Recordings of motor cortical activity typically show oscillations around 10 Hz and 20 Hz; only those at 20 Hz are coherent with electromyogram (EMG) of contralateral muscles. Experimental measurements of the phase difference between ~20 Hz oscillations in cortex and muscle are often difficult to reconcile with the known corticomuscular conduction delays. We investigated the generation of corticomuscular coherence further using a biophysically-based computational model, which included a pool of motoneurons connected to motor units which generated EMG. Delays estimated from the coherence phase-frequency relationship were sensitive to the width of motor unit action potential. In addition, the non-linear properties of the motoneurons could produce complex, oscillatory phase-frequency relationships. This was due to the interaction of cortical inputs to the motoneuron pool with the intrinsic rhythmicity of the motoneurons; the response appeared more linear if the firing rate of motoneurons varied widely across the pool, such as during a strong contraction. The model was able to reproduce the smaller than expected delays between cortex and muscles seen in experiments. However, the model could not reproduce the constant phase over a frequency band sometimes seen in experiments, nor the lack of ~10 Hz coherence. Simple propagation of oscillations from cortex to muscle cannot therefore completely explain the observed corticomuscular coherence.