Factor Occupational Rating System Scale

1. Abstract

The Factor Occupational Rating System Scale (FORSS), alternatively designated as the Occupational Rating Scale of the Cincinnati Knee Rating System (CKRS), is an ergonomically grounded, patient-reported and clinician-administered assessment instrument designed to quantify physical mechanical exposure and occupational joint demands placed upon the human knee joint within the workplace environment. Developed initially by Frank R. Noyes, Linda A. Mooar, and Sue D. Barber in 1991, and subsequently adapted and cross-culturally validated for European Dutch-speaking populations by Strik, Aufdemkampe, Neeb, and Mastenbroek in 1998, the instrument operationalizes work-related joint loading along the International Classification of Functioning, Disability and Health (ICF) participation dimension. The scale systematically evaluates seven distinct ergonomic variables: walking, standing, sitting, climbing stairs or ladders, running, squatting or kneeling, and heavy lifting/carrying. Each occupational factor is scored through a composite matrix capturing three core mechanical exposure parameters: frequency (how often an activity occurs during an average shift), intensity (the physical load, external resistance, or biomechanical force required), and duration (the cumulative time spent executing the loading pattern per workday). The resultant score categorizes occupational demand into discrete hierarchical functional strata, ranging from sedentary work with minimal lower-extremity demand to very heavy, strenuous manual labor characterized by repetitive deep knee flexion, high axial impact, and excessive load transport. Psychometric evaluations of the scale demonstrate strong construct validity through robust correlations with established orthopedic and functional activity metrics (such as the Tegner Activity Scale, the Lysholm Knee Scoring Scale, and the Short Form-36 Physical Functioning subscale), high test-retest reliability across multiple clinical populations with anterior cruciate ligament (ACL) reconstructions, meniscal pathology, and tibiofemoral osteoarthritis (intraclass correlation coefficients generally exceeding 0.85), and robust discriminative capacity distinguishing light-duty desk workers from heavy industrial or construction personnel. The FORSS serves as an indispensable tool in occupational health, orthopedic surgery, sports medicine, and vocational rehabilitation, providing an objective, standardized metric to guide return-to-work clearance, establish ergonomic accommodations, and quantify occupational biomechanical risk.

2. Keywords

Factor Occupational Rating System Scale, FORSS, Cincinnati Knee Rating System, occupational knee load, biomechanical exposure, orthopedic assessment, ergonomic demand, work participation, anterior cruciate ligament, functional capacity evaluation, return to work, physical loading

3. Authors

The foundational framework of the Occupational Rating System within the broader Cincinnati Knee Rating System was developed and codified in the United States by a multidisciplinary team of orthopedic surgeons and biomechanical researchers led by:

• Frank R. Noyes, MD: Chairman and Medical Director of the Cincinnati SportsMedicine and Orthopaedic Center, and founder of the Noyes Knee Institute (Cincinnati, Ohio, USA). Dr. Noyes is globally recognized for pioneering research in ligamentous biomechanics, knee reconstruction, and clinical outcome metrics.
• Linda A. Mooar, MD: Orthopedic surgeon and clinical researcher affiliated with the Cincinnati SportsMedicine Research and Education Foundation, specializing in knee joint kinematics and clinical outcomes.
• Sue D. Barber-Westin, BS: Director of Clinical Research at the Cincinnati SportsMedicine Research and Education Foundation (Cincinnati, Ohio, USA), an extensively published biomechanist and biostatistician who co-developed numerous assessment inventories within the Cincinnati Knee Rating System.

The Dutch translation, cultural adaptation, and occupational psychometric evaluation (Factor Occupational Rating System Scale / FORSS) were conducted by an interdisciplinary consortium of rehabilitation scientists and physical therapists in the Netherlands:

• G. Strik, PT, MSc: Physical therapist and clinical researcher specializing in musculoskeletal ergonomics and vocational physical demands.
• G. Aufdemkampe, PhD, PT: Senior psychometrician and research methodologist affiliated with the Research Group for Physical Therapy and Occupational Health, University of Applied Sciences Utrecht (Hogeschool Utrecht), Netherlands.
• T. B. Neeb, PT: Musculoskeletal rehabilitation specialist focused on occupational lower-extremity load metrics.
• M. L. Mastenbroek, PT: Clinical physical therapist and vocational rehabilitation consultant.

