• 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.