Rigidity in Parkinson's Disease: The Objective Effect of Levodopa.

Background: Quantitative evidence of levodopa-induced beneficial effects on parkinsonian rigidity in Parkinson's disease (PD) is lacking. Recent research has demonstrated the velocity-dependent nature of objective rigidity in PD and revealed its neural underpinning.

Objective: The present study aimed to examine the effect of levodopa on objective rigidity in PD.

Methods: Eighteen patients with PD underwent clinical and instrumental evaluations of muscle tone in the OFF and ON states. The clinical assessments focused on rigidity in the most affected upper limb. The biomechanical components of objective rigidity were assessed using robot-assisted wrist extensions at seven angular velocities (5-280°/s). Surface electromyography of the flexor carpi radialis muscle enabled the concurrent evaluation of short- and long-latency stretch reflexes (SLR and LLR).

Results: Levodopa improved the clinical scores of rigidity. Biomechanical measurements showed that levodopa reduced the total and neural components of force but had no effect on viscoelastic components. Levodopa reduced the velocity dependence of the LLRs but did not affect the SLRs. Finally, we found significant clinical-instrumental correlations between levodopa-induced changes and biomechanical and neurophysiological measures of objective rigidity in PD.

Conclusions: Levodopa improved objective rigidity in PD by decreasing its biomechanical neural component as well as the size of LLRs. The beneficial effect of levodopa on biomechanical and neurophysiological features of objective rigidity was related to the specific angular velocity of wrist extensions; that is, the higher the angular velocity, the greater the beneficial impact of levodopa on objective rigidity. These findings allowed the description of a new pathophysiological model of rigidity in PD. © 2025 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.