ʿUlūb is a plural denoting marks or raised/indented lines. Al-nasʿ is the strap binding the saddle or travelling equipment. Daʾayāt names parts of the animal’s back where these marks are visible.
The verse need not be read as a report of disease or injury. It primarily depicts traces made by the travelling harness. Their resemblance to markings in stone simultaneously suggests the hardness and endurance of the animal’s body.