...Muscle Cramps...
Quinine is not commonly used these days for the treatment of malaria. Instead, it is more widely used as a pain reliever of nighttime leg cramps.
Technically, it increases the tension response when a muscle is exposed to some form of direct stimulus. By doing this, the pains felt from an overworked muscle group, or tetanic stimulation, are lowered.
Quinine is in the family of alkaloids, which are known to be both stimulants and pain relievers. Some other alkaloids are:
Caffeine
Nicotine
Morphine
Cocaine