Which preposition to use with flexures

of Occurrences 9%

Sometimes the efflorescence is continuous and universal; but more generally on the trunk of the body there are intervals of a natural hue between the patches, with papulous dots scattered over them, the colour being most deep on the loins and neighbouring parts, at the flexure of the joints, and upon those parts of the body which are subjected to pressure.

Which preposition to use with  flexures