A statistical mechanical model is used to describe the effect of restrictions, due to confinement by crystals, on the segmental relaxation dyamics of semicrystalline polymers. It is shown that the emergence of a rearrangement free energy barrier is intrinsically associated with the dynamic heterogeneity observable in relaxation experiments such as those from dielectric spectroscopy. The relaxation behaviour of a number of systems has been considered; the analysis shows that chain orientation has a significant effect on the degree of cooperativity. Moreover, the relaxation features appear to be influenced by the thermodynamic state of the embedding crystalline domains.