"Less tightly bound" is true at the molecular level whether or not the spring is expanded or contracted from a neutral position. At a molecular level, the rest

"Less tightly bound" is true at the molecular level whether or not the spring is expanded or contracted from a neutral position. At a molecular level, the rest state is for the molecules is in a kind of bathtub, loosely analogous to the effect that a camlock has in a mechanical system. Strain in either the expansion or compression direction introduces an opposing stress. Until the force exceeds enough of the bonds' strength to cause plastic deformation, both directions are "uphill" along the potential energy curve.