Electromyograms are repeatable: precautions and limitations.

Electromyograms recorded by bipolar, fine wire electrodes placed into anatomically equivalent sites in skeletal muscles of vertebrates are repeatable when the animals use the muscles in a similar way. Repeatability applies to the number of spikes recorded from a given site and to their average amplitude as well as to the root-mean-square value, though the values obtained for these descriptors differ among muscles, and perhaps fascicles, of particular animals even when the animals are performing equivalent actions. Tests suggest that these results are not affected by the nature of most kinds of recording equipment. Also, substantial differences in electrode tip configuration and wire diameter induce relatively minor, less than 8 percent, differences in electrode resistance and impedance. Doubling the length of the fine wire leads produces less than an 8 percent (15 percent when the length is tripled) effect; however, the effect of electrode material may be as much as 85 percent in resistance and 20 percent in impedance. Reports of nonreproducibility or variability of electromyograms apparently result mainly from anatomically inexact placement into physiologically and histochemically different fascicles of compound muscles, from recordings of muscles that are active at very low levels, and perhaps from comparison among recordings of muscles that really differ in their activity level.