Two kinds of EMG

There are two kinds of EMG in wide­spread use: sur­face EMG and intra­mus­cu­lar (nee­dle and fine-​wire) EMG. To per­form intra­mus­cu­lar EMG, a nee­dle elec­trode or a nee­dle con­tain­ing two fine-​wire elec­trodes is inserted through the skin into the mus­cle tis­sue. A trained pro­fes­sional (such as a neu­rol­o­gist, physi­a­trist, or phys­i­cal ther­a­pist) observes the elec­tri­cal activ­ity while insert­ing the elec­trode. The inser­tional activ­ity pro­vides valu­able infor­ma­tion about the state of the mus­cle and its inner­vat­ing nerve. Nor­mal mus­cles at rest make cer­tain, nor­mal elec­tri­cal sig­nals when the nee­dle is inserted into them. Then the elec­tri­cal activ­ity when the mus­cle is at rest is stud­ied. Abnor­mal spon­ta­neous activ­ity might indi­cate some nerve and/​or mus­cle dam­age. Then the patient is asked to con­tract the mus­cle smoothly. The shape, size, and fre­quency of the result­ing motor unit poten­tials are judged. Then the elec­trode is retracted a few mil­lime­ters, and again the activ­ity is ana­lyzed until at least 10 – 20 units have been col­lected. Each elec­trode track gives only a very local pic­ture of the activ­ity of the whole mus­cle. Because skele­tal mus­cles dif­fer in the inner struc­ture, the elec­trode has to be placed at var­i­ous loca­tions to obtain an accu­rate study.

Intra­mus­cu­lar EMG may be con­sid­ered too inva­sive or unnec­es­sary in some cases. Instead, a sur­face elec­trode may be used to mon­i­tor the gen­eral pic­ture of mus­cle acti­va­tion, as opposed to the activ­ity of only a few fibres as observed using an intra­mus­cu­lar EMG. This tech­nique is used in a num­ber of set­tings; for exam­ple, in the phys­io­ther­apy clinic, motion analy­sis lab­o­ra­to­ries, research cen­ters, etc.