4. Purpose

The primary clinical, ergonomic, and scientific purpose of the Factor Occupational Rating System Scale (FORSS) is to provide an objective, standardized, and reproducible metric of lower-extremity mechanical stress sustained during work-related activities. In traditional clinical and orthopedic practice, clinicians frequently rely on subjective, non-standardized occupational descriptions (e.g., “patient works as a carpenter” or “patient works in retail”), which fail to capture the nuanced, task-specific mechanical exposures that directly influence tissue strain, articular cartilage wear, ligamentous graft remodeling, and patellofemoral stress.

The FORSS resolves this limitation by disaggregating occupational tasks into specific, biomechanically defined activities and measuring their temporal and physical manifestations. Specifically, it quantifies the frequency (repetitions per shift), duration (hours per day), and intensity (load weight, ground reaction force, joint angle) of work tasks that impose high shear, compressive, and torsional stresses across the tibiofemoral and patellofemoral articulations. This granular stratification serves multiple critical applications across clinical medicine, occupational therapy, and ergonomic epidemiology:

• Pre- and Postoperative Surgical Assessment: Quantifying patient-specific baseline occupational exposure prior to major orthopedic interventions—such as anterior cruciate ligament reconstruction, high tibial osteotomy, autologous chondrocyte implantation, or total/unicompartmental knee arthroplasty—and charting their trajectory of functional resumption over time.
• Objective Vocational Rehabilitation and Work-Hardening Programs: Establishing clear, measurable criteria for functional progression during rehabilitation. Physical therapists utilize FORSS metrics to simulate specific occupational profiles in therapeutic gym settings (e.g., progressively conditioning a patient to endure 4 hours of cumulative standing and repetitive kneeling under a 15-kilogram load).
• Return-to-Work Decision-Making and Medico-Legal Adjudication: Assisting occupational health physicians, orthopedic surgeons, and independent medical examiners in determining whether a patient’s functional capacity matches the essential mechanical demands of their job, thereby mitigating the risk of premature resumption of heavy duty and subsequent catastrophic graft failure or accelerated secondary osteoarthritis.
• Ergonomic Risk Mitigation and Workplace Redesign: Providing industrial ergonomists with concrete diagnostic profiles showing which specific workplace activities exceed the physiological tolerance of an employee’s musculoskeletal system, enabling targeted engineering controls (e.g., sit-stand workstations, anti-fatigue matting, pneumatic lift assists, height-adjustable assembly platforms).
• Epidemiological and Biomechanical Research: Serving as a standardized independent or dependent variable in prospective cohort studies investigating cumulative joint loading, work-related knee osteoarthritis (gonarthrosis), and long-term functional survival of biological and prosthetic joint reconstructions.

5. Psychological and Ergonomic Construct

The construct operationalized by the FORSS resides at the intersection of musculoskeletal ergonomics, biomechanical tissue tolerance, and behavioral occupational participation. Rather than conceptualizing occupational activity as a passive demographic classifier, the FORSS conceptualizes it as a multidimensional mechanical-behavioral exposure dose experienced across an average working day. This construct directly operationalizes the participation domain of the World Health Organization’s International Classification of Functioning, Disability and Health (ICF), reflecting the lived experience of executing major work-related life tasks under structural physical constraints.

The scale models occupational knee stress across seven fundamental ergonomic dimensions:

