The purpose of this study was to investigate the behaviour of the human soleus muscle during isometric contraction. A model taking into account the musculoskeletal geometry, the musculotendon architecture and the neural excitation input has been developed. The neural excitation input was simulated using a recruitment and firing rate organisation model. The musculotendon actuator was modelled as a tendon inserted in series with fibres defined by a contractile component in parallel with an elastic component. At maximal neural excitation, the model highlighted the functional significance of tendon stiffness and pennation angle. These architectural parameters tended to increase the operative ankle joint angle range of the soleus actuator, either to the maximal plantarflexion positions for the pennation angle or to the maximal dorsiflexion positions for tendon elasticity. When the model was simulated under various neural excitation levels, it predicted a displacement of the soleus fibre optimal length towards the soleus long length (maximal dorsiflexion position) with increasing neural excitation. The study concluded that the effect of muscular architecture should be taken into account to analyse the effect of neural excitation level on isometric force output.