Why do i get severe leg cramps right below the back of my knees to the point that if i am not careful they will lock?

The Charlie horse: Muscle fatigue/injury, over-used, poor blood cirulation, medication use, neurologic disease, low potassium/calcium etc...Can cause muscle spasm. At times, no cause can be found. Charlie horse or leg cramp often occurs at night and can be quite severe. The true mechanism is not well known. There are treatments, but you should get evaluated by doc first. Keep well hydrated. Good luck.