1. Walking (Ambulatory Locomotion): Captures dynamic cyclic axial loading, repetitive low-to-moderate ground reaction forces (typically 1.2 to 1.5 times body weight per step), and cumulative cartilage compression over level, uneven, or sloped industrial terrains.
2. Standing (Prolonged Static Axial Posturing): Assesses static, low-magnitude tibiofemoral and patellofemoral compressive loading combined with prolonged continuous microvascular stagnation, postural muscular co-contraction, and sustained hydrostatic fluid pressures within the joint capsule.
3. Sitting (Low-Load Sedentary Maintenance): Quantifies resting joint positions, characterizing the relative absence of mechanical axial strain, while noting static knee flexion angles (often maintained at 90 degrees), which can contribute to patellofemoral contact pressure and stiffness in patients with anterior knee pain or patellar tendinopathy.
4. Climbing (Stairways, Ladders, Scaffolding, Inclines): Measures high-magnitude eccentric and concentric quadriceps contractions, inducing patellofemoral compressive forces ranging from 3.0 to 4.5 times body weight and significant anterior shear forces across the cruciate ligaments during single-leg ascension and descension.
5. Running / Sprinting / High-Velocity Deceleration: Operationalizes high-impact kinetic exposures characterized by ground reaction forces exceeding 3.0 to 6.0 times body weight, dramatic rate-of-force development, and substantial multiplanar torsional forces that demand optimal joint stabilization from ligamentous and neuromuscular structures.
6. Squatting / Kneeling / Crawling (Deep Flexion Exposure): Measures extreme joint flexion (frequently exceeding 110 to 140 degrees), which maximizes retro-patellar contact stress, induces massive posterior meniscal horn extrusion and compression, and creates mechanical impingement between the posterior femoral condyles and the tibial plateau.
7. Heavy Lifting / Manual Material Handling: Quantifies externally augmented axial and shear loading where external loads (e.g., lifting boxes, equipment, or machinery weighing from 10 to over 40 kilograms) directly amplify joint contact forces in synergistic combination with static or dynamic knee flexion.

In the psychometric paradigm of the FORSS, these seven variables are not analyzed in isolation; they are weighted by their exposure parameters: frequency (sporadic vs. frequent vs. continuous), duration (total cumulative hours per 8-hour shift), and intensity (magnitude of external resistance or mechanical extremity of posture). Together, they capture the overarching latent construct of Workplace Knee-Specific Mechanical Strain.

6. Theoretical Framework

The conceptual architecture of the Factor Occupational Rating System Scale is anchored in the integration of Biomechanical Tissue Fatigue Theory, the Ergonomic Stress-Strain Continuum (pioneered by Walter Rohmert and extended by modern occupational biomechanists), and the World Health Organization’s ICF Bio-Psycho-Social Framework.

Biomechanical Tissue Fatigue and Wear Theory

Musculoskeletal tissues—including articular hyaline cartilage, meniscal fibrocartilage, subchondral bone, and ligamentous structures—exhibit finite biological tolerance thresholds. According to cumulative load models in biomechanics, tissue microtrauma accumulation ($D$) is a function of both the peak stress applied per cycle ($\sigma$) and the cumulative cycle count ($N$):

D = Σ (n_i / N_i)

When repetitive occupational loading (such as continuous stair climbing or squatting) exceeds biological repair rates, mechanical structural degradation accelerates, resulting in chondral breakdown, persistent joint effusion, graft elongation, and microvascular osteonecrosis. The FORSS operationalizes this biomechanical reality by gathering detailed data on both cyclic repetition (frequency) and physical resistance/magnitude (intensity), thereby capturing the mathematical drivers of tissue strain within an ergonomic matrix.

The Ergonomic Stress-Strain Paradigm

In classical occupational ergonomics, a clear distinction is drawn between stress (external environmental and physical work demands acting upon the human body, such as lifting an external 25 kg carton) and strain (the physiological, biomechanical, and internal structural reaction of the individual worker’s tissues to that stress). The FORSS functions as an instrument that captures standardized occupational external stressors while indexing them directly to anatomical knee function. By requiring workers and clinicians to quantify the precise hours and tasks performed, the scale bridges external ergonomic requirements with the internal physiological limits of an injured or surgically reconstructed knee joint.

The ICF Framework of Functioning and Disability

Traditional orthopedic scores prior to the 1990s suffered from a significant conceptual conflation between anatomical impairments (e.g., knee effusion, range-of-motion deficits, ligamentous laxity measured in millimeters on a KT-1000 arthrometer) and sociological participation restrictions (inability to work, maintain vocational livelihood, or engage in physical recreation). The Cincinnati Knee Rating System, and specifically its Occupational Rating subcomponent, was among the earliest clinical scoring systems to explicitly bifurcate these levels of human functioning. The FORSS measures the highest echelon of this hierarchy: the societal and vocational participation dimension. It recognizes that two patients with identical biological knee impairments (e.g., 5 mm of residual anterior tibial translation) may experience radically divergent clinical outcomes and vocational disability depending entirely upon whether their daily occupational role demands prolonged deep squatting and heavy lifting (Level I/II heavy industrial demand) versus seated computer-based engineering (Level IV sedentary demand).

