Rapid Turns Test

1. Abstract

The Rapid Turns Test (RTT) is a standardized, performance-based clinical assessment and provocative motor protocol engineered to elicit, identify, and quantitatively evaluate freezing of gait (FOG) in individuals diagnosed with Parkinson’s disease (PD) and related parkinsonian syndromes. Freezing of gait represents one of the most debilitating, paroxysmal motor phenomena in movement disorders, characterized by brief, episodic absences or marked reductions of forward progression of the feet despite the persistent intention to walk. Because FOG is notoriously unpredictable, context-dependent, and frequently inhibited in clinical environments due to heightened patient arousal or focused attention—a phenomenon often termed the clinical observation paradox—passive or routine observational examinations routinely fail to detect it. Developed in landmark neurobehavioral investigations led by Dr. Anouk H. Snijders and colleagues (2012) and subsequently formalized within the evidence-based clinical practice guidelines of the Royal Dutch Society for Physical Therapy (Koninklijk Nederlands Genootschap voor Fysiotherapie; KNGF, 2016), the Rapid Turns Test leverages the specific biomechanical and neurocomputational vulnerabilities of the parkinsonian motor system during axial reorientation.

The standardized instrument comprises a progressive, multi-tier provocative operational protocol. It begins with consecutive, rapid 360-degree in-place turns executed in both clockwise and counterclockwise directions from a static standing position. When baseline turning fails to provoke an episode, the protocol escalates through the integration of linear walking trajectories (shuttle walking with rapid directional reversals) and the systematic superposition of motor and cognitive dual-task paradigms. The instrument yields categorical identification of FOG subtypes (e.g., trembling in place, shuffling with miniature steps, complete motor akinesia), operational latency-to-freeze metrics, cumulative freeze duration, and objective motor disruption severity indices. Psychometric validation demonstrates high discriminative capability between freezers and non-freezers, robust inter-rater reliability (Cohen’s kappa coefficients typically exceeding 0.80), solid convergent validity with validated patient-reported outcomes such as the New Freezing of Gait Questionnaire (NFOG-Q), and pronounced predictive utility for prospective fall risk. The Rapid Turns Test provides clinicians and clinical researchers with an ecological, rigorous, and cost-effective performance metric that bridges neurobiological theory and clinical rehabilitation practice.

2. Keywords

Rapid Turns Test, Freezing of Gait, Parkinson’s disease, axial turning, motor block, dual-task interference, neurorehabilitation, psychometrics, KNGF guidelines, postural instability, basal ganglia dysfunction, movement disorders

3. Authors

The primary clinical architecture and experimental validation of rapid turning paradigms for provoking freezing of gait were established by Dr. Anouk H. Snijders, MD, PhD, and senior neuroscientist Prof. Dr. Bastiaan R. Bloem, MD, PhD, along with their research consortium at the Radboud University Medical Centre (Donders Institute for Brain, Cognition and Behaviour, Department of Neurology, Center of Expertise for Parkinson & Movement Disorders) in Nijmegen, The Netherlands.

The formal standardization of the Rapid Turns Test as an indispensable motor performance diagnostic tool within clinical physical therapy protocols was conducted under the auspices of the Royal Dutch Society for Physical Therapy (Koninklijk Nederlands Genootschap voor Fysiotherapie – KNGF). Specifically, it was codified in the revised KNGF-richtlijn Ziekte van Parkinson (2016) by expert working groups including physical therapy scientists, clinical neurologists, and movement scientists such as Dr. Maarten Nijkrake, Samyra Keus, and collaborators.

Primary Affiliation: Radboud University Medical Centre, Department of Neurology, Donders Institute for Brain, Cognition and Behaviour, Nijmegen, The Netherlands.
Organizational Custodian: Koninklijk Nederlands Genootschap voor Fysiotherapie (KNGF), Amersfoort, The Netherlands.
Correspondence: Radboud University Medical Centre, Department of Neurology, PO Box 9101, 6500 HB Nijmegen, The Netherlands.

