Abstract
Objectives: The application of well-controlled, quantitative measurement systems has challenged the traditional notion that rigidity in Parkinson's disease (PD) is a velocity-independent phenomenon. This review aimed to evaluate whether rigidity in PD is velocity-dependent or velocity-independent across different joints, body regions, testing speeds, and methodologies. Methods: This systematic review followed the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines. Methodological quality of cross-sectional studies was assessed using the Appraisal Tool for Cross-Sectional Studies (AXIS), and reporting completeness was evaluated with the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) checklist. Results: Seventeen studies were included and analyzed by the body part assessed (wrist, elbow, hand, knee, trunk). Rigidity quantification in PD used various biomechanical technologies, sometimes combined with neurophysiological methods. Although rigidity is classically considered velocity-independent, experimental evidence suggests a more complex behavior, partially velocity-dependent, especially at moderate to high angular velocities. Methodological quality was variable but generally acceptable, with more recent studies showing stronger adherence to AXIS. However, compliance with STROBE reporting standards remained inconsistent. Conclusions: While rigidity in PD has not been classically defined as velocity-dependent, current evidence indicates that, under specific testing conditions, rigidity increases with passive movement velocity. These findings challenge traditional clinical assumptions and emphasize the need for standardized measurement protocols.