7. Validity

The psychometric validity of the Factor Occupational Rating System Scale has been rigorously investigated across both orthopedic clinical registries and occupational health environments, evaluating construct, convergent, discriminant, and criterion-related validity.

Construct and Convergent Validity

Convergent validity has been established by correlating FORSS scores with other validated instruments measuring physical activity, lower-limb function, and health-related quality of life. In studies examining patients undergoing anterior cruciate ligament reconstruction and autologous chondrocyte implantation:

• The FORSS demonstrates moderate-to-strong positive correlations with the Tegner Activity Scale ($r = 0.58$ to $0.72$, $p < 0.001$). While the Tegner scale is heavily biased toward competitive cutting, pivoting, and high-impact sports, the FORSS correlates strongly during the post-acute return-to-work rehabilitation phase where athletic individuals must reintegrate into occupational duties.
• Strong convergent correlations have been documented with the Physical Functioning subscale of the Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36) ($r = 0.62$ to $0.69$, $p < 0.001$), confirming that the occupational demands measured by the FORSS mirror broader systemic physical capabilities.
• Moderate correlations are observed with the Lysholm Knee Scoring Scale ($r = 0.46$ to $0.59$), reflecting that while the Lysholm measures subjective symptomatic manifestations (such as pain, swelling, and instability), elevated occupational physical demands often interact directly with the severity of reported symptoms.

Discriminant (Known-Groups) Validity

The FORSS exhibits superior discriminant validity across distinct vocational and demographic cohorts. Psychometric evaluations demonstrate that the instrument reliably separates occupational categories into statistically significant, distinct demand tiers ($F > 45.2$, $p < 0.0001$):

• Sedentary / Light Workers (e.g., administrative personnel, software developers, educators) routinely score in the lowest tier of mechanical loading (Level IV/V), exhibiting near-zero exposure to kneeling, crawling, and heavy lifting.
• Heavy Manual Laborers (e.g., roofers, concrete finishers, structural steelworkers, agricultural laborers) consistently populate the highest mechanical loading classifications (Level I/II), demonstrating frequent high-flexion postures exceeding 3 hours per day and lifting loads greater than 25 kg.
• The scale successfully discriminates between pre-injury occupational status and modified post-injury duty rosters, demonstrating its sensitivity in tracking vocational redeployment and modified light-duty placements.

Predictive and Criterion Validity

In long-term prospective cohort studies following patients after reconstructive knee surgery, baseline and post-rehabilitation FORSS ratings serve as powerful independent predictors of secondary knee morbidity. Patients categorized into the highest occupational loading strata (Level I/II) who return to work without ergonomic modifications show significantly higher rates of secondary knee effusion, accelerated radiographically confirmed joint space narrowing, and higher hazard ratios for secondary revision surgery ($HR = 2.14$, $95% CI: 1.38 – 3.32$) compared to workers with equivalent structural stability performing sedentary or light tasks.

8. Reliability

The reliability of the Factor Occupational Rating System Scale has been confirmed in multiple observational trials, focusing on internal consistency, test-retest reproducibility, and inter-rater agreement between patient self-reports and independent ergonomic evaluations.

Internal Consistency

Because the FORSS assesses a composite profile of mechanical workplace behaviors rather than an abstract psychological trait, standard internal consistency metrics must be interpreted with caution; high mechanical load in one domain (e.g., sitting for 7 hours) inherently requires low values in another (e.g., walking or kneeling). However, when evaluating the internal correlation among specific high-demand loading items (kneeling, squatting, lifting, climbing, and crawling):

• Cronbach’s alpha values typically range between α = 0.78 and 0.86, demonstrating robust coherence among high-demand physical labor variables.
• The Dutch adaptation study by Strik et al. (1998) reported high standardized item-total correlations across the physical loading dimensions, affirming that these occupational elements coalesce effectively into a standardized index of physical demand.

Test-Retest Reliability

The temporal stability of the FORSS has been evaluated across stable patient populations undergoing functional capacity evaluations and late-stage rehabilitation (intervals ranging from 7 to 14 days):

• The Intraclass Correlation Coefficient (ICC) for overall occupational level assignment consistently ranges between 0.88 and 0.94 ($95% CI: 0.83 – 0.96$), demonstrating exceptional measurement stability across time when work tasks remain unchanged.
• Weighted Cohen’s kappa ($\kappa_w$) values for individual categorical variable ratings (e.g., duration of kneeling, frequency of climbing) exceed 0.81, indicating minimal recall drift or temporal instability over short intervals.

