What causes spasms in a person's leg?

An overwhelming majority of the muscle cramps or spasms that I see are caused by muscle fatigue. Usually with muscle strain, so ... If an individual hadn't stretched out properly before exercise, or if somebody just overdid an activity one day, that's usually the cause of a muscle spasm, and what I usually tell people is to make sure you stretch before any activity. Other causes would be dehydration, which can affect your electrolytes. Usually it's magnesium or potassium. So eating a balanced diet, staying hydrated, are things that can prevent that cause of muscle cramps. But those are the 2 biggest.