What causes nighttime leg cramps that result in extremly painful temporary contractures called?

No definite answer. Night time leg cramps are very common especially in middle aged folks and more so in the elderly. There has been no cause documented. Mineral deficiency is debatable as there's no proven relief from use of magnesium that is advertised as anti-cramp remedy. Quinine had worked the best until it was pulled off the market a few years ago.
# of things. Venous incompetence (leaky vein valves) is one of the causes many are not familiar with. Abnormalities in blood minerals (potassium, calcium, magnesium), dehydration, tired muscles from exercise and /or prolonged standing. Seek help of phlebologist (vein doctor) who can help you.

Related Questions

Nighttime leg cramps. How to prevent and alleviate?

Leg cramps. Low magnesium MAY be the cause of your leg cramps. Also, drinking Club Soda relieves leg cramps for some people. Please see your doc to make sure all is well. Peace and good health.

I'm tapering Pamelor (40, 30, 20mg) with panic attacks & nighttime leg cramps. DR wants me to start Zoloft (sertraline) (50mg). How should I transition these meds?

Talk with your docto. Your doctor should provide very specific instructions on how to do this. Typically you slowly increase the zoloft (sertraline) while at the same time slowly decreasing the Pamelor. The time frame would depend on you specifically. If your doctor did not provide specific instructions then I would schedule another appointment to discuss this.