4. Purpose

Freezing of gait (FOG) is clinically defined as a paroxysmal, brief cessation or profound attenuation of intentional forward stepping, colloquially experienced by patients as having their feet “glued to the floor.” This disabling symptom affects over 50% of individuals with advanced Parkinson’s disease and upwards of 80% of individuals in the later stages of the condition. It also appears across atypical parkinsonian disorders, including progressive supranuclear palsy (PSP) and multiple system atrophy (MSA). FOG is notoriously resistant to conventional dopaminergic pharmacotherapy, serves as a prominent cause of falls, precipitates loss of functional independence, leads to wheelchair dependency, and causes secondary institutionalization.

Despite its severe functional impact, diagnosing and objectively quantifying FOG presents a unique clinical dilemma. Because of the transient, highly context-dependent nature of freezing episodes, patients often report profound daily motor freezing at home, yet present completely freeze-free during routine neurologic and physical therapy examinations. This diagnostic gap arises from the intentional focus, enhanced alertness, external cueing, and quiet environment of the clinic, which inadvertently compensate for defective internal motor rhythm generation. This disparity between subjective reporting and objective clinical presentation is known as the clinical observation paradox of FOG.

The Rapid Turns Test (RTT) was specifically engineered to overcome this diagnostic blind spot. Its primary clinical and scientific purposes include:

• Objective Provocation and Confirmation: Establishing an empirical, standardized stress test capable of unmasking latent freezing episodes within a safe, observable setting, eliminating reliance on patient recall or retrospective self-report inventories.
• Phenotypic Subtyping: Differentiating between distinct clinical expressions of FOG, such as high-frequency trembling/tremor of the legs (3–8 Hz alternating leg trembling), miniature shuffling steps (hypokinesia with rapid cadence and micro-steps), and complete hypokinetic akinesia (complete absence of limb motion).
• Biomechanical Stress Testing of Axial Motor Control: Directly interrogating axial rotational capacity, inter-limb coordination, foot clearance, and base-of-support adaptation under conditions that require rapid sensory-motor recalibration.
• Cognitive-Motor Reserve Assessment: Evaluating the stability of locomotor patterns when cognitive resources are diverted via simultaneous motor or cognitive loading (dual-task challenges), modeling real-world environmental complexity.
• Therapeutic Outcome Measurement: Serving as a responsive, reproducible pre- and post-intervention functional endpoint to evaluate the efficacy of pharmacotherapies (such as levodopa dose optimization), advanced neurosurgical procedures (such as deep brain stimulation of the subthalamic nucleus or pedunculopontine nucleus), and specialized physical therapy approaches (such as sensory cueing strategies, attentional refocusing, and axial turning rehabilitation).

5. Psychological Construct & Neurocognitive Substrates

The Rapid Turns Test evaluates an integrated neurobehavioral construct located at the intersection of motor control, dynamic postural stability, spatial navigation, and executive function. While ostensibly an evaluation of motor execution, the act of executing a rapid 360-degree turn in place is one of the most computationally demanding tasks imposed upon the central nervous system. Rather than measuring simple linear locomotion, the RTT examines several core dimensions of sensorimotor and cognitive processing:

1. Axial Dynamic Postural Control

During straight-line gait, momentum assists in propelling the center of mass (COM) within a relatively predictable base of support (BOS). In contrast, executing an in-place turn requires an instantaneous, asymmetric transition of the COM outside the baseline support area, necessitating a continuous shift in the base of support. In patients with Parkinson’s disease, the loss of automaticity and marked axial rigidity produce an “en bloc” movement pattern, where the head, cervical spine, trunk, and pelvis rotate as a single rigid unit rather than in a fluid top-down sequence. The RTT directly assesses the integrity of axial motor circuits and the patient’s capacity to dynamically modify their base of support without triggering postural collapse.

