptoms. ROC analysis demonstrated excellent discriminative performance of the logistic regression model for predicting nonindependence in this task, with an AUC of 0.92. The optimal cutoff probability value, determined using the Youden index, was 0.222, yielding a sensitivity of 94.1% and specificity of 71.7% (Figure 2C). This model was constructed with upper limb rigidity and axial symptoms as explanatory variables, indicating that the severity of these motor symptoms is strongly associated with the likelihood of nonindependence. The regression equation and corresponding coefficients are presented in Figure 2C.

Analysis focused on PwPD at HY stage 4

In the sitting-to-supine task, the nonindependent group had significantly greater values for axial symptoms than did the independent group, but no significant differences in axial symptoms were observed between the turning in bed and supine-to-sitting tasks. The nonindependent PwPD at HY stage 4 also showed significant differences in several clinical variables from the independent group (Table 5). In the supine-to-sitting and sitting-to-supine tasks, the nonindependent group had significantly lower MMSE scores and higher TMT-A times than the independent group did.

Although statistical analyses were not performed for patients at HY stages 2–3 due to the very small number of nonindependent cases in each bed mobility task (Table 2), these patients appeared to exhibit similar clinical features, such as increased axial symptoms, to those observed at HY stage 4.

DISCUSSION

This cross-sectional study is the first to comprehensively investigate factors associated with nonindependence in turning in bed, supine-to-sitting, and sitting-to-supine movements in individuals with PD. Surprisingly, upper limb rigidity and axial symptoms were identified as common factors despite the distinct nature of these movements.

Axial symptoms were significantly greater in the nonindependent group across all three bed mobility tasks. Both univariate and multiple logistic regression analyses identified axial symptoms as common factors associated with nonindependence in PwPD. These findings suggest that bed mobility, such as standing and walking, requires substantial postural control. Therefore, axial symptoms may also contribute to bed mobility impairments. Because axial symptoms reflect postural control deficits, we speculate that 1) increased axial symptoms impair postural control in all three bed mobility tasks and that 2) these symptoms contribute to nonindependence in these movements.

Bed mobility tasks commonly involve postural transitions where the movement of the body’s center of mass against a base of support demands substantial postural control. However, each task places different demands on the postural system. Turning in bed requires axial rotation, core stability, and adaptive postural control, as the base of support changes sequentially during rolling. Supine-to-sitting movement involves concentric trunk control against gravity. Sitting-to-supine movement requires controlled lowering of the trunk, demanding eccentric control. These task-specific biomechanical demands may explain the varying rates of nonindependence observed across the three movements.

PwPD can be classified into postural instability and gait difficulty (PIGD), tremor-dominant (TD), and intermediate subtypes [34]. The PIGD subtype typically has higher axial symptom scores than the TD subtype does, suggesting a greater likelihood of bed mobility impairments. Given the strong association between axial symptoms and postural control deficits, PwPD with the PIGD subtype may be particularly vulnerable to difficulties in bed mobility. Notably, in our overall analyses spanning HY stages 2–4, axial symptoms were associated with all bed mobility tasks. However, in the HY stage 4 subgroup analysis, no significant differences in axial symptoms were observed between the independent and nonindependent groups for the turning in bed and supine-to-sitting tasks. Axial symptoms tend to worsen as PD progresses [35], suggesting that this progression may be linked to decreased independence in turning in bed and supine-to-sitting movements. Notably, among PwPD at HY stage 4, those unable to independently perform sitting-to-supine movements presented more severe axial symptoms, indicating that postural control deficits have a greater impact on sitting-to-supine movements in advanced PD.

Upper-limb rigidity was a key factor contributing to nonindependence across all bed mobility tasks. These findings suggest that PwPD may rely on compensatory upper-limb movements to generate turning force or control the center of mass movements. Increased upper-limb rigidity may compromise such compensatory strategies, resulting in movement difficulties and loss of independence.

Previous studies have reported weak correlations between nighttime turning in bed movement speed and rigidity, likely due to differences in assessment timing and medication states [2]. A notable strength of the present study is its clear demonstration of a relationship between nonindependence in turning in bed movements and upper-limb rigidity, which were all assessed under consistent medication conditions. Furthermore, a previous study on supine-to-sitting movements [26] reported that upper-limb rigidity was associated with the time required for supine-to-sitting movements in PwPD who could perform the task independently. While our study focused on movement independence rather than movement time, our findings suggest that upper-limb rigidity is equally important in determining task performance [26]. These findings suggest that upper-limb rigidity affects not only movement speed but also independence in supine-to-sitting movements.

Given the role of upper-limb compensatory mechanisms, rigidity in this region may interfere with bed mobility in task-specific ways: it may hinder effective force transmission and coordination during turning, reduce the drive required for trunk flexion in the supine-to-sitting movement, and impair the controlled descent of the trunk during the sitting-to-supine movement.

Trunk extension strength, identified as a factor related to nonindependence in turning in bed, is crucial for controlling trunk rotation in the latter half of movement. Reduced trunk extension strength may impair braking capacity in the latter part of turning, making it difficult to generate sufficient rotational force and leading to nonindependence.

