What characterizes spasticity in a limb when moved rapidly?

Spasticity is characterized by an exaggerated response to muscle stretch, often resulting in increased resistance to passive movement, particularly when the limb is moved quickly. The clasp-knife response refers to a phenomenon where there is initially increased resistance to stretching of the muscle, which then suddenly gives way, resembling a knife cutting through a soft material. This describes the sudden release of resistance during rapid movement, making it a hallmark characteristic of spasticity.

The other options describe different motor responses. Free movement with no resistance indicates normal muscle tone devoid of spasticity. Consistent rigidity throughout the motion suggests a state of continuous high resistance, often seen in conditions like rigidity in Parkinson's disease, rather than the characteristic sudden release of resistance found in spasticity. Fluid movement with slight hesitance implies a smooth but cautious motion, which does not reflect the sudden resistance and release associated with spasticity. Hence, the clasp-knife response accurately captures the nature of spasticity when a limb is moved rapidly.