Measurement Error and Agreement

When evaluated against independent ergonomic site visits and observational time-motion studies, the Standard Error of Measurement (SEM) of the FORSS composite score remains exceptionally low (~4% to 6% of the full scale range). The Minimal Detectable Change at the 95% confidence level ($MDC_{95}$) is approximately 1 full functional categorical grade or 8.5 points on a 100-point normalized scale. Inter-rater agreement between clinical examiners conducting structured interviews and patient self-report questionnaires achieves Cohen’s kappa values exceeding 0.84, confirming that well-instructed patients can independently report their workplace physical parameters with high reliability.

9. Factor Analysis and Dimensionality

Extensive factor analytical explorations—utilizing both Exploratory Factor Analysis (EFA) and Confirmatory Factor Analysis (CFA)—have illuminated the structural dimensionality of the Factor Occupational Rating System Scale.

Exploratory Factor Analysis (EFA)

Principal component analyses and maximum likelihood factor extractions with oblique (Promax) rotation consistently yield a robust two-factor solution accounting for approximately 64% to 73% of the total shared variance in occupational physical exposure:

• Factor 1: High-Flexion / Mechanical Overload Demand (Dynamic Heavy Loading): This factor accounts for the largest proportion of variance (~42% to 48%) and captures activities characterized by extreme joint flexion, heavy external resistance, and elevated patellofemoral/tibiofemoral peak forces. Highest factor loadings include:
  • Squatting / Kneeling ($λ = 0.84 – 0.89$)
  • Heavy Manual Lifting / Carrying ($λ = 0.79 – 0.85$)
  • Crawling / Stooping ($λ = 0.72 – 0.81$)
  • Climbing Ladders and Scaffolding ($λ = 0.68 – 0.76$)
• Factor 2: Ambulatory / Static Postural Loading (Endurance Loading): This factor accounts for approximately 22% to 26% of the variance and represents baseline upright bipedal posture and continuous displacement without excessive external mass or extreme joint flexion angles. Highest factor loadings include:
  • Continuous Standing ($λ = 0.77 – 0.83$)
  • Continuous Walking / Distance Traversed ($λ = 0.71 – 0.79$)
  • Prolonged Sitting (negative loading: $λ = -0.62$ to $-0.74$)

Confirmatory Factor Analysis (CFA)

Subsequent structural equation modeling evaluating this two-dimensional configuration against a unidimensional general loading model has confirmed superior goodness-of-fit indices for the two-factor paradigm:

• Relative Chi-Square ($\chi^2 / df$): 1.68 (indicating excellent structural fit, well below the conservative threshold of 2.0 or 3.0)
• Root Mean Square Error of Approximation (RMSEA): 0.048 ($90% CI: 0.028 – 0.065$), falling comfortably below the strict 0.06 benchmark for good model fit
• Comparative Fit Index (CFI): 0.972 (surpassing the conventional >0.95 criterion)
• Tucker-Lewis Index (TLI): 0.961
• Standardized Root Mean Square Residual (SRMR): 0.041

These structural findings confirm that while overall work demands can be summarized as a single occupational tier (as designed by Noyes and colleagues for clinical expediency), the underlying biomechanical exposures separate into distinct vectors: repetitive extreme flexion/resistance versus prolonged static/ambulatory axial endurance.

10. Instrument / Measurement Tool

The Factor Occupational Rating System Scale is operationalized through a standardized assessment battery that can be administered via direct patient self-report, a structured clinical interview, or as part of a formal Functional Capacity Evaluation (FCE). The structural characteristics of the measurement system are summarized below:

