A sprain and a break affect completely different structures within your lower limbs. Sprains are soft tissue injuries. They occur when the ligaments that stabilize your ankle or other foot structures are stretched beyond their normal flexibility, damaging them. In some cases, the connective tissue may even tear. A broken bone affects your hard tissue. Damage strains the bony structure beyond what it can support, so it cracks. Both of these are common problems for athletes and other active people. Typically they occur suddenly and painfully, and need professional intervention to heal correctly.