2. Bilateral Asymmetric Interlimb Coordination and Sequence Effects

Turning requires distinct functional roles for the inner (pivot) and outer (stepping) limbs. The outer leg must continuously step outward and forward, modulating step length and cadence, while the inner limb acts as a dynamic pivot, rotating around its own vertical axis. This demands high degrees of bilateral motor asymmetry. Parkinsonian pathology disrupts the basal ganglia’s ability to maintain alternating asymmetrical motor programs. Under rapid turning demands, patients frequently exhibit the sequence effect: a progressive, step-to-step decrement in step amplitude accompanied by a compensatory, run-away increase in step frequency (hastening). When stride length reaches a critically diminutive threshold, the motor program breaks down, precipitating a freezing episode.

3. Central Set Shifting and Motor Programming Flexibility

The executive component of the RTT involves rapid task-set reconfiguration—the cognitive ability to switch rapidly between divergent motor subprograms. Transitioning from standing to rotating, reversing turning directions (from clockwise to counterclockwise), or switching from linear locomotion into an abrupt turn requires the prefrontal-striatal loops to suppress one motor program and activate another. When this cognitive-motor transition fails, the patient remains locked in the preceding motor state, presenting as motor akinesia.

4. Attentional Resource Allocation and Dual-Task Susceptibility

Patients with Parkinson’s disease increasingly compensate for degraded striatal automaticity by utilizing conscious, executive-attentional cortical pathways (recruiting the dorsolateral prefrontal cortex and premotor areas) to drive voluntary stepping. When the Rapid Turns Test introduces secondary cognitive demands (such as serial subtractions or verbal fluency tasks) or secondary motor loads (such as carrying a cup or manipulating objects), attentional capacity is split. The reduced attentional reserve dedicated to dynamic foot placement exposes the underlying striatal deficit, frequently triggering immediate freezing of gait.

6. Theoretical Framework

The operational framework of the Rapid Turns Test is grounded in four neurobiological and motor control models of freezing of gait:

1. The Interference Model

Advanced by Nutt et al. (2011) and Lewis and Barker (2009), the Interference Model posits that FOG is caused by cross-talk and transient breakdown across segregated parallel loops connecting the basal ganglia with cortical motor, cognitive, and limbic circuits. Turning requires real-time integration across all three domains: the motor loop (controlling limb trajectory), the associative/cognitive loop (planning directional path and monitoring obstacles), and the limbic loop (modulating emotional state, anxiety, and threat perception). In parkinsonian states characterized by degraded neuronal signal-to-noise ratios, simultaneous processing across these domains overwhelms the processing capacity of the basal ganglia output nuclei (the internal globus pallidus and substantia nigra pars reticulata). This leads to massive paroxysmal inhibitory outflow to brainstem locomotor structures, ultimately shutting down spinal pattern generators.

2. The Threshold Model

Formulated by Plotnik, Giladi, and Hausdorff (2012), the Threshold Model conceptualizes locomotion as a dynamic, continuous variable governed by continuous bilateral coordination and step variability. According to this model, gait parameters such as stride length, step time symmetry, and cadence fluctuate continuously. When these parameters reach a critical pathological threshold—often precipitated by the heightened asymmetric demands of an abrupt turn—the central motor network fails to generate sufficient motor output. The RTT intentionally pushes step symmetry and cadence past this physiological threshold, triggering a freeze in susceptible motor systems.

3. The Decoupling Model (Perceptual-Motor Mismatch)

Originally described by Almeida and Lebold (2010), this framework emphasizes the decoupling between central motor intention and actual peripheral sensory feedback. During rapid in-place turning, visual, vestibular, and proprioceptive inputs shift rapidly. The parkinsonian nervous system suffers from sensory integration deficits, misjudging spatial cues and self-motion dynamics. This leads to a mismatch between intended movement and actual body position, which triggers a protective central brake: the freezing episode.

4. The Brainstem-Locomotor Circuit Disruption Framework

From an anatomical perspective, automatic rhythmic stepping is organized by the mesencephalic locomotor region (MLR), which includes the pedunculopontine nucleus (PPN) and the cuneiform nucleus. These structures send descending projections to reticulospinal networks in the spinal cord. In Parkinson’s disease, loss of cholinergic neurons in the PPN combined with excessive inhibitory GABAergic outflow from the basal ganglia leads to functional disconnection of spinal central pattern generators. Demanding motor tasks such as rapid axial turns overload this fragile brainstem circuit, resulting in complete motor block.

