We present a model based on k.p theory which is able to capture the subband structure effects present in ultra-thin strained silicon nanowires. For electrons, the effective mass and valley minima are calculated for different crystal orientations, thicknesses, and strains. The actual enhancement of the transport properties depends highly on the crystal orientation of the nanowire axis; for certain orientations strain and confinement can play together to give a significant increase of the electron mobility. We also show that the effects of both strain and confinement on mobility are generally more pronounced in nanowires than in thin films. We show that optimal transport properties can be expected to be achieved through a mix of confinement and strain. Our results are in good agreement with recent experimental findings. (C) 2011 Elsevier Ltd. All rights reserved.