What is shear locking in 1st order solid elements

1st order solid elements do not behave well in bending.  Fully-integrated 1st order solids have overly-stiff behavior known as 'shear locking'.  One must be wary of this behavior.  Many codes have addressed this problem with element formulations. Fully integrated elements in Abaqus/Standard do not hourglass (shear) but may suffer from “locking” behavior: both shear and volumetric locking. Shear locking occurs in first-order, fully integrated elements (CPS4, CPE4, C3D8, etc.) that are subjected to bending.