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Spasticity and rigidity are common symptoms resulting from central nervous system injuries (e.g., spinal cord injury and Parkinson's disease). During passive joint movement, spasticity and rigidity manifest as two distinct patterns of increased resistance. Spasticity is a type of hypertonia characterized by a stretch reflex that increases with speed, accompanied by exaggerated tendon reflexes. Rigidity, on the other hand, is another form of hypertonia, where resistance increases during passive movement and remains consistent throughout the range of motion. The degree of rigidity is traditionally considered independent of stretch velocity, which is one of the key differences from spasticity. However, recent studies have found that rigidity may also increase with stretch velocity. Despite attempts to distinguish different types of hypertonia based on stretch velocity, these efforts have largely been unsuccessful. Many factors influence muscle tone, which can be broadly categorized into changes in neural and biomechanical properties. The Modified Ashworth Scale and the Unified Parkinson's Disease Rating Scale are the most commonly used clinical tools for assessing spasticity and rigidity. Additionally, devices such as the Myoton or laboratory parameters like Post-Activation Depression (PAD) are also used for assessment.