Bicoherence analysis is applied to electromyogram (EMG) data for vastus medialis obliques (VM), rectus femoris (RF) and vastus lateralis (VL) quadriceps muscles of 18 adult male subjects for isometric knee extension exercise. Mean average bicoherence for VM, RF and VL is 30.9 +/- 5.8, 26.0 +/- 1.2 and 25.4 +/- 1.4% respectively and repeated measures ANOVA differentiates the muscles on the basis of average bicoherence (F = 16.2 (1, 17), p = 0.0009, VM cf. VL and F = 15.4 (1, 17), p = 0.0011, VM cf. RF). Prominent regions representative of strong second-order phase coupling between constituent EMG frequencies are identified within VM and RF bicoherence spectra. No such prominent regions are identified for VL which is thought to be less activated than VM during the specified task. Hence, the degree of second-order phase coupling may increase as the level of muscle activation increases. The subject group consists of young (24.0 +/- 0.9 years) and elderly (68.9 +/- 0.9 years) subgroups that cannot be differentiated by standard indices (median and spectral edge frequency) to within p < 0.05 using the Mann-Whitney test. Average bicoherence differentiates the subgroups for RF (T = 9 (8,10), p < 0.005) but not for VM or VL. The application of a bicoherence threshold that takes harmonic amplitude into account graphically differentiates the subgroups for all muscle types. The findings suggest that nonlinear processes play a role within the EMG generation process and support a mechanomyogram bicoherence analysis study that shows nonlinear processes occur within active muscle fibre twitch summation patterns. A potential exists for bicoherence analysis to complement standard EMG frequency analysis techniques.