As noted above, these injuries occur when the ankle is twisted underneath the leg, called inversion. Risk factors are those activities, such as basketball and jumping sports, in which an athlete can come down on and turn the ankle or step on an opponent's foot.
Some people are predisposed to ankle sprains. In people with a hindfoot varus, which means that the general nature or posture of the heels is slightly turned toward the inside, these injuries are more common. This is because it is easier to turn on the ankle.
In those who have had a severe sprain in the past, it is also easier to turn the ankle and cause a sprain. Therefore, one of the risk factors of spraining the ankle is having instability. Those who have weak muscles, especially those called the peroneals which run along the outside of the ankle, may be more predisposed.