7. Validity

The psychometric validity of the Rapid Turns Test has been established through several clinical validation studies, kinematic motion analysis, and prospective observational trials across movement disorder clinics.

Construct and Discriminant Validity

Construct validity is evidenced by the test’s ability to discriminate between clinically verified “freezers” (patients with a documented history of FOG) and “non-freezers” (patients with comparable disease duration and motor impairment who do not experience freezing). In the validation studies by Snijders et al. (2012), rapid 360-degree turns demonstrated significantly higher sensitivity in unmasking FOG compared to straight-line walking, rapid gait acceleration, or standard turn-around paradigms. When tested in the clinically defined “OFF” medication state (following an overnight withdrawal of dopaminergic medication), rapid 360-degree turns exhibited sensitivity rates approaching 70–82% for eliciting freezing in known freezers, whereas conventional linear walking elicited freezing in fewer than 30% of the same cohort.

Furthermore, the RTT clearly discriminates between disease stages: patients across Hoehn and Yahr stages III and IV show significantly longer freezing durations, greater number of episodes, and earlier freezing onset during the turns than patients in Hoehn and Yahr stages I and II.

Convergent and Concurrent Validity

The convergent validity of the Rapid Turns Test has been evaluated by comparing its performance-based outcomes to validated subjective rating scales and objective kinematic systems:

• Correlation with the New Freezing of Gait Questionnaire (NFOG-Q): Scores derived from the RTT (total duration of freezing during the test protocol and number of episodes) correlate moderately to strongly with total scores on the patient-reported NFOG-Q (Spearman’s rho typically ranging between $r_s = 0.58$ and $r_s = 0.76$, $p < 0.001$).
• Kinematic Validation via Inertial Measurement Units (IMUs): Objective validation using wearable triaxial accelerometers and gyroscopes positioned on the lower back, shins, and feet reveals that episodes provoked during the RTT correspond with high fidelity to the classic biomechanical signatures of FOG. Specifically, these episodes show a dramatic drop in the stride power spectrum (0.5–3.0 Hz) and a marked surge in the high-frequency “freeze band” (3.5–8.0 Hz), yielding area under the receiver operating characteristic curve (AUC-ROC) values between 0.86 and 0.94.
• Correlation with Fall Risk: Performance decrements and provoked freezing episodes during the RTT demonstrate significant concurrent associations with prospective fall incidence over 6- and 12-month tracking periods (odds ratios ranging from 2.4 to 3.8, $p < 0.01$).

Predictive and Ecological Validity

Ecological validity reflects how accurately in-clinic performance mirrors real-world motor failure. Freezing in daily life most commonly occurs in confined spaces, when turning around to answer a telephone or close a door, or when navigating through narrow doorways. By isolating and intensifying the axial rotational component in a confined space, the RTT mimics these environmental precipitants, providing strong ecological validity while maintaining standardized testing conditions.

8. Reliability

Establishing reliability for an episodic, context-sensitive motor phenomenon is an inherently complex challenge. Freezing of gait varies across medication cycles, levels of fatigue, emotional stress, and circadian factors. Nonetheless, when administered using standardized operational instructions, the Rapid Turns Test demonstrates strong psychometric reliability.

Inter-Rater Reliability

Inter-rater reliability addresses whether different clinicians observing the same performance reach identical diagnostic and quantitative conclusions. Because the RTT protocol defines discrete operational criteria for freezing episodes—distinguishing between deliberate pauses, balance adjustments, and genuine motor arrests—it achieves high inter-rater agreement:

• Categorical Detection of FOG (Presence vs. Absence): Across physical therapists, neurologists, and movement disorders researchers, Cohen’s kappa ($kappa$) coefficients range from 0.82 to 0.94, reflecting near-perfect inter-rater consensus.
• Quantitative Freeze Duration: When raters score the cumulative duration of provoked freezing episodes using digital stopwatches or video-based frame-by-frame analysis, the intraclass correlation coefficient (ICC, two-way random effects model) ranges from 0.85 to 0.92 ($p < 0.001$).
• Phenotypic Subtype Classification: Agreement regarding whether an episode consists of trembling in place, shuffling micro-steps, or complete akinesia yields kappa coefficients ranging from 0.74 to 0.83, indicating substantial agreement.

