The contribution of central and peripheral factors to the frequency content of the electromyogram was examined in 10 subjects performing maximal 45-s contractions of thenar muscles. The median frequencies (Fm) of surface-recorded electromyograms and compound action potentials were similar early (P greater than 0.6) and late (P greater than 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 greater than 0.1). The Fm of electromyograms increased 11% (P less than 0.02) in only the 1st s of contraction as force was raised from 25 to 100% of maximum. Only one of five 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. The findings indicated that the spectral content of muscle electrical activity, and its shift during contraction, primarily reflects intrinsic muscle properties.