I went to my new neurologist doctor and i have 4 tests to do and he said that the reason i am having gait abnormality issues is because of spasticity issues. what causes that to happen?

The spasticity can be infrequently from primary muscular issues but commonly from neurological pathology in the brain or spinal cord. . The neurologist can give you a differential diagnosis and,I'm sure, has ordered appropriate tests.

Spasticity is a response to damage to the corticospinal tracts, most often within the spinal cord. Extrinsic compressive lesions and intrinsic (within cord) can be present. A multitude of disorders could be considered.