Spasticity (Clonus) vs Rigidity

Spasticity = Hyperreflexia = Clonus. All are the same for test purposes, though there are some subtle technical differences. Think of it as a degree of magnitude along the +1 to +4 scale of reflex testing. This could make you think UMN lesions, hypocalcemia, serotonin syndrome, etc.
Rigidity is more closely associated with specific pathologies like Parkinson's, acute dystonia, and neuroleptic malignant syndrome. Unlike spasticity, it's not along the spectrum of reflex testing -- just kind of it's own thing. I believe it's accurate to think of it as a distinct sign of higher-pathway neurological pathology, as you can tell by the dopaminergic pathways involved in my examples.

Tip

In patients with spasticity, quick movements present more resistance than slow ones; in patients with rigidity, resistance remains the same.