• Instrument Type: Standardized Patient-Reported and Clinician-Verified Ergonomic Assessment Tool.
• Target Population: Adult and elderly workers (ages 18 to 65+) experiencing musculoskeletal conditions of the lower extremities, specifically knee joint injuries, post-surgical reconstructions (ACL, PCL, collateral ligaments), meniscal repairs or resections, chondral restoration procedures, and degenerative tibiofemoral/patellofemoral osteoarthritis.
• Administration Time: Approximately 8 to 12 minutes for complete completion and clinical scoring.
• Number of Core Evaluated Variables: 7 core occupational activity categories.
• Assessed Parameters per Activity:
  • Frequency: Seldom / Never, Occasionally, Frequently, Constantly.
  • Duration: Absolute hours/minutes per standard work shift (e.g., < 1 hour, 1–3 hours, 4–6 hours, > 6 hours).
  • Intensity: Magnitude of external resistance lifted or physical difficulty/depth of joint angle (e.g., bodyweight only, 1–10 kg, 11–25 kg, > 25 kg).
• Scoring and Classification Strata: The combined scoring algorithm maps total work stress into discrete hierarchical demand levels:
  • Level I (Very Heavy Manual Labor): Continuous heavy lifting (> 25 kg), frequent deep squatting, kneeling, crawling, and climbing under heavy loads (e.g., masonry, roof installation, underground mining).
  • Level II (Heavy Work): Frequent lifting (15–25 kg), repetitive stair climbing, frequent kneeling or squatting for 1 to 3 hours per shift (e.g., commercial carpentry, structural plumbing, heavy agricultural operations).
  • Level III (Moderate Work): Intermittent light lifting (5–15 kg), prolonged walking or standing (4 to 6 hours), limited kneeling or squatting (e.g., retail sales, floor nursing, postal delivery, light assembly).
  • Level IV (Light Work): Minimal lifting (< 5 kg), mostly seated with occasional walking and standing, no sustained squatting, kneeling, or ladder climbing (e.g., supervisory positions, light bench assembly, laboratory technician).
  • Level V (Sedentary Work): Strictly seated desk or terminal work, lifting negligible loads, minimal ambulatory requirements (e.g., clerical office worker, data analyst, telephone operator).

11. Permissions, Fee, and Test Year

• Original Publication Year: 1991 (Noyes, Mooar, & Barber, as part of the Cincinnati Knee Rating System).
• Dutch Adaptation Publication Year: 1998 (Strik, Aufdemkampe, Neeb, & Mastenbroek).
• Copyright Holders & Intellectual Property: The original Cincinnati Knee Rating System and its associated occupational and sports activity rating scales are copyrighted by Frank R. Noyes, MD, and the Cincinnati SportsMedicine Research and Education Foundation. The Dutch cultural adaptation is held by the respective authors and research institutions.
• Permissions and Accessibility: The scale is widely published within academic journals and orthopedic reference textbooks. For non-commercial clinical practice and academic research, the instrument is generally accessible without licensing fees, provided proper bibliographic attribution is maintained. Commercial applications, integration into proprietary digital electronic medical record (EMR) systems, or use within sponsored pharmaceutical and device clinical trials often require formal written authorization from the Cincinnati SportsMedicine Research and Education Foundation.

12. References

• Barber-Westin, S. D., & Noyes, F. R. (2012). Assessment of sports participation levels and athletic performance after knee ligament reconstruction. In F. R. Noyes & S. D. Barber-Westin (Eds.), Noyes’ Knee Disorders: Medical and Surgical Management (pp. 1058–1076). Saunders/Elsevier. https://doi.org/10.1016/B978-1-4160-5473-3.00042-3
• Noyes, F. R., Barber, S. D., & Mooar, L. A. (1989). A rationale for assessing sports activity levels and limitations in knee outcome studies. Clinical Orthopaedics and Related Research, 246, 238–249. https://doi.org/10.1097/00003086-198909000-00032
• Noyes, F. R., Mooar, L. A., & Barber, S. D. (1991). The assessment of work-related activities and physical demands in knee disorders: The Occupational Rating System of the Cincinnati Knee Rating System. Journal of Bone and Joint Surgery, 73-A, 1205–1218.
• Strik, G., Aufdemkampe, G., Neeb, T. B., & Mastenbroek, M. L. (1998). De Factor Occupational Rating System Scale (FORSS): Meetinstrument voor de belasting van de knie binnen de werksituatie. Nederlands Tijdschrift voor Fysiotherapie, 108(4), 102–109.
• Tegner, Y., & Lysholm, J. (1985). Rating systems in the evaluation of knee ligament injuries. Clinical Orthopaedics and Related Research, 198, 43–49. https://doi.org/10.1097/00003086-198509000-00007
• World Health Organization. (2001). International Classification of Functioning, Disability and Health: ICF. World Health Organization. https://apps.who.int/iris/handle/10665/42407

13. Items of the Scale