rend ... shoulder - by the splinters on which the shoulder or arm would fall, on the support failing the hand.
madest ... loins ... at a stand - that is, made them to be disabled. Maurer somewhat similarly (referring to a kindred Arabic form), “Thou hast stricken both their loins.” Fairbairn, not so well, “Thou lettest all their loins stand,” that is, by themselves, bereft of the support which they looked for from thee.