Quantitative Analysis of Axial Rigidity at Different Passive Movement Velocities in Parkinson’s Disease: A Cross-Sectional Study

Background/Objectives: Rigidity has been defined as an increase in muscle tone that is independent of the velocity of the stretch in Parkinson’s disease (PD). However, there is an ongoing debate about this non-velocity-dependent nature of rigidity in PD. To investigate the behaviour of axial muscle tone at different examination velocities using isokinetic dynamometry, and to determine whether trunk muscle resistance is velocity-dependent in people with PD compared with healthy controls (HCs). Methods: A cross-sectional study was conducted with HC and people with PD (I–III stages of Hoehn and Yahr and assessed by the UPDRS, Section III: motor aspects) by a senior neurologist. The trunk extension–flexion component of an isokinetic dynamometer measured axial muscle tone over a range of 50° (S: 30-50-80). The continuous passive mode with three angular speeds (30°/s, 45°/s and 60°/s) was used to assess muscle tone. Peak torque (N), work (J) and work recorded in the first and in the last third of the explored trunk range of motion were calculated (J) were registered. All these outcomes were performed within 1–3 h of the administration of anti-Parkinsonian medication (ON phase) in the PD sample. Results: People with PD (N = 36) and healthy controls (N = 20) completed the study. Our results showed largely similar behaviour in work and peak torque registered in both groups, by which, resistance measures, like peak torque, weakly increased with mobilisation speed from 30°/s to 45°/s, without reaching statistical significance, but increased from 45°/s to 60°/s, only in the flexors. No clear increase was observed in the work. Furthermore, greater torque measures in PD than controls were only observed for peak torque at 30°/s. Conclusions: Peak torque of trunk flexors–extensors tends to increase as the angular speed increases in both PD and controls. This may suggest that the (relatively slow) tested speeds were likely evaluating the non-neural component of muscle tone. This has implications for the clinical assessment of axial rigidity in PD.