Surface versus intramuscular electrodes for electromyography of superficial and deep muscles.

The relative selectivity of surface electrodes placed over the gastrocnemius and soleus muscles in the standard manner was determined by comparing the electromyogram from these electrodes to the electromyogram from wire electrodes inserted into the soleus, gastrocnemius, and tibialis posterior muscles. Muscle activity was elicited in 11 normal subjects by performing six manual muscle tests. Three of the tests followed the standard technique and three were modifications designed to provide better differentiation of muscle action. All electromyographic data were quantified by computer integration and normalized to cancel out sampling inconsistency. None of the muscle tests totally restricted activity to the designated muscle, but the tests did determine which was the strongest participant. During the standard specific gastrocnemius and soleus muscle tests, the corresponding surface electrode provided a lower electromyogram than did the matching wire electrode; that is, the surface-to-wire-electrode ratio was less than one (gastrocnemius = 0.74, soleus = 0.58). This ratio was greater than one when the electromyogram was being recorded from one muscle while testing another muscle (gastrocnemius = 1.55, soleus = 1.92). The mathematical model relating surface-electrode values to the wire-electrode data from all three muscles identified 60 percent of the surface gastrocnemius electrode electromyogram as arising from that muscle, while only 36 percent of the soleus surface electrode data related to the activity of the soleus: Sg = .19Ws + 60Wg + .13Wt + .08Wo and Ss = .36Ws + .31Wg + .22Wt + .11Wo.