Notably, in both the overall analysis and the HY stage 4 subgroup, the nonindependent group for supine-to-sitting and sitting-to-supine movements had lower MMSE scores and longer TMT-A times than the independent group did. These movements involve sequential actions with multiple components, requiring a high degree of cognitive control and sustained attention. Because cognitive function declines in advanced PD [36], it is likely that reduced cognitive function and attention contribute to nonindependence in supine-to-sitting and sitting-to-supine movements.

This study has several limitations. First, as a cross-sectional study, causal relationships cannot be established. Second, bed mobility assessment and clinical evaluation were performed only during the “ON” phase. PwPD who are nonindependent in bed during the “ON” phase are likely to remain so during the “OFF” phase. However, as bed mobility during the “OFF” phase was not assessed, factors influencing independence in the “OFF” phase remain unclear. Third, although this study evaluated the strength of the neck, trunk, and proximal lower limbs, it did not assess other potentially relevant strengths, such as upper-limb strength or other lower-limb strengths beyond the hip flexors (e.g., quadriceps, hamstrings, calves), which may also affect bed mobility. Fourth, although factors such as neuropsychiatric symptoms, sleep quality, or home environment factors could affect bed mobility, we did not include these variables in the current analysis. However, depressive symptoms have been reported to be associated with bradykinesia [37], suggesting that depression may indirectly affect bed mobility independence through these motor symptoms. Future studies should consider the possible indirect influence of neuropsychiatric symptoms, including depression, on bed mobility. In addition, evaluations were conducted in a controlled hospital setting, and bed mobility independence prior to admission was not assessed, which may limit the generalizability of the findings to daily life. However, conducting evaluations in a controlled hospital setting helped reduce confounding effects from environmental variability, which is a strength of the present study.

Future research should employ longitudinal designs incorporating clinical assessments of bed mobility during both the “ON” and “OFF” medication phases and consider a broader range of contributing factors, including nonmotor symptoms and a more comprehensive assessment of physical strength, to better elucidate the progression of bed mobility decline in PwPD. In addition, investigating the relationship between bed mobility and other functional movements, such as rising from a chair and transferring, may help elucidate shared mechanisms underlying mobility limitations.

Conclusions

This study identified factors associated with nonindependence in turning in bed, lying to sitting, and sitting to lying in PwPD. Axial symptoms and upper limb rigidity were common factors across all bed mobility tasks, whereas specific factors were identified for each movement. Early rehabilitation interventions focusing on postural control and upper limb function may help maintain bed mobility independence in PwPD. Additionally, cognitive function, as indicated by the TMT-A time, should be considered when assessing and planning rehabilitation, particularly for patients at HY stage 4. While the hospital-based design limits generalizability to daily life, addressing these factors through multidisciplinary interventions may help mitigate the decline in bed mobility independence in PwPD.

Notes

Conflicts of Interest

The authors have no financial conflicts of interest.

Funding Statement

This research was supported by the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (KAKENHI) (grant no. 23K10549).

Acknowledgments

We thank all participants and thank the physicians, staff of the Rehabilitation Department, and collaborators of Hokkaido Neurological Hospital.

Author Contributions

Conceptualization: Masaru Narita, Yohei Okada. Data curation: Masaru Narita, Kosuke Sakano. Formal analysis: Masaru Narita. Funding acquisition: Yohei Okada. Investigation: Masaru Narita. Methodology: Masaru Narita, Yohei Okada. Project administration: Yohei Okada. Resources: Yohei Okada. Supervision: Shinsuke Hamada, Fumio Moriwaka. Validation: Yuichi Nakashiro. Visualization: Masaru Narita. Writing—original draft: Masaru Narita, Yohei Okada. Writing—review & editing: all authors. Writing—review & editing: all authors.

Figure 1.

Bed mobility tasks. Turning in bed (A), supine-to-sitting (B), and sitting-to-supine (C).

Figure 2.

ROC curves of the models used to predict the nonindependence of bed mobility among patients with PD (n=109) by multiple logistic regression analyses. Turning in bed (A), supine-to-sitting (B), and sitting-to-supine (C). Prediction formula for nonindependence in turning in bed: nonindependent = -2.513+1.148×upper limb rigidity+1.194×axial symptoms+(-1.311)×trunk extension strength. Cut-off value: 0.127. Sensitivity: 0.800. Specificity: 0.674. Prediction formula for nonindependence in supine-to-sitting: nonindependent = -5.032+1.146×upper limb rigidity+1.161×axial symptoms. Cut-off value: 0.201. Sensitivity: 0.727. Specificity: 0.701. Prediction formula for nonindependence in sitting-to-supine: nonindependent = -9.467+1.922×upper limb rigidity+2.645×axial symptoms. Cut-off value: 0.222. Sensitivity: 0.941. Specificity: 0.717. AUC, the area under the curve; ROC, receiver operating characteristic; PD, Parkinson’s disease.

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