Factors determining the frequency content of the electromyogram

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Abstract
Contribution of central and peripheral factors to the frequency content of the electromyogram was examined in 10 [human] subjects performing maximal 45-s contractions of thenar muscles. Median frequencies (Fm) of surface-recorded electromyograms and compound action potentials were similar early (P > 0.6) and late (P > 0.5) in the contractions. There was a mean decrease in the Fm during contraction of 39% for electromyograms and 35% for compound potentials (P > 0.1). Fm of electromyograms increased 11% (P < 0.02) in only the 1st s of contraction as force was raised from 25 to 100% of maximum. Only 1 of 5 subjects showed evidence of increasing synchronization of motor unit discharge during contraction. There was no evidence that delay or dispersion of action potential propagation in terminal nerve fibers or at the neuromuscular junction had a significant effect on frequency content. Apparently, the spectral content of muscle electrical activity, and its shift during contraction, primarily reflects intrinsic muscle properties.