A characteristic tremor of the patellae, sometimes known as knee bobbing, juddering, or quivering, may be seen in primary orthostatic tremor (POT; "shaky legs syndrome"). It is due to rapid rhythmic contractions of the leg muscles on standing, which dampen or subside on walking, leaning against a wall, or being lifted off the ground, with disappearance of the Knee tremor; hence this is a task-specific tremor. Auscultation with the diaphragm of a stethoscope over the lower limb muscles reveals a regular thumping sound, likened to the sound of a distant helicopter. EMG studies show pathognomonic synchronous activity in the leg muscles with a frequency of 14-18Hz, thought to be generated by a central oscillator (peripheral loading does not alter tremor frequency).
A number of drugs may be helpful in POT, including phenobarbitone, primidone, clonazepam, and levodopa, but not propranolol (cf. essential tremor).
Heilman KM. Orthostatic tremor. Archives of Neurology 1984; 41: 880-881
Brown P. New clinical sign for orthostatic tremor. Lancet 1995; 346: 306-307