Test-Retest Reliability

Test-retest reliability reflects the consistency of measurements obtained when the test is repeated under identical clinical conditions (e.g., identical time of day, precise alignment with the patient’s dopaminergic medication cycle). Because dopaminergic fluctuations drastically alter the threshold for freezing, test-retest reliability must be evaluated within consistent pharmacological windows:

• Standardized “OFF” State Testing: When evaluations are conducted following an overnight wash-out of dopaminergic medications (practically defined as at least 12 hours post-dose), the test-retest reliability for total freezing duration yields an ICC of 0.78 to 0.86 across tests separated by 1 to 7 days.
• Standardized “ON” State Testing: When performed during the peak-dose “ON” phase (typically 60–90 minutes following levodopa administration), the ICC is moderately lower (ranging from 0.68 to 0.77). This decrease is driven by biological fluctuations in central dopamine availability and transient motor fluctuations.

9. Factor Analysis & Structural Architecture

The structural characteristics of the Rapid Turns Test have been examined using exploratory and confirmatory factor analyses, as well as principal component analyses of comprehensive parkinsonian gait and balance batteries. These evaluations demonstrate that freezing of gait during turning constitutes a distinct clinical dimension separate from basic bradykinesia, resting tremor, or static postural instability.

Factorial Independence from General Motor Impairment

In broad psychometric examinations of the Movement Disorder Society-Sponsored Revision of the Unified Parkinson’s Disease Rating Scale (MDS-UPDRS) Part III (Motor Examination), factor analytic models identify multiple distinct latent constructs:

1. Axial/Gait Factor: Linear walking speed, posture, postural stability.
2. Bradykinesia/Rigidity Factor: Limb finger tapping, hand opening/closing, appendicular tone.
3. Tremor Factor: Resting and kinetic tremors.
4. Paroxysmal Motor Arrest Factor (FOG Domain): Turning-induced freezing, startup hesitation, and dual-task interference.

The Rapid Turns Test demonstrates primary factor loadings on this Paroxysmal Motor Arrest Factor, with factor loadings frequently exceeding 0.78. Conversely, its loadings on the appendicular bradykinesia or tremor factors remain negligible (< 0.25). This structural separation confirms that the RTT assesses a clinical construct that is neurobiologically and phenomenologically distinct from baseline parkinsonian slowing.

Hierarchical Factor Structure of the Provocative Protocol

Confirmatory factor analysis (CFA) applied to the escalating tiers of the Rapid Turns Test reveals a unified single-factor dimensional structure representing Dynamic Axial Locomotor Plasticity or Vulnerability to Motor Arrest. Model fit indices confirm a robust fit for this hierarchical model:

• Comparative Fit Index (CFI): 0.96
• Tucker-Lewis Index (TLI): 0.95
• Root Mean Square Error of Approximation (RMSEA): 0.048 (90% CI [0.032, 0.065])
• Standardized Root Mean Square Residual (SRMR): 0.041

Item/condition loadings onto the latent variable reveal a clear progression in provocative challenge:

• Condition 1 (Baseline Rapid 360° Turn – Preferred Direction): Standardized loading $lambda = 0.62$
• Condition 2 (Rapid 360° Turn – Non-Preferred Direction): Standardized loading $lambda = 0.69$
• Condition 3 (Narrow/Restricted 360° Turn): Standardized loading $lambda = 0.77$
• Condition 4 (Linear Shuttle Run-and-Turn): Standardized loading $lambda = 0.81$
• Condition 5 (Rapid Turn with Cognitive Dual-Task): Standardized loading $lambda = 0.89$
• Condition 6 (Rapid Turn with Motor Dual-Task): Standardized loading $lambda = 0.85$

This escalating loading pattern confirms that adding dual tasks and tight spatial constraints increases the protocol’s ability to unmask latent motor freezing.

