Spasticity

Spasticity refers to increased resistance to passive movement, but the resistance is smaller when the movement is performed quickly, and the resistence depends on the limb position. In other words, the resistance to movement is velocity-dependent resistance. It's a sign of upper motor neuron lesion. Spasticity is better observed when passive movement is performed quickly. Spasticity is sort of the counterpart to rigidity.

Spasticity classically manifests as the clasp knife phenomenon, where at the beginning of the movement there is increased resistance to passive movement, but the resistance suddenly decreases.

Spasticity is best observed in the flexors of the upper extremity and extensors of lower extremity.