Spasticity and rigidity are common symptoms of central nervous system injuries, such as spinal cord injury and Parkinson's disease, and result in distinct patterns of increased resistance during passive joint movements. Spasticity is characterized by a velocity-dependent increase in stretch reflexes, accompanied by exaggerated tendon responses, while rigidity is marked by consistent resistance throughout the range of motion, traditionally considered independent of stretch velocity. However, recent studies suggest that rigidity may also be influenced by stretch velocity. This study aims to investigate muscle tone by examining spasticity, rigidity, and normal muscle function through neural and biomechanical changes. Standard clinical tools, such as the Modified Ashworth Scale and Unified Parkinson's Disease Rating Scale, along with additional assessments like the Myoton and Post-Activation Depression (PAD), will be employed.

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.

Health subjects:

Exclusion Criteria:

1. Musculoskeletal injuries on legs.
2. Osteoporosis.

SCI subjects:

Inclusion Criteria 1. Participants with chronic spinal cord injury, with injury duration greater than one year.

Exclusion Criteria

1. Current musculoskeletal or joint injuries in the lower limbs.
2. History of central or peripheral neuromuscular diseases.
3. Presence of a pacemaker.
4. Current use of antispastic or antidepressant medications.
5. Current venous thromboembolism or osteoporosis.
6. Impairment of the soleus H-reflex arc.

PD subjects:

Inclusion Criteria:

- Clinical diagnosis of Parkinson disease.

Exclusion Criteria:

1. Musculoskeletal injuries on legs
2. Osteoporosis.
3. Any peripheral or central nervous system injury or disease patients.