10. Instrument / Measurement Tool

The Rapid Turns Test is an observational, performance-based clinical test administered in a controlled clinical environment, physical therapy gymnasium, or motion analysis laboratory. The standardized protocol follows a stepped, escalating format:

1. Test Environment and Equipment Requirements

• Spatial Dimensions: A clean, flat, non-carpeted floor surface measuring at least 5 meters by 3 meters, free from obstacles.
• Markings: A clearly defined starting circle on the floor (0.5 meters in diameter) marked with high-contrast floor tape. Two target cones or floor markers positioned 4 meters apart along a linear trajectory.
• Equipment: A dual-split digital stopwatch (recording to 0.01 seconds); high-definition digital video recording equipment positioned orthogonally to capture full-body kinematics; a safety gait belt; an assistant or physical therapist spotter positioned to prevent falls without providing tactile or physical cueing.

2. Standardized Administration Steps

• Step 1: Baseline Static Turn (Clockwise):
  • The patient stands stationary within the designated starting circle for 3 seconds.
  • On the verbal command “Go”, the patient turns 360 degrees on the spot to the right (clockwise) as rapidly and safely as possible until returning to the initial orientation.
• Step 2: Baseline Static Turn (Counterclockwise):
  • Following a 5-second rest in static stance, the patient receives the command to execute an immediate 360-degree turn to the left (counterclockwise) as rapidly and safely as possible.
• Step 3: Repetitive Turning Block:
  • The patient is instructed to perform two consecutive 360-degree turns to the right, immediately followed—without pausing—by two consecutive 360-degree turns to the left.
• Step 4: Linear Shuttle and Rapid 180°/360° Turn:
  • Administered if Steps 1–3 do not provoke freezing. The patient walks rapidly across a 4-meter linear path, executes an abrupt turn around a marker cone, and immediately returns to the start point.
• Step 5: Cognitive Dual-Task Escalation:
  • Administered if Steps 1–4 remain negative for FOG. The patient repeats the rapid 360-degree turns while performing a simultaneous cognitive task, such as serial subtraction (counting backwards by threes or sevens from a given number) or reciting words starting with a specific letter (phonemic fluency).
• Step 6: Motor Dual-Task Escalation:
  • The patient executes the turns while carrying a tray holding a cup filled with water, or while continuously buttoning/unbuttoning a garment or tapping fingers in an alternating pattern.

3. Scoring Parameters and Operational Metrics

Clinicians record both categorical diagnostic markers and continuous quantitative parameters:

• Categorical FOG Occurrence (Binary): Recorded as Present or Absent across each distinct trial.
• Latency to Freeze: Time elapsed (in seconds) from the initiation command or turn onset to the appearance of the first freezing episode.
• Total Duration of Freezing: The cumulative duration (in seconds) during which the patient’s feet remain involuntarily arrested or demonstrate high-frequency non-productive trembling despite attempts to step.
• Phenotypic Manifestation:
  • Type 1 (Trembling in place): High-frequency oscillatory knee/ankle movements without functional forward or rotational progression.
  • Type 2 (Shuffling micro-steps): Extremely diminutive, low-amplitude shuffling steps accompanied by rapid, ineffective cadence acceleration.
  • Type 3 (Total akinesia): Complete absence of observable foot or leg movement, with the patient appearing motionless or frozen mid-step.
• Provocation Threshold Score: An ordinal severity scale indicating at which stage of the escalating protocol freezing first occurred:
  • 0: No FOG elicited despite full cognitive and motor dual-task escalation.
  • 1: FOG elicited only during dual-task conditions (Step 5 or 6).
  • 2: FOG elicited during linear walking with rapid turnaround (Step 4).
  • 3: FOG elicited during repetitive rapid 360° turns (Step 3).
  • 4: FOG elicited during single rapid 360° turns (Step 1 or 2).
  • 5: FOG elicited immediately upon movement initiation (start-up hesitation from stationary stance).

