The crush bands that form during plastic deformation of closed-cell metal foams are often inclined at 11-20° to the loading axis, allowing for shear displacement of one part of the foam with respect to the other. Such displacement is prevented by the presence of a lateral constraint. This was analysed in this study, which shows that resistance against shear by the constraint leads to the strain-hardening effect in the foam that has been reported in a recent experimental study. © 2009 Acta Materialia Inc.