I sprained my ankle last summer and now every once in a while, my ankle just gives out from under me. I never know when it's going to happen. Why does it do this?

The mechanics and biomechanics of ankle sprains are not fully understood. Studies using force plates and 3-D high-speed motion cameras have provided many new insights. But these are still under investigation.

What we do know is that the position of the foot and ankle just before making contact with the floor or ground and at the point of impact is extremely important. If the rear foot is in a position of inversion (slightly turned inward), there is an increased risk of spraining the lateral (talofibular) ligament.

If ankle inversion is also accompanied by a position of plantar flexion (toes pointed), the risk of ankle injury goes up as well. As your foot lands and makes contact with the floor or ground, the inverted position of the rear foot strains the lateral (talofibular) ligament of the ankle. Stepping on to an uneven surface or landing with more force than the foot and ankle were prepared for can cause hyperinversion (increased inversion) and ankle sprain. Likewise, any unexpected contact with the ground or floor can have a similar reaction.

Once the soft tissues of the ankle joint have been injured, tiny receptors within the joint can also be damaged. These receptors signal to the brain the precise location of the joint in order to prepare the ankle for the next movement. But many times, the lack of signalling feedback results in yet another injury. That's why rehab is so important to regain normal motion and strength and to restore the signaling features of this very sensitive joint.