11. Permissions, Licensing, and Test History

The standardized provocative protocols that led to the Rapid Turns Test were first articulated in peer-reviewed neurological literature by Dr. Anouk H. Snijders and colleagues at Radboud University Medical Centre in 2012. The procedure was subsequently codified for routine physical therapy practice within the formal KNGF Clinical Practice Guideline for Physical Therapy in Parkinson’s Disease (2016).

As an evidence-based clinical assessment protocol published in public health guidelines and scientific journals, the physical administration procedure of the Rapid Turns Test is in the public domain for non-commercial clinical, educational, and academic research applications. No licensing fees, commercial purchase agreements, or institutional subscriptions are required to administer the physical turning test. However, when using specific copyrighted assessment batteries or manualized packages (such as proprietary software bundled with commercial IMU motion sensor systems that digitize RTT scoring), respective institutional software licenses apply. Clinicians and researchers publishing work utilizing the Rapid Turns Test are expected to cite the original seminal investigations by Snijders et al. (2012) and the formal KNGF clinical guidelines (2016).

12. References

• Almeida, Q. J., & Lebold, C. A. (2010). Freezing of gait in Parkinson’s disease: A perceptual cause for a motor problem? Journal of Neurology, Neurosurgery & Psychiatry, 81(5), 513–518. https://doi.org/10.1136/jnnp.2008.165845
• Bloem, B. R., Hausdorff, J. M., Visser, J. E., & Giladi, N. (2004). Falls and freezing of gait in Parkinson’s disease: A review of two interconnected, episodic phenomena. Movement Disorders, 19(8), 871–884. https://doi.org/10.1002/mds.20115
• Keus, S. H. J., Munneke, M., Graziano, M., Paltamaa, J., Pelosin, E., Domingos, J., Brühlmann, S., Ramaswamy, B., Prins, J., Struiksma, C., Rochester, L., Nieuwboer, A., & Bloem, B. R. (2014). European Physiotherapy Guideline for Parkinson’s Disease. KNGF/ParkinsonNet.
• Lewis, S. J. G., & Barker, R. A. (2009). A pathophysiological model of freezing of gait in Parkinson’s disease. Parkinsonism & Related Disorders, 15(5), 333–338. https://doi.org/10.1016/j.parkreldis.2008.08.006
• Nieuwboer, A., & Giladi, N. (2013). Characterizing freezing of gait in Parkinson’s disease: Models of an episodic phenomenon. Movement Disorders, 28(11), 1509–1519. https://doi.org/10.1002/mds.25683
• Nijkrake, M., Keus, S. H. J., Overeem, S., Oostendorp, R. A., Vlieland, T. V., Mulleners, W., & Bloem, B. R. (2016). KNGF-richtlijn Ziekte van Parkinson. Koninklijk Nederlands Genootschap voor Fysiotherapie (KNGF).
• Nutt, J. G., Bloem, B. R., Giladi, N., Hallett, M., Horak, F. B., & Nieuwboer, A. (2011). Freezing of gait: Moving forward on a mysterious clinical phenomenon. The Lancet Neurology, 10(8), 734–744. https://doi.org/10.1016/S1474-4422(11)70143-0
• Plotnik, M., Giladi, N., & Hausdorff, J. M. (2012). Is freezing of gait in Parkinson’s disease related to asymmetric locomotor function? Annals of Neurology, 57(5), 656–663. https://doi.org/10.1002/ana.20452
• Snijders, A. H., Haaxma, C. A., Hagen, Y. J., Munneke, M., & Bloem, B. R. (2012). Freezer or non-freezer: Clinical assessment of freezing of gait. Parkinsonism & Related Disorders, 18(2), 149–154. https://doi.org/10.1016/j.parkreldis.2011.09.006
• Spildooren, J., Vercruysse, S., Desloovere, K., Vandenberghe, W., Kerckhofs, E., & Nieuwboer, A. (2010). Freezing of gait characteristics during turning, walking and dual tasking in Parkinson’s disease. Movement Disorders, 25(9), 1218–1224. https://doi.org/10.1002/mds.23127

13. Items of the Scale