Introduction and Purpose: Muscle rigidity is one of the cardinal motor symptoms of Parkinson’s disease and is characterized by increased resistance to passive joint movement. Despite its clinical significance, the neural and biomechanical mechanisms underlying rigidity are not yet fully understood. This review aimed to explain the relationships among basal ganglia dysfunction, alterations in stretch reflexes, motor control mechanisms, and the response of muscle rigidity to levodopa.
Methodology: This study was conducted as a narrative review. Relevant studies addressing muscle rigidity in Parkinson’s disease, its neurophysiological and biomechanical mechanisms, stretch reflexes, motor control, and the effects of levodopa were examined. Their findings were descriptively and analytically synthesized, with particular emphasis on clinical, neurophysiological, and quantitative assessment studies.
Results: The reviewed evidence indicates that Parkinsonian rigidity is a multifactorial phenomenon and cannot be attributed solely to increased muscle tone or spinal reflex hyperactivity. Dysfunction of basal ganglia circuits and altered regulation of descending pathways may influence sensorimotor processing and muscular responses to stretch. Biomechanical and neurophysiological studies, particularly those employing robotic wrist stretching, have demonstrated increased long-latency reflex responses and a velocity-dependent component of objective rigidity. Furthermore, levodopa may reduce certain neural and reflex components of rigidity; however, its effects on mechanical and viscoelastic components are not necessarily uniform.
Conclusion: Muscle rigidity in Parkinson’s disease results from complex interactions among central motor control mechanisms, reflex responses, and the mechanical properties of muscles and joints. Therefore, comprehensive assessment of rigidity requires simultaneous consideration of its clinical, neurophysiological, and biomechanical components. Investigating the effects of levodopa on different neural and mechanical components may also contribute to a better understanding of this motor impairment.