Functional ankle instability is a condition that occurs most often after an ankle sprain. Like you, the patient describes a sense that the ankle is going to give out from underneath him or her. Often that’s exactly what does happen.
X-rays and ligament testing are normal. That means there are no fractures and the ligaments aren’t loose or “lax.”
A recent study of electrical activity of the peroneus longus muscle in the lower leg and ankle may offer some clues. It seems the muscle is slow in responding to changes in the foot position after an ankle sprain. This decreased activity may be part of what’s causing the ankle instability.