Isometric features of orthostatic tremor: an electromyographic analysis. A patient is described with "orthostatic" tremor. Electromyography revealed tremor bursts of 15 Hz in the lower extremities while standing and with isometric activation of the muscles, but the bursts disappeared with isotonic activation of muscles. Similar tremor was recorded in the arms with isometric, but not isotonic activation. Review of previously reported cases confirms these findings. The clinical and electrophysiologic features of this tremor distinguish it from other recognized forms of tremor. 